# Fender Twin Reverb Rebuild/Restore



## Lincoln

I spotted this old Twin Reverb chassis on eBay a while back. Now, I need a Twin Reverb about as much as I need a second vasectomy, but I put in a bid of $111.11, figuring I wouldn't end up with it anyway. 
I won.
I am now the proud owner of a 1976 Twin Reverb chassis that's been beat to hell, robbed blind, and probably left out in the rain on several occasions. It's the 100 watt, master volume, non-ultra-linear model according to the serial number. Lots of missing parts - OT, choke, verb OT, several pots, caps, jacks, face plate, couple tube sockets, etc. It's a real beauty.









I've determined the power transformer works, the chassis is ok, it has a back face plate, and a serial number. Everything else is a wash. Maybe save some wiring.

Looking for tips and ideas on this build. One thing I've heard a lot of is "lose the master volume".
I'm thinking of building it as an AB763 blackface. All opinions & ideas welcome


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## Frenchy99

Lincoln said:


> It's a real beauty.




MJF$#

I ll be following this !!! The PT is good news to start!

Good luck !


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## cboutilier

Lincoln said:


> I spotted this old Twin Reverb chassis on eBay a while back. Now, I need a Twin Reverb about as much as I need a second vasectomy, but I put in a bid of $111.11, figuring I wouldn't end up with it anyway. I won.
> I am now the proud owner of a 1976 Twin Reverb chassis that's been beat to hell, robbed blind, and probably left out in the rain on several occasions. It's the 100 watt, master volume, non-ultra-linear model according to the serial number. Lots of missing parts - OT, choke, verb OT, several pots, caps, jacks, face plate, couple tube sockets, etc. It's a real beauty.
> View attachment 206489
> 
> 
> I've determined the power transformer works, the chassis is ok, it has a back face plate, and a serial number. Everything else is a wash. Maybe save some wiring.
> 
> Looking for tips and ideas on this build. One thing I've heard a lot of is "lose the master volume".
> I'm thinking of building it as an AB763 blackface. All opinions & ideas welcome


I'd ditch the MV, and build it back to early 70s SF spec. Or sell it to me for $112, that would work too.


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## Lincoln

cboutilier said:


> I'd ditch the MV, and build it back to early 70s SF spec. Or sell it to me for $112, that would work too.


$112 sounds like a real good deal, but I wouldn't want to burden you with such a beastly beast. 

I've been kicking back and forth between BF and SF cosmetics, since I have to buy a face plate anyway. I think silver face might just win out. I'm firmly on the fence.


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## johnnyshaka

Boy, this should be an interesting thread...I admire guys like you who see "beauty" is something like that train wreck!


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## Lincoln

johnnyshaka said:


> Boy, this should be an interesting thread...I admire guys like you who see "beauty" is something like that train wreck!


Well, it's certainly not going to happen over night. Specially with summer finally upon us. I may feel the need to venture outside on occasion. Something like this will probably be more work than starting from scratch with all new parts. But I'm in this for the adventure so it's all good. I decided I'm going to go with a 4/8/16 output transformer to allow some freedom on speaker choices.


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## cboutilier

Lincoln said:


> $112 sounds like a real good deal, but I wouldn't want to burden you with such a beastly beast.
> 
> I've been kicking back and forth between BF and SF cosmetics, since I have to buy a face plate anyway. I think silver face might just win out. I'm firmly on the fence.


I love SF cosmetics! 

I'm always on the hunt for trashed Twins to fix up for myself. I have a cabinet tucked away in the basement already for one. 

I also want to find a Twin chassis to build a 4x 6V6 Double Deluxe Reverb.


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## Lincoln

cboutilier said:


> I love SF cosmetics!
> 
> I'm always on the hunt for trashed Twins to fix up for myself. I have a cabinet tucked away in the basement already for one.
> 
> I also want to find a Twin chassis to build a 4x 6V6 Double Deluxe Reverb.


The 6V6 route has crossed my mind. Probably need a different OT though. You think about 50 watts? 40?


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## cboutilier

Lincoln said:


> The 6V6 route has crossed my mind. Probably need a different OT though. You think about 50 watts? 40?


By Fender's math it should be 44. I'd realistically expect 35.


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## Lincoln

cboutilier said:


> By Fender's math it should be 44. I'd realistically expect 35.


Another power cutting option I've read about is one where you toggle the Cathode ground (pin 8) to the two outer power tubes. (V7 & V10). Cutting the ground eliminates them out of the circuit, cutting the power by 50%. And if you don't like the tone, you never use it


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## StevieMac

Not sure if you actually stated this as part of your plan but my one suggestion is...turn it into a head. The reasoning behind this should be fairly obvious. I like the 4 x 6V6 idea @cboutilier mentioned.


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## cboutilier

StevieMac said:


> Not sure if you actually stated this as part of your plan but my one suggestion is...turn it into a head. The reasoning behind this should be fairly obvious. I like the 4 x 6V6 idea @cboutilier mentioned.


If I go 1 channel, I could likely fit it into a Super Reverb sized chassis. Which would be cool, as I have a Haggerty head cab for that.


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## Lincoln

StevieMac said:


> Not sure if you actually stated this as part of your plan but my one suggestion is...turn it into a head. The reasoning behind this should be fairly obvious. I like the 4 x 6V6 idea @cboutilier mentioned.


weight?


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## keithb7

Interesting project. I'm not 100% sure the $111 investment price was a deal. LOL. Free, plus all the parts and your time, still might be uneconomical.

I gutted a '76 non-UL Master Volume Twin. I built an earlier 73 non-MV Twin out of it. It was modded to hell, so I yanked all the crap out and made a nice amp out of it. 

This was all the crap I pulled:










The end project looked like this when I was done:










I too like the idea mentioned above. Turn it into a head. You'll end up with a Showman Reverb. They do sound killer these Twins. There is nothing like them.


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## Lincoln

keithb7 said:


> Interesting project. I'm not 100% sure the $111 investment price was a deal. LOL. Free, plus all the parts and your time, still might be uneconomical.
> 
> I gutted a '76 non-UL Master Volume Twin. I built an earlier 73 non-MV Twin out of it. It was modded to hell, so I yanked all the crap out and made a nice amp out of it.
> 
> This was all the crap I pulled:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> I too like the idea mentioned above. Turn it into a head. You'll end up with a Showman Reverb. They do sound killer these Twins. There is nothing like them.


No Fair! You got a tube with yours!! I'll save all the parts in a box too, just for a laugh. I'm not even saving the boards on this one, they are too far gone, too dirty, to ever get a good solder connection on. Great looking amp BTW. I saved your picture to my files for "reference" purposes, hope you don't mind. I didn't think about going head until StevieMac mentioned it. Makes sense. Better to carry 50lbs in each hand, than to carry 100lbs in one hand. 

I'm almost afraid to ask, but what were the 2 hose clamps doing in your amp?


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## greco

Lincoln said:


> I'm almost afraid to ask, but what were the 2 hose clamps doing in your amp?


I looked at those for a long time and wondered exactly the same thing. Then I thought maybe @keithb7 mixed them in as a way of messing with us and having a good laugh.


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## keithb7

The Twin I was discussing earlier was not my amp. It was brought to me by a local musician to repair and un-mod. A previous owner had loose tube sockets. They thought it would be a good idea to put a hose clamp around the tube base and socket to help retain everything. LOL. I re-indexed the tube sockets and threw away the hose clamps.

@Lincoln I have a crap ton of great close up pics of my personal 1973 non-push-pull MV Twin reverb. I documented everything. If you would like these pics, let me know. I can share my google-pics folder with you. Some examples here. These are great for studying when you are putting your amp back together ala early 70's non-MV style.



















If you decide to go this route, let me know through PM for shared access.

I have good pics of a 1971 Twin Rev as well. Seen here from a distance.


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## Lincoln

That would be great @keithb7 . When I get to that point, I know where to turn if I get into a jamb. Thank you.


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## LaRSin

I know this has been suggested all ready ,, I would also go with a head ,, That's what I did when I rebuilt the I bought, not working ,, But not nearly as bad as that though..


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## _Knighthawk_

+1 for the AB763.


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## cboutilier

LaRSin said:


> I know this has been suggested all ready ,, I would also go with a head ,, That's what I did when I rebuilt the I bought, not working ,, But not nearly as bad as that though..
> View attachment 208449


I would build a combo cabinet for it. I like combos.


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## Dorian2

Good luck on your build Dave. I have a couple of 8 Ohm speakers I can give to you if you want! Some guy left em at my house and I haven't got them back to him, so they're free for the taking.


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## Lincoln

Dorian2 said:


> Good luck on your build Dave. I have a couple of 8 Ohm speakers I can give to you if you want! Some guy left em at my house and I haven't got them back to him, so they're free for the taking.


Don't give them away to just anybody Pete! I will be back.


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## butterknucket

I wish I knew how to do this so I could rebuild a junked out Fender.


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## greco

butterknucket said:


> I wish I knew how to do this so I could rebuild a junked out Fender.


Start reading as much as you can and consider buying a Champ (or similar) amplifier kit. Do you have the basic hand tools and soldering equipment for electronics? Having a decent multimeter is also virtually essential.


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## butterknucket

greco said:


> Start reading as much as you can and consider buying a Champ (or similar) amplifier kit. Do you have the basic hand tools and soldering equipment for electronics? Having a decent multimeter is also virtually essential.


I have all that stuff, minus a multimeter. 

My eyes aren't so good though for that kind of thing.


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## Lincoln

butterknucket said:


> I wish I knew how to do this so I could rebuild a junked out Fender.
> 
> My eyes aren't so good though for that kind of thing.


That's why they make lighted magnifying glasses! When I started, I used a magnifier almost constantly. Now I find I seldom use one, it's like my eyes are "training" to see more detail.

If you do decide to pursue the amp thing, start with a complete kit that has good instructions. Finding all those little parts when you don't really know what you're looking for is a killer. And like @greco says, read everything you can on the subject and watch lots of YouTube's explaining the fundamentals of electronics. I really like "Uncle Doug". A touch dry, but he explains things very well is an easily understandable way.


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## butterknucket

Lincoln said:


> That's why they make lighted magnifying glasses! When I started, I used a magnifier almost constantly. Now I find I seldom use one, it's like my eyes are "training" to see more detail.
> 
> If you do decide to pursue the amp thing, start with a complete kit that has good instructions. Finding all those little parts when you don't really know what you're looking for is a killer. And like @greco says, read everything you can on the subject and watch lots of YouTube's explaining the fundamentals of electronics. I really like "Uncle Doug". A touch dry, but he explains things very well is an easily understandable way.


Thanks!

It's a little bit more complicated than that though lol.


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## greco

butterknucket said:


> It's a little bit more complicated than that though lol.


Why? 
What do you mean? 
(I'm serious with these questions)


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## greco

Lincoln said:


> Finding all those little parts when you don't really know what you're looking for is a killer.


This is exactly what did me in.


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## Frenchy99

Lincoln said:


> I really like "Uncle Doug". A touch dry, but he explains things very well is an easily understandable way.


Yep! like him too.. Watched tons from him..


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## Lincoln

cboutilier said:


> I love SF cosmetics!
> 
> I'm always on the hunt for trashed Twins to fix up for myself. I have a cabinet tucked away in the basement already for one.
> 
> I also want to find a Twin chassis to build a 4x 6V6 Double Deluxe Reverb.


Well, I climbed down off the fence and bought a face plate. It's a silver face, non-master volume.

I've also decided it's going to be a head with a separate 2 x 12 cab

It's going to a while before I get going on it though. I have several other projects to finish first.


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## Lincoln

Well, I dragged this beast out of the closet tonight to have another look at it, and put the new face plate on it before it got bent or something. I was thinking of going through amp and trying to fix it "as is". 








But there's no way that could ever be productive. It's going back in the closet again, there is nothing here to save. Next time I drag it out, it will be an getting an AB763 circuit.

I do have question though. There's a screw driver adjustable pot tied into the tube heater circuit near the end of the line beside V2.
What the heck is that all about? Is it common on these? I see @keithb7 's twin has the same pot. Hum control?


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## WCGill

Hum balance control, also known as a "humdinger". Adjust to minimum noise.


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## keithb7

Yes AC circuit heater hum balance. Not a bad thing. These were a good idea.

I recall one time when I posted an ad for Vintage Fender Twin Amp wanted. A guy contacted me. Nice fellow. He said the amp needed work. $250 or so, I think he wanted. 1978 Ultra Linear Twin R. I arrived and flashed it up to test. It sounded pretty darn great aside from the noisy hum. I paid him and took it home. Once I got home I tweaked the hum balance pot. Hum was gone! Deadly quiet. The amp had been serviced a couple years earlier. I flipped the Twin for $900 in short time.

That would be a fun project. Gutting and rebuilding that Twin. Head with a 1x15 cab would likely be pretty marketable to the pedal steel crowd.


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## cboutilier

keithb7 said:


> Yes AC circuit heater hum balance. Not a bad thing. These were a good idea.
> 
> I recall one time when I posted an ad for Vintage Fender Twin Amp wanted. A guy contacted me. Nice fellow. He said the amp needed work. $250 or so, I think he wanted. 1978 Ultra Linear Twin R. I arrived and flashed it up to test. It sounded pretty darn great aside from the noisy hum. I paid him and took it home. Once I got home I tweaked the hum balance pot. Hum was gone! Deadly quiet. The amp had been serviced a couple years earlier. I flipped the Twin for $900 in short time.
> 
> That would be a fun project. Gutting and rebuilding that Twin. Head with a 1x15 cab would likely be pretty marketable to the pedal steel crowd.


Or a 1x15 combo for the Me crowd


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## keithb7

@cboutilier do I recall correctly that you already own a 1x15 combo?


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## jb welder

Lincoln said:


> There's a screw driver adjustable pot tied into the tube heater circuit near the end of the line beside V2.
> What the heck is that all about? Is it common on these? I see @keithb7 's twin has the same pot. Hum control?


Yes, like the others said it's a hum balance pot. It's used instead of the two 100ohm resistors you often see between the heater lines and ground on Fenders that don't have the center-tap in the heater winding. If you wanted to use one_ instead_ of the heater CT you would leave the heater CT disconnected. Attached shows it in the SF Twin 135W circuit. They are fairly common but also often get burnt from tube failures, as do the resistors when used instead of the pots.


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## cboutilier

keithb7 said:


> @cboutilier do I recall correctly that you already own a 1x15 combo?


Indeed, but its only 45W of too loud. 1x15 Silverface combos are my thing.


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## Lincoln

jb welder said:


> Yes, like the others said it's a hum balance pot. It's used instead of the two 100ohm resistors you often see between the heater lines and ground on Fenders that don't have the center-tap in the heater winding. If you wanted to use one_ instead_ of the heater CT you would leave the heater CT disconnected. Attached shows it in the SF Twin 135W circuit. They are fairly common but also often get burnt from tube failures, as do the resistors when used instead of the pots.
> 
> View attachment 238968


very interesting. Could this hum balance pot setup be used on other Fender amps without a heater center tap? In place of the two 100ohm resistors to ground?


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## jb welder

Lincoln said:


> Could this hum balance pot setup be used on other Fender amps without a heater center tap? In place of the two 100ohm resistors to ground?


Yes. 
Ampeg also used them a fair bit.


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## Lincoln

Well, it was a rainy day today, so I did some work on the used up/dead Twin Reverb I bought a couple years back. I had been trying to clean it off & on over the last few months. Got all the grease and grim off it, but there is still some staining I can't scrub off with anything. A sand blaster might do the trick.

Anyway, all the filter caps had been robbed before I got it, so I put new ones in and got everything under the cover squared away.








BTW, It was not I who melted those wires, they came "pre-melted".

After that, I mounted the 3 missing transformers, and the two missing tube sockets, and installed a cap can (also missing). Checked for grounds too.








Now I'm working on the board side of the chassis. Following a AA270 schematic while keeping an eye on an AB763 schematic for reference. So far I haven't found any difference between the two, But I'm sure I will at some point. As I said, a lot of components were robbed, and any work done to it was done with the cheapest of crap available.

If this old baby could talk......I would love to hear the story of its life.


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## greco

I'll be following this with much interest. Thanks for the pics and details.


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## Lincoln

Here's the underbelly shot, in all its glory








The rectifier/bias board was in really poor shape. Someone had robbed 2 diodes and a capacitor off it even. 









After shot


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## greco

I admire you for taking on such a huge project! 

It seems as though you will have to go over the entire circuit in a square inch by square inch manner just to see what is missing, loose, etc.

This alone looks VERY scary...


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## Lincoln

the plan.........is to reconnect everything on the tube side of the chassis. Then reconnect everything on the control side of the chassis. Then take a hard look at the eyelet board, and most likely replace it totally with all new components on it. I just didn't want to have all 4 "sides" undone at the same time.

What haunts me, is wondering why this amp was stripped for parts. The power transformer would be the obvious reason, but I checked it first, it's working. I've found so many things wrong, and crappy workmanship everywhere, but is there something else? I know the normal channel never worked, because the cathode resistor and capacitor weren't grounded. and it looked like they had never been grounded.


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## Gordie

Hey Linc, that middle 22uF cap inside the doghouse is rated at 450 volts and it's for the screens of the 6L6s. I wonder if that's likely to see more than 450 VDC? It may. At the very least, I would swap the outermost 22uF one that's rated for 500V


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## Gordie

Oops, your middle cap is a 33uF @ 450V. The voltage at the screens should be on your schematic, I hope. I think this chassis I have is circa '73 or '74 Quad Reverb. I can't remember if it has a 4 or 8 ohm output trans. I was going to try and run it on 6V6s, but now I'm reconsidering....


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## Lincoln

Gordie said:


> Hey Linc, that middle 22uF cap inside the doghouse is rated at 450 volts and it's for the screens of the 6L6s. I wonder if that's likely to see more than 450 VDC? It may. At the very least, I would swap the outermost 22uF one that's rated for 500V


Good catch! Thank you. The schematic clearly says 500 volts for all three of those caps. I didn't pay attention to voltages, guess I'm too used to building smaller amps where 450 is 'good enough".
I changed two 450's up to 500's so they're all 500's now.









As you can see by the "dog's breakfast" collection of caps, I struggled to find 20UF 500V caps in my stuff. Good thing there's a cap over the caps!


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## Lincoln

I keep running into that fine line.
The line between keeping it as original as possible, and making it more reliable. 

Do you fight to save those little pieces of original wire that have been baking in the heat of the power tubes for 44 years?

Do you re-pin those original tube sockets? A lot of them, the lugs are broken off and the wires just soldered onto whatever stub was left.

Do you rip everything out and just start over? It would be way faster. I know, it's only a 76 twin. Not like it's anything precious or rare.


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## Lincoln

I've been working on this epic disaster Twin Reverb for the last two nights now. I took out the old eyelet board, installed a new AB763 board I had pre-prepped. Started hooking everything back up and making sense out of it all. I have a toggle in the old ground switch hole that shuts off the ground to pin 8 on two power tubes for a 1/2 power mod. Half of way too much is still too much? 

Either this amp was already mostly converted from an AA270 to a AB763, or there is very little difference between the two. One thing that caught my attention was the phase inverter coupling capacitor. It had been replaced, and was going to the wrong side of the 220K channel mixing resistors. Mistake or meant to be? Actually that whole area is different.


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## Paul Running

Lincoln said:


> Do you rip everything out and just start over? It would be way faster. I know, it's only a 76 twin. Not like it's anything precious or rare.


That's what the Blueprinters do with an engine, rip her down and check every component to spec before assembling and testing...many hobbies are similar, including amps.


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## jb welder

Lincoln said:


> Either this amp was already mostly converted from an AA270 to a AB763, or there is very little difference between the two. One thing that caught my attention was the phase inverter coupling capacitor. It had been replaced, and was going to the wrong side of the 220K channel mixing resistors. Mistake or meant to be? Actually that whole area is different.


I'm sure you know the stickers are not reliable as far as the versions, so I guess the AA270 reference is by age? Or based on what you are finding in there?
Where exactly was that coupling cap going, and what other differences in that area?


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## Paul Running

Lincoln said:


> 've been working on this epic disaster Twin Reverb for the last two nights now. I took out the old eyelet board, installed a new AB763 board I had pre-prepped. Started hooking everything back up and making sense out of it all. I have a toggle in the old ground switch hole that shuts off the ground to pin 8 on two power tubes for a 1/2 power mod. Half of way too much is still too much?


Does the output transformer have multiple-secondaries? If so, you could run an 8 ohm cab from the 4 ohm tap, to match the impedance of a pair of output tubes versus a quad.


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## Paul Running

Keep those multi-tap transformers if you plan on experimenting...they can place a match-up center of the reactive ellipse, on a load-line, with a bit of tweaking.


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## Lincoln

Paul Running said:


> Does the output transformer have multiple-secondaries? If so, you could run an 8 ohm cab from the 4 ohm tap, to match the impedance of a pair of output tubes versus a quad.


Yes! The OT was missing anyway, so when I bought a New OT I got the multi-tap upgrade version. 16/8/4 ohms. It's going to be a head, so anything is possible.


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## Paul Running

Here's my Ampeg V5, 1985; originally came with 4 X 6550. I have re-tubed to 2 X EL34. Impedance is matched simply by switching to 4 ohms and check your voltages and plate currents.


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## Lincoln

Here's a work in progress shot of my dirty old twin. New board is in place, I started wiring from the R.H. side of the picture, working left.


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## Paul Running

Does this design look somewhat familiar to something that Dudley Craven might have imagined?


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## Lincoln

jb welder said:


> I'm sure you know the stickers are not reliable as far as the versions, so I guess the AA270 reference is by age? Or based on what you are finding in there?
> Where exactly was that coupling cap going, and what other differences in that area?


It looks like maybe that's where the master volume tied in, looking at pictures supplied by @keithb7 of his 73 TR with MV (on page 1) All that was already gone when I got this chassis. 
Here's a side by side, old board on bottom of shot, one of the .1 caps is missing in the picture. Look where the little .001 cap attaches. This is the area of the board that is different. I don't know that it was an AA270, I'm just guessing because of the age of it.


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## Paul Running

The stock TFMs in the V5 are safely operated by a dual pair...I believe that quads are pushing both TFMs past my comfort zone. I do like the aggression when this thing is tweaked. I last had it configured with a pair of Philips EL34s that I got at a tube swap...many years ago. Anyway, I piggy-backed a 120VAC TFM to the main PTFM and I was hitting 630VDC to the plates (stock 465VDC + 165VDC). The Philips like to run hot so, I don't run them often or long, otherwise, I'm looking at more than convectional cooling.


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## Paul Running

Lincoln said:


> Here's a work in progress shot of my dirty old twin. New board is in place, I started wiring from the R.H. side of the picture, working left.
> View attachment 344997


Yeah, we all have a routine that we feel comfortable with...just like regular or goofy footers...I'm a goofy-footer.


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## cboutilier

Lincoln said:


> It looks like maybe that's where the master volume tied in, looking at pictures supplied by @keithb7 of his 73 TR with MV (on page 1) All that was already gone when I got this chassis.
> Here's a side by side, old board on bottom of shot, one of the .1 caps is missing in the picture. Look where the little .001 cap attaches. This is the area of the board that is different. I don't know that it was an AA270, I'm just guessing because of the age of it.
> View attachment 344999


I have a stock '75 I could open if you needed more picture of the stock layout.


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## Lincoln

Paul Running said:


> Yeah, we all have a routine that we feel comfortable with...just like regular or goofy footers...I'm a goofy-footer.


I've always started on the pre-amp side. Am I doing it backwards?


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## Paul Running

I don't think there is a backwards. For myself, I usually build the power supplies, check and test them then I lay the LV wiring and start from the output section and work to the front, testing each stage as I go...unless it's a simple amp, I feel more comfortable building and testing in stages.


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## Lincoln

cboutilier said:


> I have a stock '75 I could open if you needed more picture of the stock layout.


Thank you for the offer, I'm not sure it would help me though. I'm building this amp into an old Blackface version, so that's more what I need to be looking at. I've got good prints, I'll get through it.


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## Paul Running

Lincoln said:


> Thank you for the offer, I'm not sure it would help me though. I'm building this amp into an old Blackface version, so that's more what I need to be looking at. I've got good prints, I'll get through it.


Are you going to run the plates of V1A and V2A at 180VDC or 210VDC?


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## Paul Running

You may have this project for the Deluxe Reverb:
If you are using the stock reverb TFM, I recommend using strong 12AT7s and double-check the value of the 12AT7's cathode resistor.


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## Lincoln

Paul Running said:


> You may have this project for the Deluxe Reverb:
> If you are using the stock reverb TFM, I recommend using strong 12AT7s and double-check the value of the 12AT7's cathode resistor.


No, I've never seen that paper before. Thank you. I have the parts for a deluxe R and a super R sitting here waiting for motivation to strike, so I'll make good use of that.

When I placed components on the board, I was using a Fender DR drawing. Now that I'm attaching wires to double checking things, I'm using a MoJo Tone TR drawing. They show wire colours, I found that helpful. Usually the plates on V1 & V2 come out as something over 200 volts. What's the deal with keeping them down to 180V?


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## Paul Running

Lincoln said:


> What's the deal with keeping them down to 180V?


The lower plate voltage will reduce the headroom for the first stage...it may not be an issue, unless you are planning on overdriven inputs to the first stage, ie hot pedals.


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## Paul Running

Lincoln said:


> Another power cutting option I've read about is one where you toggle the Cathode ground (pin 8) to the two outer power tubes. (V7 & V10). Cutting the ground eliminates them out of the circuit, cutting the power by 50%. And if you don't like the tone, you never use it


It might be safer to remove the tubes that are not in circuit...it can be an unsafe practise to leave floating inputs or any pin unterminated, when power is applied.


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## Lincoln

Paul Running said:


> It might be safer to remove the tubes that are not in circuit...it can be an unsafe practise to leave floating inputs or any pin unterminated, when power is applied.


I was already re-thinking that. Partly because even when cut down to 1/2 power, we're still talking 50 watts. What's the point? And partly because I could make much better use of that empty hole (ground switch) as a speaker impedance selector.


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## jb welder

Schematic and layout (pg.2) attached, in case anyone following along.



Lincoln said:


> Look where the little .001 cap attaches. This is the area of the board that is different. I don't know that it was an AA270, I'm just guessing because of the age of it.
> View attachment 344999


Looks like they just used the wrong eyelets, but the .001 is still going to the junction of the 220K's.
So if they had the black & white wires going to the other ends of those 220K resistors, it would still do the same job and be correct to the schematic. As long as there are no other connections on the bottom side of the board.


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## Lincoln

jb welder said:


> Schematic and layout (pg.2) attached, in case anyone following along.
> 
> 
> 
> Looks like they just used the wrong eyelets, but the .001 is still going to the junction of the 220K's.
> So if they had the black & white wires going to the other ends of those 220K resistors, it would still do the same job and be correct to the schematic. As long as there are no other connections on the bottom side of the board.


Thank you for the download.

Nothing on the bottom of the board at all. The picture of keithb7's amp shows shielded wire there also. Mine had nothing. Would that shielded wire be the feed in from channel 1? Is that shield a good idea? 

I did notice one change from the AB763 DR I was following to the AB763 TR. On the negative feedback string, after the 82K, there is another resistor just before ground. The PI bias resistor goes to ground through it too. On the DR it was a 47R, but on the TR it called for 100R. So I changed it to a good stout 100R. I have no idea what that's going to do, but there must be a reason for it.


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## Paul Running

Lincoln said:


> Thank you for the download.
> 
> Nothing on the bottom of the board at all. The picture of keithb7's amp shows shielded wire there also. Mine had nothing. Would that shielded wire be the feed in from channel 1? Is that shield a good idea?
> 
> I did notice one change from the AB763 DR I was following to the AB763 TR. On the negative feedback string, after the 82K, there is another resistor just before ground. The PI bias resistor goes to ground through it too. On the DR it was a 47R, but on the TR it called for 100R. So I changed it to a good stout 100R. I have no idea what that's going to do, but there must be a reason for it.


Here's a blurb on the LT pair...most of your questions will be answered here:


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## jb welder

Lincoln said:


> The picture of keithb7's amp shows shielded wire there also. Mine had nothing. Would that shielded wire be the feed in from channel 1? Is that shield a good idea?


The SF's (especially with master) got pretty busy and prone to noise pickup from all the wire runs, thus the shielded wire. With the basic AB763 I think you are ok without.



> I did notice one change from the AB763 DR I was following to the AB763 TR. On the negative feedback string, after the 82K, there is another resistor just before ground. The PI bias resistor goes to ground through it too. On the DR it was a 47R, but on the TR it called for 100R. So I changed it to a good stout 100R.


The 82K is part of the PI cathode circuit, but not really related to the NFB. 
The NFB is the 820 coming back from the OT secondary, and the 100 (or 47). So a voltage divider with the 820 and 100 in series. The ratio of those 2 resistors sets the NFB. 
Now with the Twin you have 4R load vs 8R load for the Deluxe. So the voltage across the load will be double for the 8R load. To achieve the same amount of NFB for a 8R load, we can either double the 820R, or halve the 100R. They chose to halve the 100R, so 47R at the bottom end of the NFB divider for the Deluxe.


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## Lincoln

well, hard for me to believe, but it's all together and hooked up. Everything Except for a toggle I want to install for 4ohm/8ohm speaker. Voltage tested without tubes, everything looked good. I'm going to wait until tomorrow when I can go get the toggle I need to see if it makes noise.


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## Lincoln

jb welder said:


> The SF's (especially with master) got pretty busy and prone to noise pickup from all the wire runs, thus the shielded wire. With the basic AB763 I think you are ok without.
> 
> 
> The 82K is part of the PI cathode circuit, but not really related to the NFB.
> The NFB is the 820 coming back from the OT secondary, and the 100 (or 47). So a voltage divider with the 820 and 100 in series. The ratio of those 2 resistors sets the NFB.
> Now with the Twin you have 4R load vs 8R load for the Deluxe. So the voltage across the load will be double for the 8R load. To achieve the same amount of NFB for a 8R load, we can either double the 820R, or halve the 100R. They chose to halve the 100R, so 47R at the bottom end of the NFB divider for the Deluxe.


Thank you for the explanation on those two questions I had. 
I meant 820R when I said 82K for that feedback resistor..........but I had to go back and check anyway, just to make sure, with my history & all.  There's an 820R in there.


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## Lincoln

the bowels of the beast


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## Paul Running

And here's a chapter on Feedback that may help you:


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## Lincoln

Paul Running said:


> And here's a chapter on Feedback that may help you:


More bedtime reading! Thank you


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## Lincoln

I did fire it up and make noise, but it wasn't a total success. The overall volume is very low, and it acts like I have some sort of funny ground issue going on. On both channels, the treble control has more effect on volume than the volume control does. The volume although very low (maybe 1 watt) builds until about 1/2 sweep and then drops off as you increase from there. Voltages are where they should be, and I set the bias at dead on 75% for 6L6's (50mA @ 450V). Still looking. Good thing is, I put everything on top of the board so I can easily follow. I just realized I did not check the values of any pots. I took for granted they had never been messed with.


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## Paul Running

Do you have a signal generator and scope?


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## Lincoln

Paul Running said:


> Do you have a signal generator and scope?


no, I have nothing other than multi-meters


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## Paul Running

Lincoln said:


> no, I have nothing other than multi-meters


Since your voltages are in spec, try the split-method for troubleshooting. Start in the middle and work one way or the other...if the first half works, work towards the output. Sorry but I'm not much help without a scope and signal gen...comparable to some people with their smart phones.


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## Lincoln

Because both channels are doing exactly the same thing, I'm wondering if I should looking in the phase inverter area. 
There were so many wires cut on this chassis as someone canalized it for parts. Everything is pretty much spelled out on a schematic except for the running of the grounds. There was one large gauge ground wire running from the board, around channel 2 grounding area to nowhere. I never did find a home for it, and eliminated it. I wonder now if it went under the can.


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## Paul Running

It could be an incomplete ground...check the ground terminations. Completely discharge all caps and begin DC resistance checks. Power and grounds are a good starting point. Check your schematic, determined what you should measure and confirm it with a measurement...it does help to proceed in a methodical manner.

DIY cap slow / fast discharge probe. Easy to build and convenient and yes, I have received a lifter...you learn to respect the electron after your first lifter and you also learn not to get ahead of yourself...stay calm and think it through:

















Connect a test speaker to the output.
This can only be performed as long as you have grid-stopper resistors on the control grids (P5) of the output tubes...nothing lower than 4K7 ohms.
If you ground the control-grid with a probe that terminates to ground, you should hear a fair-size crunch from your test speaker...a 6V6 with instantaneous 0V on the grid is going to climax guaranteed. Do it with both sides and this will provide a verdict on the output section.


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## Lincoln

The speakers are making noise each time I touch something to test a voltage, and that's pretty normal from what I've experienced. I'll be going over it again, extra close to see what I missed.

I built myself an amp chassis stand today. No more wrestling that twin reverb on the bench.


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## greco

Lincoln said:


> I built myself an amp chassis stand today. No more wrestling that twin reverb on the bench.


COOL!..Congrats!

Is this a wooden cradle style of chassis holder that allows for rotation about the long axis of the chassis and can be locked at various angles?


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## GreasyB in YYC

Speaking of homemade chassis stands, Uncle Doug posted “blueprints” to build a DIY chassis stand on his latest youtube vid....for those that are interested.


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## Lincoln

GreasyB in YYC said:


> Speaking of homemade chassis stands, Uncle Doug posted “blueprints” to build a DIY chassis stand on his latest youtube vid....for those that are interested.


Yeah, I'm a member of his fan club. His last YouTube inspired me to finally build one. It came at the perfect time for me


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## Lincoln

greco said:


> COOL!..Congrats!
> 
> Is this a wooden cradle style of chassis holder that allows for rotation about the long axis of the chassis and can be locked at various angles?


yup.


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## nonreverb

Lincoln said:


> The speakers are making noise each time I touch something to test a voltage, and that's pretty normal from what I've experienced. I'll be going over it again, extra close to see what I missed.
> 
> I built myself an amp chassis stand today. No more wrestling that twin reverb on the bench.


As stated earlier, the grounds might be a good place to start. Check the rail ground under the pots. That's a known area of trouble. The solder spots holding the ground wires can sometimes be bad. Also check the 100ohm resistor ground at the PI. I've seen them where they're so stretched across the eyelets the solder gets a halo crack.


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## loudtubeamps

After you do your trouble shooting.....If you haven’t fired it up yet, may I recommend a series load bulb limiter for a safe startup ?
FWIW...if I have a dead soldier, my first look is checking plate voltages on all tubes.
A bad circuit and or bad blocking cap will show up with at or near full B+ at the plate.
I have always liked the overall tone and performance of pre amp tubes running in the 220 to 240 v range.
The phase inverter...in the high 200’s to low 300’s


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## Paul Running

Lincoln said:


> yup.
> 
> View attachment 346298


Nice...I like the angled view.


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## nonreverb

loudtubeamps said:


> After you do your trouble shooting.....If you haven’t fired it up yet, may I recommend a series load bulb limiter for a safe startup ?


As good an idea as that is, it's not practical on a Twin IMHO. With a input wattage rating of over 300W you need a serious bulb to power it. Otherwise it won't get up to full power. It could without the tubes but that defeats the purpose.


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## Frenchy99

Lincoln said:


> yup.
> 
> View attachment 346298


Now I`m jealous...


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## Lincoln

nonreverb said:


> As stated earlier, the grounds might be a good place to start. Check the rail ground under the pots. That's a known area of trouble. The solder spots holding the ground wires can sometimes be bad. Also check the 100ohm resistor ground at the PI. I've seen them where they're so stretched across the eyelets the solder gets a halo crack.


I'm used to soldering the PT end of the brass plate to a wire, and running that wire to my common ground. This plate was soldered to the chassis, and it was cracked of course. I re-soldered it when I put the board in. A lot more ground points used on this amp than I'm used to using. I pretty much left the pots and wiring along. I need to pull them out and make sure things are as they should be.


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## Lincoln

loudtubeamps said:


> After you do your trouble shooting.....If you haven’t fired it up yet, may I recommend a series load bulb limiter for a safe startup ?
> FWIW...if I have a dead soldier, my first look is checking plate voltages on all tubes.
> A bad circuit and or bad blocking cap will show up with at or near full B+ at the plate.
> I have always liked the overall tone and performance of pre amp tubes running in the 220 to 240 v range.
> The phase inverter...in the high 200’s to low 300’s


In the last 4 months or so, I've started plugging the amps I'm testing, into a power monitor. It won't stop an overload of course, but at least I can see what's going on. This one was running about 180 to 200 watts, way more than anything I've seen so far.


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## Lincoln

Frenchy99 said:


> Now I`m jealous...


wow, that's a switch!


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## Paul Running

Aren't they a handy item!










I've had this one for 15 years now...a RS clearance purchase. Kicking myself that I didn't buy more however, the price is descent now and maybe they have a few more features, I should check that out.


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## Lincoln

Well, I don't know what to say.

I put the twin Reverb chassis on the stand, put the power tubes back in it, fired it up, and be dammed if it doesn't work like a million bucks! 

I had checked all the grounds coming out of the filter caps (4 of them), looked around, poked around a bit, put the tubes back in, and suddenly it works! 

I can't get the volume over about 2-1/2 in my room, and it's so sensitive! I was playing a strat, and had clean quack like I've never heard before. Pretty excited. 😎


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## Frenchy99

Congrats !!!

Sounds like a cold solder somewhere. You might have temp fixed it by pocking around... Or double check how clean your pre amp sockets are. Had that one happen to me several time.


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## Lincoln

Frenchy99 said:


> Congrats !!!
> 
> Sounds like a cold solder somewhere. You might have temp fixed it by pocking around... Or double check how clean your pre amp sockets are. Had that one happen to me several time.


Thanks

I was poking around the phase inverter socket, that's the only one, and it was a new socket too I'll re-flow it just in case.


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## nonreverb

Lincoln said:


> Thanks
> 
> I was poking around the phase inverter socket, that's the only one, and it was a new socket too I'll re-flow it just in case.


Great work Lincoln!


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## loudtubeamps

nonreverb said:


> As good an idea as that is, it's not practical on a Twin IMHO. With a input wattage rating of over 300W you need a serious bulb to power it. Otherwise it won't get up to full power. It could without the tubes but that defeats the purpose.


Quite correct about lower voltages...about a 10 to 15% drop.Still more than adequate to troubleshoot a sick amp that is drawing too much current. I have been using a 100 watt bulb for testing amps up to 200 watts...specifically the Hiwatt 200’s ..no issues and lots of headroom on the bulb filament.Good to hear the Twin is up and running Lincoln.


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## nonreverb

Paul Running said:


> Aren't they a handy item!
> 
> View attachment 346438
> 
> 
> I've had this one for 15 years now...a RS clearance purchase. Kicking myself that I didn't buy more however, the price is descent now and maybe they have a few more features, I should check that out.





loudtubeamps said:


> Quite correct about lower voltages...about a 10 to 15% drop.Still more than adequate to troubleshoot a sick amp that is drawing too much current. I have been using a 100 watt bulb for testing amps up to 200 watts...specifically the Hiwatt 200’s ..no issues and lots of headroom on the bulb filament.Good to hear the Twin is up and running Lincoln.


That's cool. Personally, I've never used a bulb for tube amps and never had a problem. I do use one whenever I'm repairing SS amps though.


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## Frenchy99

I only use a light bulb when I have a good idea !


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## jb welder

The idea with the bulb limiter is to save on fuses, and save the amp from damage. You want to use a lower power bulb for this reason. Using a bulb that is high enough power to allow the amp to run at volume will not offer any protection.
Somebody's uncle on youtube  got an earful for recommending a 300W limiter bulb for that reason.

Once the amp is repaired to the point you are ready to test at volume, you shouldn't be on the bulb anymore.


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## nonreverb

jb welder said:


> The idea with the bulb limiter is to save on fuses, and save the amp from damage. You want to use a lower power bulb for this reason. Using a bulb that is high enough power to allow the amp to run at volume will not offer any protection.
> Somebody's uncle on youtube  got an earful for recommending a 300W limiter bulb for that reason.
> 
> Once the amp is repaired to the point you are ready to test at volume, you shouldn't be on the bulb anymore.


The only use I've found for a series bulb is to prevent the instantaneous demolition of many dollars worth of replacement parts in a solid state amp. And always 60 watt. That way you don't run the risk of blowing something even with a bulb in circuit. It's worth it's weight in gold for that purpose.


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## Lincoln

Well, I certainly wasn't thinking straight when I centered the handle on the new cab for the TR.








When you pick it up by the handle, it hangs at about a 90 degree angle.


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## cboutilier

Lincoln said:


> Well, I certainly wasn't thinking straight when I centered the handle on the new cab for the TR.
> View attachment 347681
> 
> When you pick it up by the handle, it hangs at about a 90 degree angle.


Just as Leo would have done. Try using the single handle to pick up a 36" tall, 100lbs Bassman 10!


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## Lincoln

cboutilier said:


> Just as Leo would have done. Try using the single handle to pick up a 36" tall, 100lbs Bassman 10!


I'm going to put two more handles on the top, one of each end, running the same direction as the chassis straps. That's too heavy for one hand.


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## Paul Running

Lincoln said:


> I'm going to put two more handles on the top, one of each end, running the same direction as the chassis straps. That's too heavy for one hand.


Yeah, that's a complex skill to layout a chassis so, that it is balanced for mass and not have issues with EMI...those TFMs and chokes can be challenging.


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## loudtubeamps

Lincoln said:


> Well, I certainly wasn't thinking straight when I centered the handle on the new cab for the TR.
> View attachment 347681
> 
> When you pick it up by the handle, it hangs at about a 90 degree angle.


A length of 1” hardwood doweling under the assembled amp (on a hard surface to find the balance point ) is my high tech method when determining where the handle should go.🤓


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## KapnKrunch

Lincoln said:


> Well, I certainly wasn't thinking straight when I centered the handle on the new cab for the TR. When you pick it up by the handle, it hangs at about a 90 degree angle.


🤣😂😅 you don't wanna move that mofo anyway! Might as well look good.


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## cboutilier

Lincoln said:


> I'm going to put two more handles on the top, one of each end, running the same direction as the chassis straps. That's too heavy for one hand.


That's one of the reasons I like single speaker combos. You can use speaker placement as a counterweight


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## Lincoln

cboutilier said:


> That's one of the reasons I like single speaker combos. You can use speaker placement as a counterweight


I'm sure that was in the back of my mind too. This is the first head cabinet I've ever built, and of course it had to be the one with 35 lbs worth of transformers on one side.
I would have sworn this thing weighed 50 lbs. I must be getting old.


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## tdotrob

Lincoln said:


> I'm sure that was in the back of my mind too. This is the first head cabinet I've ever built, and of course it had to be the one with 35 lbs worth of transformers on one side.
> I would have sworn this thing weighed 50 lbs. I must be getting old.


My Bogner is the same, center handle, massive iron, pick it up and one end just drops straight down.


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## Lincoln

tdotrob said:


> My Bogner is the same, center handle, massive iron, pick it up and one end just drops straight down.



I don't feel so bad then if Bogner is doing it too. I'm going to put on side handles like an AC30 though.


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## cboutilier

Lincoln said:


> I'm sure that was in the back of my mind too. This is the first head cabinet I've ever built, and of course it had to be the one with 35 lbs worth of transformers on one side.
> I would have sworn this thing weighed 50 lbs. I must be getting old.


That sounds about right. A stock Twin was around 85 lbs or so.


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## jb welder

Nobody wants to suggest putting some counter-weight on the other end?


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## Lincoln

jb welder said:


> Nobody wants to suggest putting some counter-weight on the other end?


I've got a big old 1 to 1 isolation transformer I could add to the other end! It's almost as big as the PT.


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## cboutilier

Lincoln said:


> I've got a big old 1 to 1 isolation transformer I could add to the other end! It's almost as big as the PT.


I used an impedance matching transformer as a counterweight when my Super Reverb was head mounted!


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## player99

Can you not grab it by the handle with one hand and by the bottom of the heavy end with the other hand?


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## Lincoln

player99 said:


> Can you not grab it by the handle with one hand and by the bottom of the heavy end with the other hand?


 That's what I've been doing to haul it around. Your survival instincts kick in and make you grab it with 2 hands when you pick it up.

So I figured, why fight the feeling? I'll just put 2 handles on it.


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## Gordie

I'm building a Quad Reverb chassis into a new finger-jointed pine head cab. There'll be two dog-bone handles Vox style


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## bolero

Lincoln said:


> That's what I've been doing to haul it around. Your survival instincts kick in and make you grab it with 2 hands when you pick it up.
> 
> So I figured, why fight the feeling? I'll just put 2 handles on it.


didn't read the whole thread, but can't you just move the handle so it's over the transformer & the weight balances?


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## Lincoln

bolero said:


> didn't read the whole thread, but can't you just move the handle so it's over the transformer & the weight balances?


The handle holes are already drilled, I guess they could be plugged with something. I'll have to see where the balance point is. It must be way off center


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## Paul Running

You can use a pull scale to determine mounting points. I bought mine at a Dollar Store...luggage scales and put a quick-pin on it...fish scales will work too:









The attached file is a xlsx file (an XL spreadsheet file). Can't upload .xlsx files on this platform so, I renamed the file to .pdf...rename file to .xlsx if you download the file (it will not execute as a pdf file)


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## Lincoln

Here's the finished amp. I never did mount those other two handles I talked about. Maybe someday.


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## Paul Running

Nice and shiny.


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## greco

EXCELLENT build! VERY impressive workmanship.

CONGRATS! You have to be extremely proud of what you have accomplished.


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## Lincoln

greco said:


> EXCELLENT build! VERY impressive workmanship.
> 
> CONGRATS! You have to be extremely proud of what you have accomplished.


Thank you Dave. It turned out well. Something about the the tone of a twin reverb is really special. 
I started this amp in May of 2018, gave up and stashed in away in the closet a few times. I learned a lot of good things doing it. The experience, changed the way I build amps.


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## KapnKrunch

Extremely proud of you, my friend. And yes, the Twin is truly a classic.


----------

