# YBA-1 Project



## Telemark (Feb 9, 2006)

I am getting into my 66 YBA1 serial 421 - felt pen or 768 - Label. 
I have some questions for those tweakers out there. and Thanks for the Kevin O'Connor Book recommendation to Scottone, that book is helping me get my head around this amp.


So far I found it biased for El-34 with 6L6GC installed, an input wire brushing a B+ wire causing hum and a strange microphonic 

I added 1 meg pots a bias cct and removed an extraneous resistor on the presence cct . I have also added a 3 condutor power plug socket.

Next I need to chase down an oscillation when the presence and treble are dialled up. I think I will change out the electrolytic power supply caps as a first attempt to fix that.

My question is about "grid stoppers" I see from later YBA 1 - 4, designs the power tube grids have 1K5 with a Feed of 68K verses the '66 design of 470K feed with no grid stopper.

Is this mod worth doing?







[/IMG]


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## Scottone (Feb 10, 2006)

Telemark said:


> I am getting into my 66 YBA1 serial 421 - felt pen or 768 - Label.
> I have some questions for those tweakers out there. and Thanks for the Kevin O'Connor Book recommendation to Scottone, that book is helping me get my head around this amp.
> 
> 
> ...


one side of your heater wiring seems to be disconnected at v1.

Obviously the amp has been completely re-wired at some point and a new output transformer installed. The splices on the OT tranformer should be covered with heat shrink tubing as opposed to black electrical tape.

The oscillation may be pre-amp tube related...have you tried swapping them yet. You could try installing the grid stoppers as they are meant to suppress oscillation at higher frequencies.

I like to see the heater wiring raised a bit higher over the other wiring as well...it looks a bit tight to the chassis in the picture.

Looks like a fun project...


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## Telemark (Feb 9, 2006)

Thanks Scott, I did fix the heater wiring as it busted off while I was poking around. 

I wondered about the OPT. The previous owner is a tech and freind of mine and was teaching himself about tube ccts with this amp. He found the the OPT miswired and set it up as is.

Yah it's not pristine, but that opens the doors for me and my soldering iron.:banana: 

I have changed the tubes with others I have laying around.

Any suggestions for getting rid of the humm?
When it was biased for around 22ma there was no hum, It didn't sound very good ! No it's around 80ma and the idle 
hum is substantial.

I may try a bias balance.
:rockon2:


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## Scottone (Feb 10, 2006)

Telemark said:


> Any suggestions for getting rid of the humm?
> When it was biased for around 22ma there was no hum, It didn't sound very good ! No it's around 80ma and the idle
> hum is substantial.
> 
> ...


Make sure that the heater wiring on the output sockets is "in phase" (i.e. pin 2 of one power tube is connected to pin 2 of the other power tube and pin 7 is connected to pin 7)

Also, if the power tubes are wildly unmatched it can cause excessive residual hum.


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## Telemark (Feb 9, 2006)

> Make sure that the heater wiring on the output sockets is "in phase" (i.e. pin 2 of one power tube is connected to pin 2 of the other power tube and pin 7 is connected to pin 7)


i never would of that of that , makes sense thanks:banana:

-just checked - in phase but not very tightly wound and as you mentioned not dressed very well. I'll see how is sounds after changing the PS Filter caps.evilGuitar:


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## Scottone (Feb 10, 2006)

do you have a picture of the top of the chassis...I'm curious about the output transformer.


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## Telemark (Feb 9, 2006)

I can take one later but It is a Hammond 1664, I can't find any info on the Net.

From the wiring it looks as though the choke was also replaced. there is a splice heading to the Powertubes and the soldering on the caps looks pretty messy.

The PT has a Yorkville copper plate with the same serial number as the chassis so I think it's original.


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## Telemark (Feb 9, 2006)

*YBA Transformers*

Scottone - The Output Transformer










and the big iron line up!










:rockon:


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## Scottone (Feb 10, 2006)

Telemark said:


> Scottone - The Output Transformer
> 
> 
> 
> ...


Thanks...I strongly suspect that this isn't the original tranny, but is the same size as the original and same manufacturer. Looks like a lot of rust on the laminates, which isn't a good thing. The choke looks right, but it could have been swapped in from another amp.

I find it strange that there are the two bright yellow wires coming out of the tranformer for the primary connections (they should be brown and green). I can see the brown and green wires in the bundle at the back of the amp, and I worry that these yellow rascals were badly spliced somewhere.

I don't want to make more work for you, but I think you should disconnect the output transformer and remove it to have a look. Something just doesn't look right here.

Might also be a good opportunity to put up a wanted ad for an original Bassmaster output tranny. I know that a lot of folks switch these out for premium units (i.e. Mercury Mag, OEI, etc., so there must be some kicking around.


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## Wild Bill (May 3, 2006)

*Old Hammond info*

You might google up House of Jim in Winnipeg. His site has some info on old Hammond OT numbers. He also sells great eyelet boards.

It might mean redoing some splices but I'd be tempted to keep the Hammond, especially if you play some low hard core tunings, like open C.

Bass notes need more iron, which is why bass guitar OT's weigh about a third or so heavier than one for lead guitar. Many "Dime Bag Darryl" players will tune below standard pitch and then wonder why their SLO100 or Booger seems to crap out, especially on the lowest strings. The problem is that their OT just isn't beefy enough.

Hammond 1600 series are hifi rated down to maybe 20hz. They keep up just fine!

They are harder to saturate, but if you just play louder and louder... 

Wild Bill


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## Scottone (Feb 10, 2006)

Wild Bill said:


> You might google up House of Jim in Winnipeg. His site has some info on old Hammond OT numbers. He also sells great eyelet boards.
> 
> It might mean redoing some splices but I'd be tempted to keep the Hammond, especially if you play some low hard core tunings, like open C.
> 
> ...


Wild Bill, wouldn't you have some concerns with the amount of rust on the laminates? It looks like it sat in a damp basement for about 20 years.

It actually may be a good transformer, but I'm really suspicious about the yellow wires that have been spliced in somehow. If you look at the earlier under chassis shot, you can see the original red, green, and brown wires coming out of the other hole and bundled up at the back of the amp.


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## Telemark (Feb 9, 2006)

I'm gonna take it apart tommorrow, The rust isn't as bad as the picture showed. Under flourescent lights you have to look pretty close and there is still paint with a _rust colored_ coating. though there is a least one good dime sized spot. ( It probably has spent some time neglected, it was given to my 15 yrs ago and I let it sit for 3 or 4 years, gave away and finally got it back last year. )

I'll see if I can figure out primary, secondary and Taps, it should be interesting.

I have scoped the outuput and it was symetrical, - pushing 40 watts across 8 ohms before it started to distort visibly.


And about Drop C - I am playing a Vince Gill song in a cover band, but the singer hasn't the range so Drop C it is! doesn't quite sound rite onthe YVC-50.
I did some jammin on this amp allready in drop D and it was very good, I will be keeping this tranny if it all works out.


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## Ripper (Jul 1, 2006)

Rust on those old tranny's really isn't an issue. They are so well sealed and gooped inside the rust very seldom gets inside to do any damage. I've seen tranny's so rusty it isn't even funny and they function fine. Alot of the old hammonds have a sepia/rust coloured undercoat on them. Check your ground connections and your wire dress, those are two of the things that just looking at the pictures jump out at me.

Jim from the House of Jim is a good guy and is always willing to help. If you need a tranny let me know what you are looking for, I might have one in my stash.

Ripper


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## Scottone (Feb 10, 2006)

Telemark said:


> I'm gonna take it apart tommorrow, The rust isn't as bad as the picture showed. Under flourescent lights you have to look pretty close and there is still paint with a _rust colored_ coating. though there is a least one good dime sized spot. ( It probably has spent some time neglected, it was given to my 15 yrs ago and I let it sit for 3 or 4 years, gave away and finally got it back last year. )


sounds like it's in better shape than it looks in the photo.

The primary should have a red, green, and brown wire, and I beleive the secondary is black and yellow if it's only one impedence tap. 

I'm not sure where those bright colored yellow wires are soldered in (the one's that go to the power tubes and the speaker jack). You may have to remove the bell covers on the transformer to find out. 

Let us know what you find...


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## Wild Bill (May 3, 2006)

*Colour codes...*



Scottone said:


> sounds like it's in better shape than it looks in the photo.
> 
> The primary should have a red, green, and brown wire, and I beleive the secondary is black and yellow if it's only one impedence tap.
> 
> ...


There was an industry standard colour code for trannies since the heyday of 
Charlie Chaplin. With an OT, the plate wires are blue and brown and the CT is red. The speaker side has black for the common, grn for 4 ohms, ylw for 8 ohms and my memory stalls out about there.

Power trannies have black for the primary, with white or a white stripe for a different line voltage tap. 5v filaments for a rectifier tube are yellow, 6.3v for most tubes are green, HV is red and if there's a CT it's red with a yellow stripe.

Hammond always followed the industry colour conventions. Many guitar amp trannies have oddball colours, especially Marshall. I have absolutely no idea why, except maybe as an attempt to confuse non-factory servicemen.


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## Telemark (Feb 9, 2006)

*Wow*

I had hoped to flush some good info out and you guys have delivered Thanks a lot.

I pulled the OPT - the speaker connections fell off when the iron touched the joint Then the taped splices disentegrated, but at least they were a solid connection.

The Output tube connections were originally Brown and Green, spliced to them now. Yellow.



> There was an industry standard colour code for trannies since the heyday of
> Charlie Chaplin. With an OT, the plate wires are blue and brown and the CT is red. The speaker side has black for the common, grn for 4 ohms, ylw for 8 ohms and my memory stalls out about there.


I find the Primary to Be Yellow/ Yellow and Red

The Secondary is Black / Yellow- It was wired for 8 ohms, and taped off are, white, brown, green, striped red/ yellow, striped blue yellow. Lotsa options!

Thanks for the House of Jim info: from his site I have the option of 4-8-15-250 or 500ohms! woohoo. SO now I can power that wall of 4-12's if I just wire them in Series!:banana: 

more importanly 3800ohms Ct primary and 60 watts audio.

The Color code is only partly legible: It seems to say one of the primary yellow wires should be blue!
and the secondary 8 ohm is black brown!

Except that leaves me with a mystery secondary wire the yellow!:food-smiley-004: This is making me hungry.


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## Scottone (Feb 10, 2006)

I've never encountered an OT transformer with the 2 plate windings being the same color. 

At this point (assuming you want to use this O/T), just get some wire of the same guage and attach extentions to these leads. I was taught to do this as follows:

1. strip about 1/2 " of the end of the two wires to be joined
2. twist the 2 stripped ends together as tightly as possible
3. solder the connection
4. cover the splice with heat shrink tubing. 2 layers may be better if possible.

maybe one of the "real" techs can chime in here if I'm misleading you


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## Telemark (Feb 9, 2006)

Thanks Scott, I am going to stick with this transformer pending one important item. It looks like the current might be limited to 100 ma.

a new Hammond 1650N - reccomended for this amp by London Power has a current rating of 380ma.

I'm not sure if I'm miss reading this number or if this will even matter. Anyone care to enlighten me?


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## Scottone (Feb 10, 2006)

Telemark said:


> Thanks Scott, I am going to stick with this transformer pending one important item. It looks like the current might be limited to 100 ma.
> 
> a new Hammond 1650N - reccomended for this amp by London Power has a current rating of 380ma.
> 
> I'm not sure if I'm miss reading this number or if this will even matter. Anyone care to enlighten me?


I took the liberty of putting an ad up on a local bulletin board and found a guy who has a complete transformer set for a 60's YBA-1. Shoot me an email if you are interested in the output tranny.

[email protected]


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## Wild Bill (May 3, 2006)

*"As always, it ain't that simple..."*



Telemark said:


> Thanks Scott, I am going to stick with this transformer pending one important item. It looks like the current might be limited to 100 ma.
> 
> a new Hammond 1650N - reccomended for this amp by London Power has a current rating of 380ma.
> 
> I'm not sure if I'm miss reading this number or if this will even matter. Anyone care to enlighten me?


We need to nail down a few more factors. That 100 ma rating without more specifics is like a one-armed fisherman showing us the size of the fish that got away! 

First off, transformers can't limit current. Current is drawn and not pushed. What that rating means is that you should keep the AVERAGE current less than 100 ma, PER SIDE (that is, for each tube in a push-pull circuit) or the transformer will start to get warmer than it should.

Now, the amount of current a 6L6 or whatever will try to draw through the OT winding is dependent on how strongly it's being driven from the preamp/PI stages and by how high a plate voltage is being applied.

Power= volts x plate current in amperes. In the typical class AB1 guitar amp we expect to get maybe a third of that amount in output power, what with circuit efficiency and losses of parts that couple the power. So if the tubes are capable of drawing 150 watts from the power supply (average) then we'd expect to get a max of 50 watts of power into the speaker load.

There's so many factors. The wires inside a transformer might get hot with a strong current load but that's only on signal peaks. On softer parts of the signal or between notes they get a chance to cool down.

Usually designers look at the tube data sheets, which often show all the factors at typical voltages and power levels.

One of the most important factors with Hammond 1600 series OT's is that they are way under-rated and spec'd to deliver full power down to 20 hz. This is far more than an OT in a lead guitar amp would ever have to handle. That's why when you look at the OT in a Traynor bass amp it's much bigger and heavier than in a Marshall or Fender lead amp of equivalent power. It needs that extra iron to handle the energy of bass notes lower than a lead guitar low E and not saturate, which is a condition that adds distortion.

That being said, with a guitar amp we WANT some distortion! So in LEAD guitar applications we can push the hell outta the Hammond ratings. If the 1650N is a 50 watt hifi speaker it will actually sound better as a 100 watt amp OT, or more! There are stories of guys running Hammond OTs so hot you can't keep your hand on the trannie and yet they never burn out...

If your amp ran a pair of EL34s before, then you're talking 60-70 watts of power. I suspect your OT will perform just fine.


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## Telemark (Feb 9, 2006)

*You should have seen one that...on the other other hand....*



> We need to nail down a few more factors. That 100 ma rating without more specifics is like a one-armed fisherman showing us the size of the fish that got away!


I hear ya WildBill, it was 100ma per side and you have cleared it up pretty well. Given the size of this beast and the size of the wires coming off of it, plus the rating of 60watts audio, I am not too concerned.

:food-smiley-004: 

I am looking for PS caps know, I was thinking of going with a 50/50 500volt, FT. This particular Amp has 4 cap positions but is missing some of the Filtering Shown in most of the schematics as they are all single Caps.

I guess I have the option of adding the missing filter sections, easy enough with a multi cap. or leaving as is.

I am having a hard time find 40/40 uF for the Preamp section, so might use 50/50. Some say this may be a problem, others say go for it. This is my highest cost so far so I would like to get it rite!

Thanks again for chimiin in.


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## Wild Bill (May 3, 2006)

*"Close enough for rock and roll..."*



Telemark said:


> I am looking for PS caps know, I was thinking of going with a 50/50 500volt, FT. This particular Amp has 4 cap positions but is missing some of the Filtering Shown in most of the schematics as they are all single Caps.
> 
> I guess I have the option of adding the missing filter sections, easy enough with a multi cap. or leaving as is.
> 
> ...


Filters are not at all critical in value. You can't go lower in voltage rating of course but upping the microfarads is no big deal, within reason.

During the golden years when everything was tubes filter electrolytics commonly had a tolerance of -20%/+80%! That's like from a bit less to nearly double! There was no need to make them to a tighter tolerance. You need a minimum amount of mfd's to ensure adequate filtering and decoupling. After that it's just overkill.

If you start doubling the value or more you might make the amp noticeably "stiffer" or "tighter". Probably you won't even notice. This refers only to the first filter that feeds the output tubes through the OT. It will be responsible for 90% of this effect. "Down the line" to the preamp tubes it will make "mice nuts" difference.

You do have to watch when drastically increasing the value of that 1st filter if you're using a tube rectifier. They have a limit on the maximum size that will not stress the tube when first charging up an empty filter. When empty a filter cap looks like a short and the current drawn is very high. As it charges the flow reduces. That's why in classic amps we see values from 20-50 mfd and with newer amps using modern silicon diodes the value might be several hundred mfd. Modern solid state diodes can handle that initial surge current.

So go ahead and use 50/50. It's well within that original "broad as a barn door" tolerance. FT's are good caps. I usually buy JJ's as they're a bit cheaper here and I've never had a problem. JJ makes a 40/20/20/20 @ 500vdc that I use as a staple on my shelf.


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## Telemark (Feb 9, 2006)

Thanks again Wildbill, I feel like I'm getting a tube amp tutorial. At the end of all this I will have at least satisfied one on my goals, Learning about tube amps.:food-smiley-004: 

Parts are in the mail, filter caps, some cathode bypass and filter values for playing with, bit of shielded wire for the input....
And the all important Chicken Beak Knobs! gotta re-inject some Mojo to this big boy.evilGuitar:


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## Telemark (Feb 9, 2006)

*In Over My Head?!!?!*

Well, I have added all the new parts inlcuding, A new Old OPT - Thanks Scottone!:food-smiley-004: New PS Filter caps , An 3 wire AC plug and a New Bassman Style Presence cct.

I fired it up cautiously last nite, set the bias at about 28ma and had listen - nuttin! I had hooked up the OPT secondary wrong. Changed that _horrendus feedback_

Starts small - hi frequency and escalates - I turned off the STBY and back on and Whammo full volume blast - sounds like powersupply noise.

I did it twice - turning down all the pots, no different, measured a cathode current of 58ma (at 470V @Plate).


Well Anysuggestions? Where do I start. I won't get back to it for a couple days, time to regroup. maybe I'll post a pic for anyone curious.

Mark.


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## Scottone (Feb 10, 2006)

Telemark said:


> Well, I have added all the new parts inlcuding, A new Old OPT - Thanks Scottone!:food-smiley-004: New PS Filter caps , An 3 wire AC plug and a New Bassman Style Presence cct.
> 
> I fired it up cautiously last nite, set the bias at about 28ma and had listen - nuttin! I had hooked up the OPT secondary wrong. Changed that _horrendus feedback_
> 
> ...


Hey Mark, try disconnecting the feedback and see if you still get the problem.

You may have the o/p transformer primary connections switched around.

Scott


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## Telemark (Feb 9, 2006)

*Ass-U-me*

I assumed that it would be correct when I discovered the new wires matched the color of the original wires - give or take 30 years of discoloration!

I also assumed that it would make no difference given the OPT I took out had primarys that were both the same color 

But of course getting it wrong would work out to pretty much 180degrees of phase shift - holy feedback batman!-

Thanks Scott I'll give that a try.


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## Scottone (Feb 10, 2006)

Telemark said:


> I assumed that it would be correct when I discovered the new wires matched the color of the original wires - give or take 30 years of discoloration!
> 
> I also assumed that it would make no difference given the OPT I took out had primarys that were both the same color
> 
> ...


That's why the wires are different colours, so you know the start winding and finish winding.

I hope that fixes it...

Scott


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## Telemark (Feb 9, 2006)

I opened the feedback loop and I can no turn on the amp without feedback!
So i must have the Primary wires reversed.

Thanks Scott.


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## Wild Bill (May 3, 2006)

*"There's more than one way to skin a ..."*



Telemark said:


> I opened the feedback loop and I can no turn on the amp without feedback!
> So i must have the Primary wires reversed.
> 
> Thanks Scott.


If it's more convenient you can reverse the phase shift a couple of other ways.

One is to flip the speaker wires coming from the OT. This of course isn't practical if the OT has multiple taps for 4,8, 16 ohms or whatever where one wire is designated "ground" or "common" but when the OT has no taps it will work just fine.

Another would be to flip the two coupling caps coming from the phase inverter, so that they each feed the opposite tube.

When you disconnected that feedback resistor from the speaker side of the OT you probably noticed that the amp has a dramatically different tone! Kinda like a VibroKing or a Vox. These amps don't use negative feedback. You might wanna install a small switch...


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## Telemark (Feb 9, 2006)

*Three lefts to make a right?*

I was thinking about just switching the OPT secondary, but, it has multi taps even though I am only using the one, and the the black wire ends up in the wrong spot, I thought it was kinda like two wrongs to make a right! 

what was the 8 ohm color - yellow?


The easiest way will be to change the primary leads on the OPT. 

I was planning a switch later between more or less feedback. but I didn't actually get to try it as I had no guitar, and It really hums without the loop, which I thought might be normal? - 

Thanks for the replies!


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## Scottone (Feb 10, 2006)

Telemark said:


> I was thinking about just switching the OPT secondary, but, it has multi taps even though I am only using the one, and the the black wire ends up in the wrong spot, I thought it was kinda like two wrongs to make a right!
> 
> what was the 8 ohm color - yellow?
> 
> ...


I would assume that the longest o/p secondary lead (other than the black) is the 8 ohm lead since that was connected in the host amp. 

I'm suprised that you would get a loud hum level with no feedback connected, although I would expect a little higher than normal. Let us know how it sounds when you get the connection changed.


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## Telemark (Feb 9, 2006)

*How Does it Sound!!!!*

Wildbill wrote:[


> There was an industry standard colour code for trannies since the heyday of
> Charlie Chaplin. With an OT, the plate wires are blue and brown and the CT is red. The speaker side has black for the common, grn for 4 ohms, ylw for 8 ohms and my memory stalls out about there.


The longest lead was green, so I figure it is 4 ohms, there was a brown cut really short and no yellow. I'm not too worried, I may buy another 8ohm cab open back, and make a vertical stack, or maybe a 2-12 partial open for variety. Either way I don't expect a problem.

:rockon2: 
Well I redid the splices and put the feedback loop back together, the hum, well when it was turned as loud as I had with no input the hum was loud! But If I played at that volume , The ringining in my ears would soon overtake the hum! At lower volumes the hum was present but not obnoxious, I will probably look at tightening up the heater wires, moving the AC input wires a bit.

I set the bias at 40ma again at 468volts. and plugged in the Strat.

Sounds pretty good! It's so subjective. The new PS filter caps have made a difference, This amp is totally usable for gigging as is nice tonal range and the guitar sounds full and even across the strings. It's very touch sensitive. I can cleary hear all the different pickup posititions.


First Channel 1 Normal - I added a point .68uF bypass cap to V2, it is subtle but adds a mid boost I like. Also a Channel 2 defeat switch. This actually adds gain to channel 1 and doesn't work as I thought it might.
ie it sounds cleaner with channel 2 in. I may remove this and use the switch for something else. It does sound good single channelled but I haven't spent enough time comparing.

I would like less gain on channel 1 as it starts to distort pretty early. 

Channel 2 has a new bright cap 120pf. This is pretty bright to my ears! But I can dial in some neat fenderish sounds.

Both channels go from fendery like clean to crunchy ACDC or maybe Stevie Tones and more. 

It gets really loud though, I'll need an attenuator or a master volume to use this without pedals.


the area I think I might look into is Bass Control, The bass tends to get carried away,-I'm using a sealed cab. I think I'll try changing the feedback, and also V1 bypass cap - maybe 100uF. And I would also like to lower the gain of V1 a bit, maybe use an 12AY7? as the Bassman did.

Fist things first I am going to put it back together, Gig with it. Maybe record it and then back to the bench.:food-smiley-004: 

thanks everyone that helped out.

Mark.


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## Scottone (Feb 10, 2006)

Telemark said:


> Well I redid the splices and put the feedback loop back together, the hum, well when it was turned as loud as I had with no input the hum was loud! But If I played at that volume , The ringining in my ears would soon overtake the hum! At lower volumes the hum was present but not obnoxious, I will probably look at tightening up the heater wires, moving the AC input wires a bit.
> 
> Mark.


Hey Mark, I'm glad you got it up and running. Definitely try re-doing those heater wires...they didn't look that well dressed to me. You could also try disconnecting the center tap of the 6.3V winding from ground and replacing it with a 100ohm resistor from each side of the winding to ground. This has worked for me in reducing hum levels in a couple of other amps.

You want to make sure that the heater wiring is as far away from signal carrying wires as possible.


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## Telemark (Feb 9, 2006)

*First Gig*

Success, I played a single nite gig without micing the Amps and brought out the YBA1 into a Single 12 Avatar cab with a Reverend Alltone-12 on one side and the other side of my Stereo set up was the trusty YCV-50 (open back V-30)

These amps show their lineage in their voicing (even with el-34 in the YCV and 6L6 inthe YBA!). I prefer the mids of the YBA and thats what I used primarily. The Volume of the YBA was an issue though it is sooooo loud. I never had it above 2.

I was using the Bright channell all nite and as much as I like it sometimes I think it was a bit much, I kept dialling in Bass and mids and had the treble almost off! If I had remembered about the bright mod I may have just changed channels! DOH.

This was the gig from Hell though  without earplugs it was a trebly washed out mess for Stage sound - cause there was no stage and we were on the concrete floor!

What I missed was being able to dial up the amp to breakup, there was no way even with my superloud drummer. I need an Attenuator or an Isobox or ????

Maybe a master volume What's the consensus on a Master Volume Mod? It is certainly cheaper than the Attenuator Route.

I'llpost Pics and clips soon.

Mark.:rockon2:


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## Wild Bill (May 3, 2006)

*"MVs are SOOO 80's!"*



Telemark said:


> What I missed was being able to dial up the amp to breakup, there was no way even with my superloud drummer. I need an Attenuator or an Isobox or ????
> 
> Maybe a master volume What's the consensus on a Master Volume Mod? It is certainly cheaper than the Attenuator Route.
> 
> ...


I think there's a thread in here somewhere about master volumes. Anyhow, it boils down to this:

There are two primary areas of distortion in a tube guitar amp. One is the preamp section and the other is the power amp, including the output transformer.

Preamp distortion is more "crunchy". Some would even say "fizzy". Power amp distortion is "warmer" and "thicker". This is the sound of classic 70's rock. Think AC/DC. The Angus brothers use nothing but old Marshalls built before MV's were first introduced - JTMs and JMPs. They wouldn't use a MV if you put a gun to their head.

MVs will let you dial in crunch at lower volumes but they are incapable of allowing power amp distortion. How can they when they reduce the signal into the power amp? 

If you're happy with only preamp distortion then great! It's all taste anyway. If you want to nail that classic power amp distortion (Mississippi Queen?) then you're beat. It's as simple as that.

If you must have a MV general concensus is that the best is what's called a post PI (phase inverter) circuit. It places the MV control AFTER the last preamp tube, the one which splits the signal and directly drives the power tubes. Usually MVs are before the PI and by placing it after you get one more stage of gain in the circuit.

It's a simple circuit. Look at the Matchless Spitfire or go to http://www.blueguitar.org and you'll find a .pdf file containing about a dozen different Trainwreck circuits. The last few use a post PI MV.

Myself, I believe that a tube amp always sounds best when cranked. If the amp is too loud for the gig then you should have a lower powered amp. A carpenter has more than one size of hammer, after all. Something in the 20 watt area and something around 50 watts will cover almost any occasion. If you ever play a large concert gig you'll be miked anyway so rarely can you use a 100 watter these days.

Then again, that's easy for me to say when I build my own amps! So do you, so why not have some more fun? Have you checked out the ax84 project for those amazing little 18 watt Marshall amps? Those circuits sound fabulous! With one of those feeding a 2-10 with Eminence or Webers that are British voiced like Celestions and you can have a whale of a time!


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## Scottone (Feb 10, 2006)

Hey Mark, glad to hear that it survived a gig. It's definitely a testament to your excellent workmanship  

Sounds like it's time to find a GuitarMate or BassMate that needs rebuilding. These are really cool lower powered Traynors and I've owned a couple of each over the years. There is a few nice Garnet models that are lower powered and break up really nice at lower volumes. My favorite is the Mini-Bass...it's really killer. Little harder to work on than the Traynors due to the terminal strip construction though.


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## Telemark (Feb 9, 2006)

*18 Watts!*

I think there is defenitley an 18Watt in my future, and/or maybe a Vox or Deluxe type amp. Thanks for the Post PI tip, that is what I am leaning to, maybe just cause I can do it, I have an extra hole in the chassis! Power amp/ OPT saturation is what I am after. I just can't take the noise!!! maybe I'll buy an attenuator after christmas.

I forgot to mention a problem that cropped up during torture testing - not at that gig! When I crank the YBA1 into a bit of growl / distortion a couple a times I got a horrendus runaway distortion, that quit as soon as I stopped the note.

I figure it could be. speaker stress sd it sounds pretty messed up - The Alltone might not be able to handle all 50 watts I can't remember. The volume is _louder_ than my YCV-50 puts out with the master cranked. Could I be stressing the voice coil / spider into a non linear region where all hell breaks loose?

Other than that I am not sure - could it be a type of blocking distortion - or a temporary bias shift? Is this the case when current is drawn thru the grid? I am not quite clear on this. I have some notes somewhere. This is defenitly not just a fully clipped waveform - ie squarewave, there is somehting funky and nasty going on.



I am considering . 1/) cranking the amp thru a 4-12 to eliminate the speaker issue. 2.) ??? getting out the test gear and putting the amp back on the bench. I just cleaned it up!

Or any other ideas? 

TIA Mark.


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## Telemark (Feb 9, 2006)

*OPT /SPeaker Mismatch*

Could using a 4 ohm tap into an 8 ohm speaker cause a problem at Higher levels - like when trying to push the YBA into distortion?

Mark.


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## Wild Bill (May 3, 2006)

*"Maybe yes, maybe no! What's in the bag, Danger?"*



Telemark said:


> Could using a 4 ohm tap into an 8 ohm speaker cause a problem at Higher levels - like when trying to push the YBA into distortion?
> 
> Mark.


Depends! Probably not - tubes are tough and usually handle a little speaker mismatch. Plugging a 16 ohm cab into a 4 ohm tap might get more exciting...

Such mismatches usually just reduce tube life a few months rather than cause a rapid failure.

What you really need to know to make an intelligent guess is the primary impedance of that output trannie. You can easily measure it. If you don't already know how you could check out http://www.aikenamps.com You'll find some great FAQS including one on output transformers that will tell you how to do it. A low AC voltage (some of those "wall wart" supplies that you find today even in garbage cans deliver AC), some measurements and a little math and Bob's your uncle!

I plugged the math long ago into an Excel spreadsheet and saved it. I'm always scrounging used OT's and the spreadsheet is more convenient.

If the OT was designed to run at say 6k primary impedance then the mismatch you describe will double that to put 12k on the tubes. That's a little high but not likely not enough to do any lasting harm. If the design point was higher like maybe 8k or even 10k then doubling the speaker load would double the primary load higher than most would like to see for 6V6's.

We really have to know the actual facts. Otherwise we're just "jawin'".


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## Telemark (Feb 9, 2006)

*Mods - Optional....*

I just demodded my YBA - back to stock in an attempt to get to the bottom of some weird problems. 

Channel 1 and two had vastly different tone and there was some other nasty things going on.

I removed the Channel defeat mod for channel 2 and the .68uF cathode bypass cap on channel 2. While I was at it I discover one of my Bias filter caps had let some smoke out of the grounded (+) end~!

It's all fixed up and wow! way better both channels are quiet and smooth, and start to crunch up after about 5 . I am still playing with the Bias to find the point I like it at. Two low and it sounds like yuk when I turn it up - crossover distortion I guess. 

Too Hi - hmmm I am at about 17 watts idle right now at 35ma and 475Volts.
I could likely take it up to about 40ma or so. If that means less crossover yukiness I'm in.... ! 23 watts max would be about 48ma so I am still safe


Starting to hear some of tone I was hoping for.

-mark.


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## Telemark (Feb 9, 2006)

*Feedback*

I was checking out the source schematic of the '56 Bassman and noticed two main differences in the YBA1 and Bassman - one was the AY7 in the VA position, the other is the feedback resistor is a 25K in the bassman and 100k in the Traynor.

I decided to try the 25K resistor and I like it! It may be overkill and I think I will play with other values and no feed back just to hear what it sounds like.

It has tighened up the bass and the amp is now much harder to distort, which isn't a terrible thing as it kinda fellapart at times, now it seems much more controlled and I might just have to try and put the 12AX7 back in V1

I am very slow with these changes as I need to digest and consider each step and play the amp out and all. Still having fun and learning!

<Mark.


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## Telemark (Feb 9, 2006)

*STOP the madness*

:bow: Petey didn't do too bad a job eh!:bow: 

I finally buttoned the amp back up with the following mods:

Input V1 coupling 1K5/22uF
V1 - 12ax7
Volume pots 1Meg

bright cap switched onChannel 2 120pf Sliver Mica

Tone Stack - changed out the existing values for Bassman values 56A - don't have the numbers here - didn't change the resistor.

Bias added a pot and resistor for variable bias.

Left the screen resistors stock.

Feedback loop - added a 50K resistor in parrallel with the 100K - tightend up the bass and controlled distortion a bit.


Output stage 6L6.


That's it, All the playing with 6V6 and cathode bias was fun but I really like this amp as it is now.


If I do anything more to this amp it might be the following:

1. post PI Master - did I mention it's loud!
2. Variable feedback - what a huge change to the feel I would love to be able to play with this.
3. single channel or mod to make channel 1 and two a little different.

While playing around I found the 6V6 sound to be totally floating my boat, I just couldn't quite be happy with the high volatages and kept expecting smoke at any minute.

This work was slow and I often made several steps sideways and back for every step forward. Still feel that I learned a lot.


:wave: Many thanks to Scotone , WildBill and others that offered their help.

I will get around to clips and Pics.:rockon: 

Mark.


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## Michelle (Aug 21, 2006)

And I'm learnin' from this too Mark, I just need a 'project' amp. But I have a few questions;

When you are measuring the output, (with scope), what are you using for a dummy load? A bunch of 5W/10W power-resistors connected to approx. 8-ohm, big 8-ohm heat-sinked dummys, or will a 'hot-plate' do it? And which is the preferred way to go?

Great work BTW
Mich


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## Wild Bill (May 3, 2006)

Michelle said:


> And I'm learnin' from this too Mark, I just need a 'project' amp. But I have a few questions;
> 
> When you are measuring the output, (with scope), what are you using for a dummy load? A bunch of 5W/10W power-resistors connected to approx. 8-ohm, big 8-ohm heat-sinked dummys, or will a 'hot-plate' do it? And which is the preferred way to go?
> 
> ...


Hey Mich, do you travel in any electrican or construction circles?

Reason I ask is that I inherited an old baseboard heater. It was a small one with two long elements like a giant version of some of those 1930's several hundred watt power resistors.

Just for kicks I measured the cold resistance of one of the elements. 7.8 ohms! I put the meter leads together and found .2 ohms for the lead resistance. That gave 7.6 ohms actual for the element!

Perfect as a load! Might drop a bit at full power but I don't have any kilowatt or two amps to use with it anyway. It'll never get hot enough to change its resistance value as a dummy load.

These things are often ripped out or replaced in old buildings. They're mostly dual units so even if one failed the other would be useful.

If you have a maintenance man in the family a couple of beer might prove useful...

Anyhow, it doesn't matter if the ohms are off a few at the speaker side for power testing. As long as you can accurately measure the value of the dummy load and see the voltage with your 'scope you can calculate the output power. Keep in mind that you have to get more than 3-4% distortion/clipping before you can see it on the 'scope. For guitar that's not a problem. Who cares if there's a couple of percent distortion happening? The human ear can't hear less than 3 or 4 % anyway. To keep anal retentive hifi geeks happy you need to build what's called a differential circuit. It's easiest with an IC op amp so you might have to scale down the input voltage if it's too much for the chip. Anyhow, when you differentiate a sine wave you get a sine wave. When you differentiate a sine wave that's not pure you get a BIG showing of the distortion! It stands out like a sore thumb on the screen. Let's you find the output power when the amp is very clean.

:food-smiley-004:


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## Telemark (Feb 9, 2006)

Michelle



> When you are measuring the output, (with scope), what are you using for a dummy load? A bunch of 5W/10W power-resistors connected to approx. 8-ohm, big 8-ohm heat-sinked dummys, or will a 'hot-plate' do it? And which is the preferred way to go?


I have a very large power resistor that is something like 100 ohms and it has this slide-able lead that lets you set it to a different value. I got as close as I could to 8 ohms and then calculated power as V(squared) / Measured ohms.

I don't think it was ever very accurate - On top of this I was measuring peak to peak not RMS so more math required.... 

I learned all of this in school but I forget and it doesn't really affect the sound.!

As to what is prefferred I guess that depends on what results you are looking for.

For power measurements:
TO make things easier and more accurate I am going to get one of those 8 ohm 100 watt resistors, or maybe two 50's so I can do 4 ohms.

I don't think a Hotplate is resistive - but I could be wrong.
I have read that is important to use a resistive load when examining the waveform, makes sense because a complex load would change the shape of the waveform and might smooth out some crossover distortion wrinkles.

I don't think you should use a Hotplate for power meausurements but I could be wrong. A purely resistive box should be fine.

a bunch o resitors in parrallel!
Why not, I have examples of this all over my work place. One of my coworkers was a stump Farmer - very ingenious at finding solutions that were cheap! Just watch the impedeance doesn't change with Heat/ current!

Good Luck.


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## Michelle (Aug 21, 2006)

Thanks Mark, Bill;
I took all this stuff 28yrs ago too at Tech so it's kinda weird getting back into it now. Funny how it comes back and I kept a lot of my books, one in particular that I'm glad I didn't chuck is 'Audio Cyclopedia' 2nd edition, I 'borrowed' that from the library and 'forgot' to return it.  For the last 28yrs I have been involved in telephony switching, mux, transport, power, data, etc.

I'm not familiar with the heater that you are talking about Bill and I was just over to the 'Tower Bldg', (tel eqpt bldg), but didn't find anything so I think I'll stop at the elect. dist. on the way home and just get some big ohmite pwr resistors. Newark has them for like $40ea but with a 98 day lead-time, and those are 300W, I would only need 50 or 100W.

At any rate, I have this forum to thank for getting me more involved in doing this sort of thing rather than fixing pc's.

Have a great day!
Michelle


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## Gunny (Feb 21, 2006)

I built a network of 20 Watt flameproof power resistors into a ventilated chassis and used banana jacks for input and output connections. Also put in a switch for 4 Ohm and 8 Ohm net resistance. The box was not expensive and the resistors were about $2 each - new items but from a surplus store in Toronto.
My point is don't spend a fortune on the load box. Even when I run an amp to the onset of waveform distortion when determining true power output, the load box does not heat up to staggering temperatures.
To oversimplify the discussion, a 100 Watt light bulb gets hot but not excessive. Of course the light bulb is not the right impedance for a load...just an analogy for the relationship of load box temperature.


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