# Newly built amp - plays radio well



## Lincoln (Jun 2, 2008)

or at least it did.......

I finished a tweed, low power twin build last night, using the 5D8 circuit.
I checked it all through, did all my normal first fire up stuff. I had an issue with the 6 volt heater wiring but it went away on it's own. More about that later.

When I took the amp off stand-by, with a guitar plugged in, it played a radio station. It played radio on 3 out of 4 inputs in fact. 4th input played guitar. And from that moment on, all the inputs stopped playing the radio and all four played guitar. 

I know it's an old circuit. I'm used to seeing a 1 meg and a 68K on inputs. This one only uses the 1 meg. No 68k's. What else makes radio? Why would it stop making radio? Or was it my guitar or cord? Confused.


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## nonreverb (Sep 19, 2006)

You might want to add those 68k resistors as they act as grid stoppers which will attenuate very high frequencies including radio ones. Do you have a 1 meg on every input Lincoln?


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## Lincoln (Jun 2, 2008)

Same amp. 
Long ago, I went through a stage when I bought up some old Hammond organ amps, a PA amp, and other things with cool, old transformers in them. Thinking was old iron had to contain unlimited mojo. I never used any in my builds, always opted for new transformers instead.
This build I decided to use one of those Lucius old babies. PT is out of a Hammond organ amp, #AO-80880. Old enough that it was all octal tubes. No 9 pins at all.
During the testing phase, one time when I tuned on the power switch, I noticed the pilot lamp was dim. I looked at my power meter and it was drawing 782 watts of power. Still on Stand-by. I shut it off, it did not blow the 2 amp fuse. Pulled the plug, started checking everything with an ohm meter, looking for a short to ground. Didn't find anything. I even checked resistance on heater circuit of all the tubes.
The draw went away. It never came back again. I didn't find or fix anything. That bothers me.

What's the chances there is some sort of short inside this old transformer? It has a CT on the heater windings.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> You might want to add those 68k resistors as they act as grid stoppers which will attenuate very high frequencies including radio ones. Do you have a 1 meg on every input Lincoln?


Yes, each input has a 1 meg to ground. 4 of them


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Same amp.
> Long ago, I went through a stage when I bought up some of old Hammond organ amps, a PA amp, and other things with cool, old transformers in them. Thinking was old iron had to contain unlimited mojo. I never used any in my builds, always opted for new transformers instead.
> This build I decided to use one of those Lucius old babies. PT is out of a Hammond organ amp, #AO-80880. Old enough that it was all octal tubes. No 9 pins at all.
> During the testing phase, one time when I tuned on the power switch, I noticed the pilot lamp was dim. I looked at my power meter and it was drawing 782 watts of power. Still on Stand-by. I shut it off, it did not blow the 2 amp fuse. Pulled the plug, started checking everything with an ohm meter, looking for a short to ground. Didn't find anything. I even checked resistance on heater circuit of all the tubes.
> ...


Oh...anything's possible although I have piles of old Hammond organ transformers and haven't had one go "yet".
Having said that, one thing to look out for is brittle insulation on the wires. I have had to check and caught a couple wires where the insulation is cracked...particularly at the point where they enter the transformer housing...that could cause a heap of trouble.


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## GreasyB in YYC (Jan 6, 2020)

Lincoln said:


> or at least it did.......
> 
> I finished a tweed, low power twin build last night, using the 5D8 circuit.
> I checked it all through, did all my normal first fire up stuff. I had an issue with the 6 volt heater wiring but it went away on it's own. More about that later.
> ...


It thinks you’re onstage with Spinal Tap. 😜


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## Lincoln (Jun 2, 2008)

GreasyB in YYC said:


> It thinks you’re onstage with Spinal Tap. 😜


either that or it's haunted!


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Oh...anything's possible although I have piles of old Hammond organ transformers and haven't had one go "yet".
> Having said that, one thing to look out for is brittle insulation on the wires. I have had to check and caught a couple wires where the insulation is cracked...particularly at the point where they enter the transformer housing...that could cause a heap of trouble.


Yeah, there is one spot I noticed where a heater wire comes out of the PT housing. It's jammed hard into the case. I'll find a way to move the wire enough to get some heat shrink around it. The wires are really stiff & old. I'm afraid of breaking them. I hope that's all it is. Sounds very hopeful. If there was a problem with those transformers, you'd know about it.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Yeah, there is one spot I noticed where a heater wire comes out of the PT housing. It's jammed hard into the case. I'll find a way to move the wire enough to get some heat shrink around it. The wires are really stiff & old. I'm afraid of breaking them. I hope that's all it is. Sounds very hopeful. If there was a problem with those transformers, you'd know about it.


Yup....and I know someone who has lots more. :-D


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## nonreverb (Sep 19, 2006)

Try disconnecting the heater lines and do a voltage test on them just to be sure once you've got heat shrink on them
BTW What did that transformer come out of? AO-80880 doesn't ring a bell... all those Hammond part numbers get jumbled in my head after a while.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Try disconnecting the heater lines and do a voltage test on them just to be sure once you've got heat shrink on them
> BTW What did that transformer come out of? AO-80880 doesn't ring a bell... all those Hammond part numbers get jumbled in my head after a while.


That was the part number stamped on the amp. I think it was out of an old M3


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## nonreverb (Sep 19, 2006)

Oh!! That looks like an M2 amp...Have you inserted a choke resistor in the power supply to mimic the FC speaker?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Oh!! That looks like an M2 amp...Have you inserted a choke resistor in the power supply to mimic the FC speaker?


no...... but it sounds like I need to do some research.  I have chokes. Never thought about it being old enough to use FC speaker.

The system voltage isn't real high. It's running at 480 volts. I was almost disappointed.


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## Latole (Aug 11, 2020)

Lincoln said:


> When I took the amp off stand-by, with a guitar plugged in, it played a radio station. It played radio on 3 out of 4 inputs in fact. 4th input played guitar. And from that moment on, all the inputs stopped playing the radio and all four played guitar.


Input jacks normaly close contact to ground ; wiring issue


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## Paul Running (Apr 12, 2020)

Lincoln said:


> Yeah, there is one spot I noticed where a heater wire comes out of the PT housing. It's jammed hard into the case. I'll find a way to move the wire enough to get some heat shrink around it. The wires are really stiff & old. I'm afraid of breaking them. I hope that's all it is. Sounds very hopeful. If there was a problem with those transformers, you'd know about it.


If you are careful with your heat-gun, you can warm-up the insulation and it will become more pliable...route the wire before it begins to cool down...it hardens fast.


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## Paul Running (Apr 12, 2020)

Lincoln said:


> no...... but it sounds like I need to do some research.  I have chokes. Never thought about it being old enough to use FC speaker.
> 
> The system voltage isn't real high. It's running at 480 volts. I was almost disappointed.


What type of output tube are you using? If you want more B+, you could cascade the high tension with another TFM and rectifier circuit. I've done it with 6146s to raise the B+ to 600+ volts...lots of headroom, if that's what you are looking for.


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## Latole (Aug 11, 2020)

Lincoln said:


> That was the part number stamped on the amp. I think it was out of an old M3
> View attachment 340368



Please can you show inside, wiring and parts ?


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## nonreverb (Sep 19, 2006)

Be careful with the B+ as Hammond used the FC speakers magnet coil as a choke. The voltages may be too high. When I was converting M2's and early M3's to permanent magnet speakers, I used a 1Kohm 20 watt resistor. Of course a proper choke will do as you mentioned.
Here's part of the schematic to give you an idea of how it was inserted in the circuit.


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## nonreverb (Sep 19, 2006)

I have an FC Jensen from an early M3. If you like I can measure the DCR of the coil for you and you can go from there.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Be careful with the B+ as Hammond used the FC speakers magnet coil as a choke. The voltages may be too high. When I was converting M2's and early M3's to permanent magnet speakers, I used a 1Kohm 20 watt resistor. Of course a proper choke will do as you mentioned.
> Here's part of the schematic to give you an idea of how it was inserted in the circuit.
> View attachment 340412


Thank you. What's going on with what I thought was the CT for HV winding? I've got it grounded (used it for stand-by). Looks like they have it going through the 700 ohm field coil before it grounds.
I see the HV starts out as 680V and they knock it all the way down to 280V. I bet you could heat a house with that thing!


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Thank you. What's going on with what I thought was the CT for HV winding? I've got it grounded (used it for stand-by)


Yeah....found out the hard way many years ago when I bought an old M3 with the speaker missing. The amp got REALLY hot when i bridged the center tap to ground. It took me some time investigating what was wrong....committed to the memory banks for a lifetime.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Yeah....found out the hard way many years ago when I bought an old M3 with the speaker missing. The amp got REALLY hot when i bridged the center tap to ground. It took me some time investigating what was wrong....committed to the memory banks for a lifetime.


I'll be playing with that. I've got 680R, so I'll try one of those to ground. See what it does. Thanks


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## Lincoln (Jun 2, 2008)

Latole said:


> Input jacks normaly close contact to ground ; wiring issue


No, the amp was dead quiet until I plugged into a jack and then the radio started to play, They are shorting as intended.


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## Lincoln (Jun 2, 2008)

Paul Running said:


> What type of output tube are you using? If you want more B+, you could cascade the high tension with another TFM and rectifier circuit. I've done it with 6146s to raise the B+ to 600+ volts...lots of headroom, if that's what you are looking for.


I had 6L6's in there at first, just in case. After I saw the voltages however, I put in 6V6's. I think @nonreverb is onto something with the HV CT running through a resistance. I have an unexplained voltage drop. I'm at 680V on standby. Then it drops to 480V. It's drawing 110 watts, that's a bit more than I'm used to seeing on a 6V6 amp.


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## nonreverb (Sep 19, 2006)

I remember how hot the 6V6's got when I ran that amp without the choke. It was amazing they didn't burn up.


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## nonreverb (Sep 19, 2006)

BTW, the DCR of that speaker coil is 687 ohms.


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## Paul Running (Apr 12, 2020)

Lincoln said:


> Thank you. What's going on with what I thought was the CT for HV winding? I've got it grounded (used it for stand-by). Looks like they have it going through the 700 ohm field coil before it grounds.
> I see the HV starts out as 680V and they knock it all the way down to 280V. I bet you could heat a house with that thing!


With a FW-bridge across 680VAC with a choke-input filter, will produce 612VDC for B+


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## Paul Running (Apr 12, 2020)

Paul Running said:


> With a FW-bridge across 680VAC with a choke-input filter, will produce 612VDC for B+


If you use capacitor-input filter, it produces 960V peaks...transmitter tube type. That 340VAC will be 350+ with today's hydro.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> BTW, the DCR of that speaker coil is 687 ohms.


I put a 680R resistor to ground on the CT of that M2 transformer. It dropped the B+ by about 100 volts to 390ish. It dropped the draw down from 120 watts to about 65 watts. The 5D8 wants 380 volts on the B+, so we're right in the ball park. No idea why I thought I needed more voltage. 

One problem. Pretty sure it was a 1 watt 680R resistor I used, and it got hot real fast. Colour was changing, smoke was rising after about 30 seconds.. I'll need about a 5 or 10 watt ceramic to make this work.


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## nonreverb (Sep 19, 2006)

Yes, that will result in an instant smoke show! I think I indicated in one of my my previous posts that it needs to be in the 20 watt range.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Yes, that will result in an instant smoke show! I think I indicated in one of my my previous posts that it needs to be in the 20 watt range.


Ah.....I missed that part. I don't know enough about chokes to choose one without using the "trial & error" method. But I've got about 5 different chokes to play with. Learning time!

Sorta looked around, didn't find any 20 watt, 680R resistors that were rated for more than 300 volts.


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## Lincoln (Jun 2, 2008)

but if it's running to ground.....it's not seeing high voltage, right?


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## nonreverb (Sep 19, 2006)

Lincoln said:


> but if it's running to ground.....it's not seeing high voltage, right?


I believe on the original schematic it's says around 100v


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## nonreverb (Sep 19, 2006)

You could use a choke if it has close DCR reading. The trick is finding one with sufficient current handling.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> I believe on the original schematic it's says around 100v


That will be the plan then.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> You could use a choke if it has close DCR reading. The trick is finding one with sufficient current handling.


the biggest choke I have is only 150mA and about 100 ohms. (Hammond 159Q)

Back to the resistor idea......


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## nonreverb (Sep 19, 2006)

I remember that first one i did. I had some crazy 50 watt resistor in it.....was about 6 inches long. Had a hell of a time trying to find a good place to mount it. ;-P


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## Lincoln (Jun 2, 2008)

nonreverb said:


> I remember that first one i did. I had some crazy 50 watt resistor in it.....was about 6 inches long. Had a hell of a time trying to find a good place to mount it. ;-P


Reason #53 why I've been buying new transformers as per application rather that using these old ones. 

These are cool. We use them at work to keep controller cards warm in the winter time. 
Ohmite 680


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Reason #53 why I've been buying new transformers as per application rather that using these old ones.
> 
> These are cool. We use them at work to keep controller cards warm in the winter time.
> Ohmite 680


PERFECT!!


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## nonreverb (Sep 19, 2006)

Chassis mount too for extra heat dissipation.


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## Paul Running (Apr 12, 2020)

Lincoln said:


> no...... but it sounds like I need to do some research.  I have chokes. Never thought about it being old enough to use FC speaker.
> 
> The system voltage isn't real high. It's running at 480 volts. I was almost disappointed.


If your B+ is 480VDC and it isn't real high (sounds like you are looking for more headroom)...that back-biasing network is going to decrease the headroom...back-biasing is used to reduce the plate supply. For maximum headroom, ground the center-tap of the PTFM and used fixed-bias.




__





What is Back-Biasing?


Guitar amplifiers




www.aikenamps.com


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## nonreverb (Sep 19, 2006)

Paul Running said:


> If your B+ is 480VDC and it isn't real high (sounds like you are looking for more headroom)...that back-biasing network is going to decrease the headroom...back-biasing is used to reduce the plate supply. For maximum headroom, ground the center-tap of the PTFM and used fixed-bias.
> 
> 
> 
> ...


Only concern I'd have with that is the current handling of that transformer. I don't know it's rating which could cause trouble if the circuit is pulling too much.


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## Paul Running (Apr 12, 2020)

Yes. If the original 6V6s are used, 480VDC raw is plenty potential. Without knowing the objective, the solutions are endless.


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## nonreverb (Sep 19, 2006)

I'm waiting to hear if he's got his heaters back. That will determine if this transformer is a go or not.


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## Lincoln (Jun 2, 2008)

Paul Running said:


> Yes. If the original 6V6s are used, 480VDC raw is plenty potential. Without knowing the objective, the solutions are endless.


The objective is learning. 
The secondary objective was to build an amp without tremolo, using a pair of organ donor transformers. That's how I ended up in tweed land - Fender amp without tremolo. I wanted to try a twin, because of the extra preamp tube (4 instead of the normal 3). Because the PT was meant for a pair of 6V6's, that's what I intended to use in place of the 6L6's this 5D8 used. I was surprised that a PT, as physically large as this one is, and showing almost 700 open circuit volts, came down in voltage that far when loaded up. At the same time, I was concerned about it drawing over 120 watts just idling with the tubes on. I learned a new way to control B+ voltage. Back biasing as you called it, totally new to me. 
Overall, a success.


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## greco (Jul 15, 2007)

Lincoln said:


> "organ donor transformers"


LOL! Good one!

Dedicated to @nonreverb ?


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## nonreverb (Sep 19, 2006)

Lincoln said:


> The objective is learning.
> The secondary objective was to build an amp without tremolo, using a pair of organ donor transformers. That's how I ended up in tweed land - Fender amp without tremolo. I wanted to try a twin, because of the extra preamp tube (4 instead of the normal 3). Because the PT was meant for a pair of 6V6's, that's what I intended to use in place of the 6L6's this 5D8 used. I was surprised that a PT, as physically large as this one is, and showing almost 700 open circuit volts, came down in voltage that far when loaded up. At the same time, I was concerned about it drawing over 120 watts just idling with the tubes on. I learned a new way to control B+ voltage. Back biasing as you called it, totally new to me.
> Overall, a success.


Indeed and also the coil/resistor add to the total impedance of the secondary which in this case, is probably essential to it operating in @ rated spec. The later version using a permanent magnet speaker had an different transformer although it too had a lifted ground but much less resistance. Just enough to create the proper negative bias for the output tubes.


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## nonreverb (Sep 19, 2006)

These are excellent EL84 output trannies from various Hammond amps. Would work well with 6V6's too.


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## Paul Running (Apr 12, 2020)

Lincoln said:


> The objective is learning.
> The secondary objective was to build an amp without tremolo, using a pair of organ donor transformers. That's how I ended up in tweed land - Fender amp without tremolo. I wanted to try a twin, because of the extra preamp tube (4 instead of the normal 3). Because the PT was meant for a pair of 6V6's, that's what I intended to use in place of the 6L6's this 5D8 used. I was surprised that a PT, as physically large as this one is, and showing almost 700 open circuit volts, came down in voltage that far when loaded up. At the same time, I was concerned about it drawing over 120 watts just idling with the tubes on. I learned a new way to control B+ voltage. Back biasing as you called it, totally new to me.
> Overall, a success.


That is the admirable quality, the desire to learn...that is the key to your success. Realizing the potential for the parts you have and your ability to learn, will create a successful project.
I remember when I first began experimenting with tube amps. I was always anxious to get to the bench...the theory took a long time to mature...I always wanted to be at the bench, it's that visual dimension. Anyway, after I settled down, I realised that it would be a lot easier to hit the books and learn...it reduced the trial and error periods.
Beware of the obsession periods...it may creep-in. When you begin to eat, drink and sleep tube amps, begin to ponder moderation...it's a syndrome.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> I'm waiting to hear if he's got his heaters back. That will determine if this transformer is a go or not.


I did get all the wiring coming out the transformer moved around, and protected. That big short/draw in the heaters only happened the once, thankfully. Also found a bad spot in one of the primary leads.
The amp works good, sounds great for the little I've been able to run it before the resistor on the HV CT starts to smoke. The Ohmite chassis mount is on the way.

I've got 2 more sets of Hammond organ transformers. These must be much newer however, judging by the plastic insulation on the wires. One set I know came from an M100. No ideas yet on what to build with them.


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## greco (Jul 15, 2007)

Great thread! Many thanks to all of the contributors.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> I did get all the wiring coming out the transformer moved around, and protected. That big short/draw in the heaters only happened the once, thankfully. Also found a bad spot in one of the primary leads.
> The amp works good, sounds great for the little I've been able to run it before the resistor on the HV CT starts to smoke. The Ohmite chassis mount is on the way.
> 
> I've got 2 more sets of Hammond organ transformers. These must be much newer however, judging by the plastic insulation on the wires. One set I know came from an M100. No ideas yet on what to build with them.


So they're from the main amp an the reverb amp?


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## Lincoln (Jun 2, 2008)

nonreverb said:


> So they're from the main amp or the reverb amp?


Both were the main amps. They looked just like the M2 amp I posted, only newer? 1960's looking caps instead of the old wax ones.

The reverb amps always go for way more money.


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## nonreverb (Sep 19, 2006)

Lincoln said:


> Both were the main amps. They looked just like the M2 amp I posted, only newer? 1960's looking caps instead of the old wax ones.
> 
> The reverb amps always go for way more money.


Those transformers that you have there, are the later version of the one that we've been discussing. The reverb amp from the M100 series is the later AO-44 which is significantly less valuable than the earlier AO-35. As it didn't have the same preamp circuit and used the rare 6GW8 power/preamp tube configuration.


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## Lincoln (Jun 2, 2008)

nonreverb said:


> Those transformers that you have there, are the later version of the one that we've been discussing. The reverb amp from the M100 series is the later AO-44 which is significantly less valuable than the earlier AO-35. As it didn't have the same preamp circuit and used the rare 6GW8 power/preamp tube configuration.


Those stand-up style OT's in the picture you posted, are those from the reverb amps? All three of the OT's I have are lay-down.


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