Gibson GA-20 power transformer

[dkevin]

Hello all,
I recently acquired a 1956 Gibson GA-20. My version is a dual 6v6 push-pull amp rated at 14-16 watts. It has 3ea nine-pins tubes in the preamp. This one came to me non-operable with the admission that the owner had tried to repair it himself and the last time he powered it up, there was smoke rising from the off/on switch. I was aware of what smoke means to a startup. I took the amp apart to discover a shorted power transformer which also was a non-original transformer. The OT was also a replacement part.
First of all, I am looking for an original GA-20 power transformer with "GA-20-P" stamped on its cover. This is a 320-0-320 @ 100+MA 6.3 @ 2A and 5.0@ 2A. It is a standup mounted transformer with mounting centers of 2" x 2.5". If you have one, let me know.
If I cannot find this original part, I will have to buy a close match from the transformers currently being manufactured. I am considering a Magnetic Components 40-18085 PT. I am also considering a 40-18022 Tweed Deluxe OT to go with it. I would appreciate it if you have advice, opinions and/or wisdom to impart about my choices. I will attach a link to pictures of the victim and possible a link to the PT/OT as well. Thanks for your help!
Dwight

http://www.classictone.net/40-18022.html
http://www.classictone.net/40-18085.html
https://imgur.com/a/T1ZZnVB

[nickb]

As the fuse didn't blow I wonder how you are sure the power transformer is bad. How did you test it? Measuring DC resistances can be misleading. Disconnect all the secondaries and power up using a light bulb limiter. If the bulb is bright then you know the transformer is bad.

Smoke from the power switch is likely just and indication that it's dirty and worn out. Time to replace it.

PS: I see the GA20A uses a double pole switch. An worn switch can arc and cause a short L to N or even earth when voltage is applied. Replace or disconnect and bypass for further testing.

[dkevin]

The PT involved was bubbling internally when power was applied and generated a lot of external heat. I tested it with no tubes installed and I am fairly confident that it had an internal short and was no good. If I had to guess why it failed, I would blame the Electrolytic caps. This amp still had the cardboard party caps installed at the factory in 1956. There was a fair amount of melted wax accumulated inside the chassis. On top of that, this was not the original PT and produced 350-0-350 volts, way beyond what the schematic called for. All-in-all, I would not reuse this PT even if it was good. The fuse was MIA as the owner/seller removed it to keep the amp from being powered up when he had a suspicion that all was not right. The a/c switch tested out just fine.

[nickb]

The PT involved was bubbling internally when power was applied and generated a lot of external heat. I tested it with no tubes installed and I am fairly confident that it had an internal short and was no good. If I had to guess why it failed, I would blame the Electrolytic caps. This amp still had the cardboard party caps installed at the factory in 1956. There was a fair amount of melted wax accumulated inside the chassis. On top of that, this was not the original PT and produced 350-0-350 volts, way beyond what the schematic called for. All-in-all, I would not reuse this PT even if it was good. The fuse was MIA as the owner/seller removed it to keep the amp from being powered up when he had a suspicion that all was not right. The a/c switch tested out just fine.

Bubbling? Run!!! That is defo a do not use. Even if it was the secondary load that was the primary cause the damage is almost certainly too great.

Also, before you install the new one check the fuse rating, eliminate all possible secondary issues, use a bulb limiter ( or variac if you have one) as you do not want to toast a new one. Consider adding fuses to the B+ and heater secondaries for better protection.

[pdf64]

The stock connection of the heater winding CT to the power tube cathodes might have confused an incompetent repairer, so I’d check that something wacky hasn’t been done that is causing the problem.

[dkevin]

I ordered a Magnetic Components 40-18085 power transformer and a 40-18022 output transformer to rebuild this beast. They seemed to fill the bill as far as matching the operating voltages of the schematic. The PT mounts perfectly into the footprint of the original PT which means no extras holes need be drilled. The OT needed mounting holes since the original was located on the speaker frame. I put it near the PT at 90 degrees. After I wired the PT, I moved on to hand draw the layout for the preamp and PI. The components for these sections are mounted on a 14 position terminal strip. I will update my imgur account to show this drawing. There were a few surprises in the preamp. It seems a resistor (R15) was simply omitted from the circuit. And I see now what pdf64 was referring to; the 6.3 vac heater center tap seems to be mounted on the output tubes shared cathode before the cathode resistor/bypass capacitor. I wonder what the thinking was in that design feature? I mean, what would the net effect be if one grounded the heater CT at the incoming ground wire instead of the output cathodes. What would the 200 ohm resistor accomplish to benefit the 6.3 heaters? Hmmmm....

https://imgur.com/a/KKTolZg

[nickb]

And I see now what pdf64 was referring to; the 6.3 vac heater center tap seems to be mounted on the output tubes shared cathode before the cathode resistor/bypass capacitor. I wonder what the thinking was in that design feature? I mean, what would the net effect be if one grounded the heater CT at the incoming ground wire instead of the output cathodes. What would the 200 ohm resistor accomplish to benefit the 6.3 heaters? Hmmmm....

https://imgur.com/a/KKTolZg

The reason is that the leakage (hum) current than flows between the heaters and the cathode depends on the heater to cathode voltage difference in a non linear way. If you elevate the DC voltage of the heaters then the two currents on the two sides of the heater will have about the same magnitude but opposite phase and so will cancel and reduce the hum. Connecting the heater center tap to the output cathode resistor is one way to to this.

[dkevin]

The beast lives. Voltages very close and sounds very good! More testing and then I'll wrap this up...meanwhile, look at some more pictures...




https://imgur.com/a/xcEyRck

[dkevin]

I've got the chassis reinstalled into the cab with the warehouse G12Q and it sounds really nice! I had to tighten up the octal socket pins. The third time I played through the amp, I turned it on and ....no sound. I checked my rail voltages and they were all high and not dropping into normal operating range. I chopsticked the output tubes and when I touched the 6V6 plates (3) I got a audible pop. I checked the solder joints and they were all clean. So I removed the tubes and looked at the pins inside the sockets. They were spread out so I closed them up a but with a small machinist's scribe. The amp started and played from then on with no more symptoms. I do have one question for the wiser heads on this forum. I omitted a capacitor tying together the PI plates and the amp doesn't seem to miss it. It is supposed to be a 100uuf capacitor and it is supposed to connect the V3 plates before the interstage capacitors appear. Is this an equalizing capacitor to balance the two plates or is it an attempt to head off parasitic oscillation? I am making a back cover for the chassis innards...the seller said he lost it somewhere. I happen to have a GA-20T that has the back cover so I'll just copy that one. The two amps are of the same vintage but the circuits and layouts are vastly different. The 20T is a horror story to work on with components mounted on both the upper and lower surface of the component board. It is the sort of Gibson amp that leaves techs shaking their heads and swearing never to work on another Gibson amp. Compared to the 20T, this rebuild of a GA-20 is a "walk in the park".

[Helmholtz]

I've got the chassis reinstalled into the cab with the warehouse G12Q and it sounds really nice! I had to tighten up the octal socket pins. The third time I played through the amp, I turned it on and ....no sound. I checked my rail voltages and they were all high and not dropping into normal operating range. I chopsticked the output tubes and when I touched the 6V6 plates (3) I got a audible pop. I checked the solder joints and they were all clean. So I removed the tubes and looked at the pins inside the sockets. They were spread out so I closed them up a but with a small machinist's scribe. The amp started and played from then on with no more symptoms. I do have one question for the wiser heads on this forum. I omitted a capacitor tying together the PI plates and the amp doesn't seem to miss it. It is supposed to be a 100uuf capacitor and it is supposed to connect the V3 plates before the interstage capacitors appear. Is this an equalizing capacitor to balance the two plates or is it an attempt to head off parasitic oscillation? I am making a back cover for the chassis innards...the seller said he lost it somewhere. I happen to have a GA-20T that has the back cover so I'll just copy that one. The two amps are of the same vintage but the circuits and layouts are vastly different. The 20T is a horror story to work on with components mounted on both the upper and lower surface of the component board. It is the sort of Gibson amp that leaves techs shaking their heads and swearing never to work on another Gibson amp. Compared to the 20T, this rebuild of a GA-20 is a "walk in the park".

The 100pF capacitor between the PI plates shorts high frequencies (above the audio range). May have been necessary to prevent oscillation.