Restoring a Gibson GA-CB "Custom" amp...

[longshadow]

Hi,

I am currently in the process of restoring my 1950s vintage Gibson GA-CB "Custom" amplifier, at least functionally if not aesthetically, and could use some advice on troubleshooting.

(Photo and audio file attached)

After being in unheated storage for the past 40+ years, it runs, but is exhibiting odd behavior:

1. the PS choke over heats after about 20-30 minutes, and starts melting the insulating goo inside, which then bubbles up from between the windings. An ammeter indicates that when I add the 6L6s to the circuit, (with no signal input) the current draw goes from about a third of an amp to well over an amp (@ 110VAC.) The specs on the 6L6s indicate the filament should draw 0.9 amps @ 6.3v, so something else is causing the vast bulk of the current draw in the 6L6s.

2. weird hum/intermodulation distortion and "breaking up" sounds, depending on the position of volume pots and tone pots (listen to attached audio file.)

Things that are already fixed: all PS filter electrolytic caps have been replaced with modern components.


Things I've measured:

All the voltages at the pins of the 6L6s, and everything seems to be okay, except: pin 5 -- control grid, on which I'm getting a positive DC voltage in the 20-30v range, instead of the negative value.

Other things to consider: the tubes are the original tubes that were in the amp when I got it in the early 1970s (it was working at the time.) One of the 6L6s has a date code that comes back as mid-1949.

All new tubes are on order, should arrive in a few days

I have the schematic diagram, such that it is.


Any suggestions on trouble shooting this beast?

[Jazz P Bass]

If the choke does not overheat without the output tubes installed then you are on the correct path.
Replace the 6L6 tubes.
Edit: the heater current has nothing to do with the current passing through the choke.

[longshadow]

If the choke does not overheat without the output tubes installed then you are on the correct path.
Replace the 6L6 tubes.
Edit: the heater current has nothing to do with the current passing through the choke.

Agreed on the heater current; that's a separate 6.3v AC supply that doesn't have filtering, but my point, aside from the melting choke, is that the total current draw of the amp -- as measured at the fuse -- goes up by almost an amp when the 6L6s are added to the circuit. The melting choke, of course, tells me that the excess current draw is on the DC supply. It took me a while to realize what you pointed out already: the heaters do not draw current through the DC supply.


I should add the 6L6s seem to be putting out a furious amount of heat on their own -- it feels hot enough to cook pizza. None of the other tubes are getting very hot, except the rectifier tube.

[g1]

Don't put 6L6's in until you get rid of the DC on the grids (pin5). I'm guessing it is probably cathode biased. If so, the grids should be around 0 volts. If fixed bias, the grids should be at some negative voltage. You may have leaky coupling caps from the phase splitter or a faulty bias supply if it is fixed bias.
If you have a schematic please post it.
As JazzP mentioned, the choke is overheating due to the lack of bias for the 6L6's. It is DC through the choke, the AC for the heaters does not go through the choke.

Edit: sorry, I didn't see your reply when I posted this.

[longshadow]

Don't put 6L6's in until you get rid of the DC on the grids (pin5). I'm guessing it is probably cathode biased. If so, the grids should be around 0 volts. If fixed bias, the grids should be at some negative voltage. You may have leaky coupling caps from the phase splitter or a faulty bias supply if it is fixed bias.
If you have a schematic please post it.
As JazzP mentioned, the choke is overheating due to the lack of bias for the 6L6's. It is DC through the choke, the AC for the heaters does not go through the choke.

Schematic added (pdf).

I believe the amp is a push-pull class A design (or so Gibson marketing material claimed at the time.)

[g1]

So the output tubes are cathode biased. With the 6L6's removed measure the voltage at the grids (pin5). You should get around 0 volts.
Don't put in any power tubes till you get rid of the positive voltage on the grids. The .05uf coupling caps supposed to block the DC and are probably leaky.

[longshadow]

So the output tubes are cathode biased. With the 6L6's removed measure the voltage at the grids (pin5). You should get around 0 volts.
Don't put in any power tubes till you get rid of the positive voltage on the grids. The .05uf coupling caps supposed to block the DC and are probably leaky.

I had exactly the same thought -- measuring voltage at pin 5 w/o the 6L6s in circuit.

I'll check the .05 uf caps next.

Excellent suggestions; much appreciated.

[longshadow]

Making progress:

With 6L6s removed, I measure 56vdc at on pin 5, and 60 vdc at the other.

This tells me the problem is not internal to the 6L6s (?)



Measuring across the .05 uf caps, I get open circuit (infinite resistance) -- that's with the cap still in the circuit.

I assume if they were leaking, I'd see some finite resistance.

[The Dude]

Those caps can break down when you "put the juice to it". Lift one side of the caps and see if your voltage on pin 5 of the sockets goes away.

[longshadow]

Will do. good idea.

Bingo!

80mv dc with one lead of the .05uf cap lifted. It was nearly 60v when connected.


Is that likely all that is wrong?

Also, are these electrolytic caps? they do not have a plus or minus, but they do have a stripe on one end.

[The Dude]

No, electrolytics are generally not used in this position. I'd probably go with ceramic or mylar. Some of the builders can likely make better recommendations than myself at this point.

[longshadow]

No, electrolytics are generally not used in this position. I'd probably go with ceramic or mylar. Some of the builders can likely make better recommendations than myself at this point.

Great. Many thanks.

[longshadow]

Another measurement:

I measured voltage on each side of the 05uf cap with the power on and the 6L6s out of the sockets: 200vdc+ on the 6J5 side of the cap; 50-60 volts on the 6L6 pin 5 side of the cap.

Also, the volume control affects the voltage being measured: as the volume is turned up, the voltage measured above drops dramatically, on both sides of the cap; the high side goes from about 220vdc to 50vdc as the volume pots are advanced to full volume setting; the low side of the cap goes from about +60vdc to about +9vdc.

[The Dude]

So effectively what you're saying is that the percentage of leakage goes up as the voltage goes up. That is exactly what you should expect from a leaky capacitor.

[longshadow]

So effectively what you're saying is that the percentage of leakage goes up as the voltage goes up. That is exactly what you should expect from a leaky capacitor.

I hadn't thought of it that way, but yes, that makes sense, now that you mention it.