Gibson GA-80 Restoration Help

[misterc57]

gibson_ga-80_varitone_schematic.pdf Hoping to restore a GA-80. First I would like to get the power supply working. Schematic is attached.

The amp is pulling excessive current with just a few volts applied from the variac. All tubes are removed except the rectifier.

If no tubes are installed there is no excessive current and the power transformer secondary is reading good voltages.

There are 6 filter caps. The two larger 20 uF filter caps do not look good. I disconnected the positive ends and clipped in two new 22 uF caps. Still getting excessive current. Could the smaller (4) 10 uF caps be causing this? I have not tried to clip them out yet.

Anything else to look for?

Thank you! MC

[Helmholtz]

Could the smaller (4) 10 uF caps be causing this?

No, check the GZ34.

[misterc57]

No, check the GZ34.

The rectifier tests good on my tube tester, both sides.

[Helmholtz]

Disconnect the OT CT and see what happens.

[misterc57]

The rectifier tests good on my tube tester, both sides.

Well I tried a different rectifier tube and no longer pulling excessive current. I guess a rectifier that is good on the tube tester is not always good in the amp!?!?

Thank you Helmholtz!

I am sure more questions and issues will arise as I continue.

MC

[Helmholtz]

. I guess a rectifier that is good on the tube tester is not always good in the amp!?!?

Depends on the tester (and its condition).

Both internal diodes may test fine, when tested individually.
But there might be a short between the 2 anodes.

[Enzo]

As has been said many times, tube testers are good at verifying bad tubes as bad, but they are not good at telling you they are good tubes.

[misterc57]

Have my new filter caps installed. Having a strange problem.

Channel 1 sounds okay to me. That does not mean it is.

Channel 2 (which includes the push button tones and the tremolo circuit) has a strange sound. On low notes on the low E string I hear a metallic rattle/shake sound that follows the note. Sounds like a bad speaker.

What I have tried and no changes include

• swap V1 12AX7
• swap V2 and V3, 5879 tubes
• a different speaker

So next I tried using a scope at different points. Looks good at input all the way to input of volume pot, but output of volume pot looks strange. All pots were sprayed clean and make no noise when turning.

Below is scope image of channel two volume pot output




However channel one looks odd also. Below is scope image of channel one volume pot output

[Helmholtz]

What was the level of your input signal?

[misterc57]

What was the level of your input signal?

About 250 mV

Channel 2 volume around 7, was not seeing much until past 5 on the scope.
Channel 1 volume around 3 to 4.

[Helmholtz]

About 250 mV

Channel 2 volume around 7, was not seeing much until past 5 on the scope.
Channel 1 volume around 3 to 4.

How do signals change when you reduce to 100mVrms or lower?

[misterc57]

100 mV input, both volume pots at 5.

Input to volume pots looks clean on scope. Outputs from volume pots look odd.

Channel 2 output




Channel 1 output



Pots...

[g1]

On low notes on the low E string I hear a metallic rattle/shake sound that follows the note. Sounds like a bad speaker.

This can be fairly common with power tube rattle. I know it's not doing it with ch.1, but it's worth trying other power tubes anyway. Or tap on the power tubes and listen for rattle.

[Helmholtz]

100 mV input, both volume pots at 5.

Input to volume pots looks clean on scope. Outputs from volume pots look odd.

Sorry, can't interpret your scope pics.

Where did you connect the ground clip of the scope probe?
I guess with pot output you mean the pot wipers which connect to the 5879 grids?

Did you verify the tube plate and cathode DC voltages (without signal)?

[misterc57]

Sorry, can't interpret your scope pics.

Where did you connect the ground clip of the scope probe?
I guess with pot output you mean the pot wipers which connect to the 5879 grids?

Did you verify the tube plate and cathode DC voltages (without signal)?

Ground is clipped to amps input jack ground.

Yes pot wiper that connects to 5879.

All tube plates, cathode DCVs are either spot on or close. Only one exception, the V4B plate reads 130 and schematic shows 85. V4B is the tremolo and the tremolo sounds great. Should I try to lower the plate VDC?

The input signal sine wave looks perfect until the volume wipers. Then it looks crazy. I am wondering if I should bypass the volume pot and clip in a new volume pot to test the scope signal, or is that a waste of time? Should I normally be seeing a clean sine wave after the volume pot?

Thank you