Gibson GA-15RVT oscillation? Strange one!

[Indyryder02]

Ok. Gibson GA-15RVT. Schematic attached, and it seems to actually match! Rare for these!

concern. When playing with volume and reverb on. sound cuts out after hard strumming and a min or so… disconnecting reverb tank no change, changing 12au7 and swapping the two 6EU7 (didn’t have any spares on hand so just checking obvious stuff, or trying to. So, while feeding a signal.. bringing up the volume towards max and slowly down will start to make the sound go away.. it’ll keep coming back, if you keep rolling it down slowly the sound will continue to be cut out, always trying to come back though. I can only assume a parasitic oscillation as I’ve never experienced anything else quite like it. I can probe with the scope and watch the signal diminish. I can get the signal to cut out by either leaving volume on amp at max and adjusting amplitude of signal generator up and down, or leaving generator up and using amp volume. I’ve chopsticked all the socket wires, trying to ensure grid and plate wires are apart etc. And thought before I go any further down the rabbit hole.. I better ask the legends on here that may have already had this experience and can shed some light on this for me.
I had to step away from it this evening as I wasn’t seeming to make any progress. And that’s always a good time to step away and ponder I find.

[The Dude]

My first thought would be an intermittent vibration sensitive connection. I'd try chop sticking around in the unit, see if you can duplicate the symptom, and maybe narrow it down to a particular connection.

[Indyryder02]

My first thought would be an intermittent vibration sensitive connection. I'd try chop sticking around in the unit, see if you can duplicate the symptom, and maybe narrow it down to a particular connection.

i would think so… but this is doing it into a dummy load.. reverb tank disconnected.. only when reverb is on. so no vibrations to deal with.
(odd speaker driven setup and transformer PI)
crosstalk was first guess. Initial design is a bit of a mess anyway layout wise from the board… but still. Shouldn’t be doing this.

[Chuck H]

reverb tank disconnected.. only when reverb is on. so no vibrations to deal with.
(odd speaker driven setup and transformer PI)

I know Gibson did some speaker driven reverbs, if that is what the above "speaker driven setup" means.?. But the schematic is not that circuit. Since the problem seems associated with the reverb an accurate schematic would help. This if the above schematic is not actually accurate for the reverb circuit.

[Indyryder02]

I know Gibson did some speaker driven reverbs, if that is what the above "speaker driven setup" means.?. But the schematic is not that circuit. Since the problem seems associated with the reverb an accurate schematic would help. This if the above schematic is not actually accurate for the reverb circuit.

my mistake.. confusion when reading about these. The schematic above does match what I have though as far as I’ve seen.

[Helmholtz]

odd speaker driven setup...

Could you post pics?

[Helmholtz]

I can only assume a parasitic oscillation as I’ve never experienced anything else quite like it. I can probe with the scope and watch the signal diminish.

If there was oscillation you should be able to see it on the scope.

[Helmholtz]

When playing with volume and reverb on. sound cuts out after hard strumming

So it doesn't happen with reverb switched off?

[Chuck H]

It WAS indicated that this was ONLY with the reverb on. I assume that means it doesn't happen with the reverb off since the word "only" was used.

I'm suspecting a poor ground or failing/bad decoupling cap causing interaction between circuits. But it's too early to call it.

[Helmholtz]

The reverb footswitch grounds the node between R15 and C15 for reverb off.
So the question is what does this change for the dry signal path.
If for instance C15 is leaky shifting the V2A bias, turning the reverb off would stop that.
In that case V2A cathode voltage would change with the reverb switch setting.

Also if V2A doesn't have a good grid leak path, R15 would provide that with reverb off.

[Indyryder02]

The reverb footswitch grounds the node between R15 and C15 for reverb off.
So the question is what does this change for the dry signal path.
If for instance C15 is leaky shifting the V2A bias, turning the reverb off would stop that.
In that case V2A cathode voltage would change with the reverb switch setting.

Also if V2A doesn't have a good grid leak path, R15 would provide that with reverb off.

Got a chance to look at this again.

So you’re definitely on the right track with this. Smart.

the voltages of the plate and cathode change on v2 a when it cuts out.

C15 was ok. Even swapped to be sure.

R25 was half a meg. Replaced. No change.

R13, R14, and C7 are missing. (Is this it? No path to ground?) And haven’t found any other schematics or pics to match what I have. (I’ll attach pics). V2 a grid goes to a .01 c6 instead of a 500pf. Possible someone’s mod. Then right to the loudness pot.

probing v2a grid node stops the problem. I assume making a small path to ground. I can take a 10 meg resistor to ground from here and stop the issue. (Tempting) haha. But I’d like to find the real culprit.
As I always learn more.

[Helmholtz]

Grid leak current is DC, so the grid leak path must be resistive and caps don't count.
The grid leak path makes sure the grid is referenced to 0 VDC.
If it's missing tube bias is undefined(floating) and can vary with signal.

[Indyryder02]

Grid leak current is DC, so the grid leak path must be resistive and caps don't count.

ahh, right. I knew that. Duh.



The grid leak path makes sure the grid is referenced to 0 VDC.
If it's missing tube bias is undefined and varies with signal.

where should it be getting its grid leak path from?
I haven’t been able to see it on the schematic. but that makes sense why adding a resistor to ground at the grid stops it .

also I noticed T3 has previously been replaced with a Hammond 124A..

[Helmholtz]

Normal grid leak path would be via R13, R14 and R12 wiper to ground.

[Indyryder02]

Normal grid leak path would be via R13, R14 and R12 wiper to ground.

ok. That makes sense. And R13 and R14, as well as C7 are missing on this amp.. the grid goes to R15 and a .01 cap (c6 I presume ) then to the loudness pot.

hmm. Wondering now if those components were removed as someone thought it was like the tone sucking orange couplet in the Falcon annps everyone clips out.