Gibson GA-5T Oscillation 1961? 6AQ5 6EU7 Version - How to Fix

[plumcrazyfx]

So I have one of the GA5T Crestline Versions - the one with the PP design 2x 6EU7, 2x 6AQ5 (cathode biased with 270 ohm), and 6X4 rectifier.  I've done a couple of updates (the thing was a basket case).  I recapped it, added a 3 prong cord, and replaced the 2.2k on the second gain stage with a 1.5k.  The thing sounds good (cleaner) BUT going from 9 to 10 on the volume the volume drops a little, it gets a little weird-sounding distortion, and I'm pretty sure it's oscillating causing it. 

I'd like to do some additional mods to get the most out of it - replacing the .01ufs to the power tubes with something bigger changing the 100ks on the input with a 68k, maybe elevating the heaters but don't want to do anything before the volume drop/stability is taken care of.  The amp is mostly point to point and has wires all over the place like those early Gibsons and there isn't a shielded wire in sight.  Recommendations?

[sluckey]

I'm pretty sure it's oscillating causing it. 

Look at it with a scope to be sure.

[shooter]

+1

I recapped it

did that include the bypass caps?
Did you disconnect the trem?, how does trem effect, or not your issues?
I've only fixed 1 with 6EU7's and it got re-wired for AU7's and new sockets, problems gone

[plumcrazyfx]

Yes - bypass caps were also replaced.

I don't have a scope.

Was hoping for something simple.

AUs are a significant decrease in gain, correct?

[shooter]

AUs are a significant decrease in gain, correct?

I went back to my notes, put new socket and  an AU in V1, V2 was a 6C4.  comments from owner "I don't remember this amp ever being this loud"  (It also had a blown interstage PI transformer and bad PA tube)

also re-wired filaments to a 2 wire arrangement with 2 100R to ground, capped and 3 prong

GA5T I think is the most varied amp model I have ever seen, no 2 are alike!! 

what do you want from the amp?, without a scope, you're limited to shot-gunning til it works, which is pretty easy to do

here's a gut shot of another one I fixed, cleaned up and flipped;

[plumcrazyfx]

I will live with it as is until I can get a scope and check it out.

I jumpered in the larger coupling caps and it doesn't make the issue any worse but the trem intensity went to near nothing.  Will be leaving it.  Have other projects to work on.

[plumcrazyfx]

I also noticed that the volume drop (oscill?) only occurs on Input 1 of the amp.  This amp was made for two instruments and doesn't have a high/low jack - instead it has 2 parallel 100Ks to the grid.  Input 1 is a stereo with a 1Meg and input 2 is a mono with only the 100K wired to it.  May be more useful to have a high low.

I shielded the wire from the volume back to the 2nd side of the 6EU7 and it didn't change the behavior.

[sluckey]

I also noticed that the volume drop (oscill?) only occurs on Input 1 of the amp.  This amp was made for two instruments and doesn't have a high/low jack - instead it has 2 parallel 100Ks to the grid.  Input 1 is a stereo with a 1Meg and input 2 is a mono with only the 100K wired to it.  May be more useful to have a high low.

The schematic shows a switched jack (not a stereo jack) for the top input and a standard mono jack on the bottom, The wiring is such that the bottom jack will operate as a low input and the top will operate as a high input. If you have two guitars plugged in each jack behaves the same, IOW, no hi/lo function. This is functionally the same as the Fender hi/lo inputs.

The hi/lo jacks would explain why you only have the issue on the high jack #1. Take a look at the jacks in Hoffman's store. If you still think you have a stereo jack, please post a pic. Someone that didn't know what they were doing could have put a stereo jack in the #1 hole.

BTW, changing those 100K resistors to 68K will not accomplish anything. I would leave them alone.

[plumcrazyfx]

No you're right. Switched jack.

[Colas LeGrippa]

put a 50uF in parallel with the power tube cathode resistor and leave it on. Can remove oscillation or great part of it.

[plumcrazyfx]

put a 50uF in parallel with the power tube cathode resistor and leave it on. Can remove oscillation or great part of it.

Didn't work this time.  I'm thinking it's preamp related.  The grounds are all over the place and wire lengths are wherever they felt like it.  I may have made it worse changing the 2.2k on the second triode's cathode to 1.5k for more gain with the original fix I did a while back.

[plumcrazyfx]

Just a quick update on this one.  I got it 98% fixed.  I lifted all of the grounds that were connected to the chassis using the lugs of the strips using nylon nuts and screws.  Rerouted the grounds to clean them up.  Reduced the cathode bypass cap on the first triode from a 25uf to a 5uf and reduced the 2.2k on second triode to a 2.7k.  Doesn't do the decreased volume thing in the last 10% of the dial.  A little hash that sounds like radio interference when all the way up but clean and nice sounding otherwise.