Repair on a Guild T1 RVT

[johnnieboy2]

OK, this one is the model with the silver grill cloth, 12" and 8" speakers, reverb and tremolo. No ground switch and no separate boards, all in one chassis. Pots marked '64 as are all other parts with I'd. Had a horribly microphonic 6GW8 tube, replaced. Now it has a loud crackle when strings hit moderately hard which dies out almost immediately and returns to normal tone for rest of note duration. Power supply voltages look good. Any ideas?

[johnnieboy2]

Tubes are good.

[Helmholtz]

Are tube sockets good and making good contacts?

Schematic?

[johnnieboy2]

No issues with tubes or sockets.

[johnnieboy2]

[Helmholtz]

Ideas will come after seeing test results.

Start usual troubleshooting:
- Measure power tube idle currents
- Measure preamp tube plate and cathode voltages (in the main channel) as well as power tube screen voltages directly at tube sockets.

Do you have a scope + sig gen?

[Mick Bailey]

Are the speakers good? Try the amp though an external cabinet if you can to eliminate them.

[johnnieboy2]

I do have signal gen and scope. As info, when I was checking and replacing tubes I removed the microphonic tube V6 and tried the amp. It sounded great! No issues and the tremolo was fantastic too, clean sounds, no weird noise. But when I replaced the new 6GW8, V6...when I would hit the strings moderately hard I'd get a loud noisy output at first, then it would clean up the tone as the note decayed. Playing at low level gently playing, all is well. If I tried to get past noon on reverb control it would get a hum louder and louder, bad oscillations. Also when footswitch for reverb is depressed to engage, there is a pop and temporary squeal. On disengaging reverb there is the pop.

[Helmholtz]

Unfortunately not every "new" tube is a good tube. DC measurements should show.

Could you rule out a bad speaker?

What is the rated output of the amp and what output before clipping do measure into an 8 Ohm dummy load?

[johnnieboy2]

I don't have a dummy load at the moment. As far as the speaker goes, as I said earlier if I remove the reverb power tube and turn off reverb the speakers are clear as a bell.

[Helmholtz]

As far as the speaker goes, as I said earlier if I remove the reverb power tube and turn off reverb the speakers are clear as a bell.

Where did you say that?

Anyway, it now sounds like a problem in the reverb channel.

[johnnieboy2]

Another finding, while momentarily disconnecting the 8" reverb speaker, the 12" had the noise in it. Same procedure with the 12" speaker disconnected and no noise in the 8" reverb speaker. Not what I was expecting at all. Does anyone here have proper tube voltages with tolerances for this amp?

[johnnieboy2]

Sorry Helmholtz, I had said it in a post I was typing but the upload failed, sorry.

[Helmholtz]

Another finding, while momentarily disconnecting the 8" reverb speaker, the 12" had the noise in it. Same procedure with the 12" speaker disconnected and no noise in the 8" reverb speaker. Not what I was expecting at all. Does anyone here have proper tube voltages with tolerances for this amp?

Disconnecting the speaker load can damage the OT.

Post tube plate and cathode voltages as advised earlier and I'll try to figure out if they are ok.

[johnnieboy2]

Update...Well, after a lot of work: replaced both capacitor cans, replaced the 8" output transformer, touched up many solder joints and the amp is working great.