List voltage reading for Q8 & Q9 at  all pin's connection points. Measure all power supplies. What are your -/+40v rails? -/+12v rails? Can you scope a signal through the unit and figure out where the signal goes bezerk?

Edit: If you scope the output is the fuzz actually crossover distortion?

Hello!  I've been a pedal builder for the past 10 years, building both from PCBs in the DIY community as well as designing some of my own.  Last winter, I began to dabble in amps.  I first took on this Gibson G-105 from the 70s. I first worked at re-capping the thing.  In a poor decision by me, I used some cheap caps to replace the two 3000uF caps located in the rectifier section of the amp.  It sounded great for a while.  It was super clean unless you pushed it, and was phenomenal hooked up to an electric piano or guitar.

One night, the amp began to totally fuzz out, producing this gated, starved sound.  Sort of like when you start screwing with the voltage going into old Germanium fuzz pedals. Luckily, the amp has a signal line out right before the long tail pair phase inverter, and I validated that it sounded fine.  I did some sanity checking around the speakers to ensure that wasn't the issue. I changed the crappy caps in the rectifier to some nice Mallory screw in terminal caps.  I even changed out the 4 power transistors. 

I've gotten a little over my head in terms of what step to take next.  I'm pretty capable of testing, but I'm unsure of where to look next.  I'm hoping the people in this forum could help point me in a good direction to begin to look.  Thanks!

Schematic for reference:  https://el34world.com/charts/Schematics/files/Gibson/Gibson_g_105.pdf

Finally had a chance to get back to it.
T4 Pin 1 -37.3v
       2 +33.9v
       3 -37.8v
T5 Pin 1 +0.6v
       2 +36.2v
       3 +28.6v
T6 Pin 1 +33.9v
       2 +34.1v
       3 +33.7v
Amp is still on dim bulb tester

Quote from: Jesse on May 15, 2024, 02:21:50 PMAny book recommendations for Electronics Repair / Troubleshooting?

Lots of good suggestions in this thread:  https://www.ssguitar.com/index.php?topic=439.0

It works! Thanks again g1!
The tricky part for me was missing the fact that the replacement part I purchased for Q9 had a metal backing, where as the original did not. See attached photo.
Any book recommendations for Electronics Repair / Troubleshooting?
Thank you again everyone! I look forward learning more here

If the rectifier's are not in circuit then there would be no B+ vdc. So I can't immediately imagine the caps are the issue from that standpoint. Of course posting a schematic would be a good thing. It seems the transformer may have other windings -/+15v rails and your heater's winding. Try disconnecting all secondary connections when in doubt. Light bulb shining with secondary taps disconnected from circuit is not good. Right now from what you have written we need to clarify what you are describing and a schematic. We need voltage readings if the light bulb is not shining bright.

Hi friends; It's been a very long time away. A lot of things got put on hold including amp repairs. I have been working on a Peavey Ecoustic 112 amp lately. Everything is running smooth and quiet except the reverb which is important for the new owner. I have reverb when I turn the pot up but I also get like a 60hz hum. I tried re-soldering, cleaning the pot, switched out the tank with 2 diff ones but nothing quiets it down? Does anyone have a suggestion? Looking through the Peavey Forum I found evidence that Peavey amps are known for noisy reverbs. Someone on there suggested using an isolation transformer which I have not tried.

Thanks for all the info and hints. I'm about to order the parts that I don't have in my stash and as soon as they arrive, I'll start breadboarding. Provided nothing unexpected gets in the way, that is.
As far as the higher voltage goes, I happen to have an LT1054-based voltage doubler put together, so I'll give it a try at roughly 18V too.
Have a good one and stay tuned!
M

I'd try 10pF to 22pF, base to collector, similar to the trick employed to smooth down silicon transistors in fuzzes, but that's a matter of taste
I had a bit of downtime this arvo and threw the Vulcan on LtSpice: the frequency response is VERY similar to the Randall RG100Es, so you'll probably hot rodded marshall vibes. That doesnt mean that it will sound like a Randall, though: clipping thresholds and gain increments are very different.
I was also surprised to see that the circuit, as is, is already shaving off some dBs above 6kHz.
I'd be tempted to try it at higher voltages, with low hFE transistors, just for the sake of it.
Oh, and the diodes do mimic grid clipping: they chop almost half of the waveform before the gain stage. Very clever design with just a few parts

Thanks for the reply, Umlaut.
Looking at the schematic, I also expect it to sound like a Marshall on steroids. Like a DSL or a Bogner Ecstasy perhaps?
Speaking of snubber caps, do you mean in parallel with collector resistors?