Well I breadboarded it early this week out of curiosity. A few observations:
- 18V instead of 9 does not make much difference.
- bias is finicky, get as close to 4.5V as possible (using 2 resistors in series or a trimmer).
- while the voicing of this circuit is not exactly to my liking, it is undeniably well designed: tried other coupling caps, bypass caps and so on, and it quicky went south. Only tried stuff for about 20min, though.
- snubber caps collector-to-base -> Nope. A 2.2n cap across the last collector bias resistor worked MUCH better to tame the highs.
- somebody at DIYStompboxes suggested using other diodes. I tried BAT85 and it sounded pretty good.

I will have to revisit it at some point during summer, as it definitely has a nice vibe. Hope this helps!

Wish I could, but all my recording gear is on the other side of Europe now and I won't be able to access it until christmas at earliest    phone recordings don't do it justice, unfortunately. I might build one in a stompbox and send it to a mate to test it and record some clips during summer, we'll see...

Looks interesting. I'd love to hear some clips!

Hi all,
I have a Randall RG100HT amp head that has a really weak/quiet Channel 1 (not Input 1).  Channel 2 sounds great and is loud as I would expect for a ~100W amp.  When using Channel 1, I have to turn the gain and volume up (past 12 noon) to get a decent signal and even then, there's not much definition to the audio and it is somewhat muffled.

I have reviewed the schematic, confirmed component values, checked DC voltages within the Channel 1 circuit, and ran a signal through Channel 1 and scoped it along the audio path.  I have 24v DC along the DC rail and have good AC audio signal up to the gate of Q2.  All components are within spec and I have replaced Q2 with a new 2N5484 transistor.  Attached is the schematic for Channel 1 and I have annotated it (in blue) with measured DC voltages.  The DC voltages for Q1 are within spec but those at Q2 and Q3 seem to support the notion that something isn't right at either Q2 or Q3.

I'm more familiar with tube amps and not SS amps so I may be not recognizing obvious solutions - so I'm putting this problem to the forum.  What recommendations do you have as a solution to the weak audio in Channel 1?https://photos.google.com/photo/AF1QipM9NMuHgN6irIVMYvnkxXqrwgm2pGozITmv5MDl

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.