The Amp Garage

Interesting! I would have never guessed.
Thanks for the input!

Very interesting. Here's a closer look at the circuit.
Why would an oscillator be required to drive a lamp?!

Hello! I'm in the process of recapping the amplifier from a Bell & Howell 179 film projector. My first goal is to get it running as it was originally designed. This amplifier has THREE 6V6GT tubes. One pair is used to drive the output transformer. However, I can't figure out the function of the thir...

xtian wrote: ↑Sat Apr 06, 2019 12:05 am I notice the date on the schematic is 1964. My question is, how did they know how awesome SRV was going to be at guitar?

Someone needs to correct the title of this thread....

I just finished building a clone of the AC10 SRT. OS/71 has several errors, including the placement of R28 and R29. The schematic at the top of this thread is 100% correct. The drawing number is OS/026. The dry signal does indeed pass between the cathodes of V2 via the 22K reistor. Strange but true!...

I completely rebuilt the reverb circuit per the VOX schematic. Hum and buzz gone!



EDIT: Mod edit to embed youtube

I'm currently waiting on parts to convert the circuit as close as possible to the original. I believe the high impedance tank will arrive today, but the resistors are still a few days out. I'll keep you posted! I had mwaller over this last weekend again, and figured out the odd negative bias, this w...

pompeiisneaks wrote: ↑Tue Feb 26, 2019 5:53 pm another thing I did want to ask. The voltage above the 100k anode resistor was about 280 I think if I recall, but the voltage at the plate was only 80VDC is that also expected?

~Phil

This is in reference to the gain stage immediately before the reverb circuit, V2a.

First - a big "thank you" to pompeiisneaks for taking a peek at my misbehaving amp! The issues have not been resolved, but we he did uncover some odd behavior that may hint at a solution. There appears to be an unusual DC offset occurring near the input to the reverb circuit: Reverb Circuit.JPG With...

[/quote] Sounds like a plan. I also really need to sell an old hitatchi scope I got for 100$ since I've never used it since, I guess I should put that up here at some point, but I need to stop being so lazy :D ~Phil [/quote] Hi Phil - I sent you a PM, but it appears to be staying in my outbox. Perha...

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas.....

Stevem wrote: ↑Sun Feb 10, 2019 11:53 am A long shot but worth the try here!
How is your PT mounted?
Can you pivot it 90 degrees without a major rework to see if it helps?

Unfortunately, the PT fits into a rectangular hole in the chassis, so it can't easily be remounted. It does have a grounded hum shield, however.

I disconnected the reverb at the grid on V2 and added a 220k pull-down resistor. There is still hum and hiss unless I remove everything except the power tubes and rectifier. Can anyone recommend a tech in the Seattle area who might be willing to troubleshoot these issues? I'm running out of ideas...

Thanks for the comments! I'm remembering that before I started trying to "fix" things, the reverb actually worked (LOL!) I think I will first try to restore the original wiring scheme and see if I can't get it to sound better. The squealing and hiss are relatively new developments... Who knows... wi...

Ok, so I isolated all the remaining connectors and pots. The hum is reduced, but a prominent white noise *shhhhhh* remains at all volume levels. There is clearly something very wrong with the reverb circuit - when I turn up the reverb knob even a bit, I start to get a lot of squealing feedback. Thou...