09-04-2020, 05:10 AM
(09-03-2020, 06:32 PM)K O\Connor Wrote: The PA has a closed-loop gain of 55, so a 20V output at the speaker requires just 365mV at the PA input. This is where the 3M3||10pF meets the 470k grid-leak and the 470k from the reverb recovery stage. The attenuation through the dry path is about a factor of seven, so only 2v5 is needed from the preamp for the 20V at the speaker.
The AA1164 schematic I'm looking at looks a bit different - there's no grid leak on the PA input. I do see it in the AB763 circuit for example, though. I will add it then.
(09-03-2020, 06:48 PM)K O\Connor Wrote: Is there a signal threshold where the reverb oscillation starts?
Yes, around 20Vpp.
(09-03-2020, 06:48 PM)K O\Connor Wrote: V1A,B are cascaded stages and even Fender did not tie their grounds together. They did however tie the reverb driver cathode to the V1B ground, which is not problematic for their intended use of the amp and "reasonable" signal levels.
Good point, I will split those.
(09-03-2020, 06:48 PM)K O\Connor Wrote: V3B trem should ground to Vs cap along with bias since the amp uses bias-modulation.
It does.
(09-03-2020, 06:48 PM)K O\Connor Wrote: The concertina really should have its own filter since it provides a supply-referenced signal plus a ground-referenced one.
I think it does too, there's an additional 10K resistor after the 4th filter cap down the chain, and V3 has its own filter cap.
(09-03-2020, 06:48 PM)K O\Connor Wrote: One other thing: Since the closed-loop gain of the PA is so high, at least diagnostically it might be worth shorting out the 47R to see if stability returns? In any case, you can also hear how that sounds. bass might be a bit looser.
I'll certainly try that. I also noticed that the signal at the V3B plate is a bit wiggly when the NFB loop is closed.
Tomi