Circuit Edits

So, while turning the amp on for the first time, I found it squealing and way noisier than could be used in any kind of setting. I ran through my notes and discovered I had wired the primary of the output transformer out of phase, that is the secondary was feeding the feedback loop out of phase, so that it wasn't taming the amp - it was making it feedback on itself. Hence the squeal and super loud noise. I remedied this by swapping the transformer primary leads on the tube sockets. Problem solved.

Another fix - the control grid of the 7591 tubes was much lower than I would have liked - sitting at around 305V, rather that 370V-375V that it should be. The extra PI filter that I had running of one of the HT leads was knocking down the voltage a bit too much. I stuck a pot across the existing resistors and varied it until I had approximately 365V-370V at that point. There was a sequence of adjustments, because as I changed that value, the plate voltage changed, and so did the bias. All 3 were reacting with each other. I eventually reached a happy medium. I connected a 22k-ohm and 33k-ohm resistor in parallel with each other.

Feedback - The Fender circuit has an 820-ohm resistor feeding back into the phase inverter stage (pre-7591), while the Fisher has a 2200-ohm in parallel with a 1000pF cap. Again, a happy medium. I have a decade resistor box that I ran across the feedback point and listened to the circuit with different feedback resistors. 1k-ohm sounded good. I replaced the jumper with a 1k-ohm resistor in parallel with an 1800pF cap.

Reverb make-up gain - tube replacement. While testing out the amp, I found that a 12AX7 (at least any of the ones I had) were far to noisy to be used in the make-up gain section for the reverb return. I threw in a used 12AU7 in there and that cured my problem, without making the amp sound noticeably different. I may consider a JFET return amp, instead of the 1/2 of a 12AX7, leaving the other half for the mixer circuit. I might be able to use the unused (when modified) section for gain for the solid state preamp I intend to use as a channel option to the tube preamp.