Thanks, Joe. I'm used to that setup on my 500-C so I think I'll take your advice. The amp schematic shows many test points and their "designed-in" values so at least I can do those tests, but I'm not sure where to make adjustments should a value be off. With the plate resistor I know I can measure the health of the tubes and use the trim pot to keep them in the "safety zone".One good thing about the amp design is that it's easy to access PCB components. Getting meter probes in there is very easy as all components on the top of the PCB are exposed. The bottom of the PCB is where the soldering is done so the PCB has to be removed for replacing components.
I rather like this little amp and I've got it sounding great with my 1970 Gibson SG. Other work I've performed on this amp:
- I upgraded all the filter caps, replaced other components with better quality parts,
- replaced the reverb tank with one that has a decreased reverb "tail" (i.e. the effect is not as long in duration),
- tried an assortment of tubes (2 of EL84 and 3 of 12AX7) to get a sound I like (I settled on 1966 EL84 Mullards and new Groove Tubes for the preamp, tone stack and phase inverter circuits),
- made the Negative Feedback circuit optional by installing connectors that can easily be disconnected (might add an on/off slide switch later),
- put in a better and "darker" 12" speaker (more of an even frequency response across all frequencies but especially reduces the overly bright highs)
- "dressed" the power transformer wiring and other wires, keeping some away from lytics that could introduce noise
- replaced the output transformer with a larger, higher quality model
- adopted some suggestions about which I read in various guitar amplifier blogs and forums that improve the efficiency and sound of the amp (phase inverter oscillation is apparently a concern with this amp that can be eliminated with one capacitor across a resistor)
Unfortunately this amp (and probably other new ones by Fender) is not designed and built with the ambitious "modder" in mind because some parts, especially the multi-wire, plastic-sheathed wiring harness from the 5 tubes to the PCB, are brittle and are at risk of breaking from too much movement. It is known that the heat from soldering on the PCB can cause the traces to become separated from the PCB. After installing the trim pot, although I was so very careful with everything, somehow I damaged a separate small board with the speaker and foot pedal jacks because I have to jiggle it to get any sound from the amp. Something came loose somehow (have to go figure that one out now). There are only 1 or 2 additional changes I want to make and that is going to have to be it, as they say.
Thorne