Re: Adding A Tube Rectifier To A Guyatone S/F Fender Clone (and other upgrades)
Tues. 15/01/06 16:00 EST
.post #5
Just to be clear, there is no IC chip in the Guyatone amps, the reverb circuits are all-tube.
If you look at the Guyatone schematic, or a Fender one, you'll see that the reverb driver valve is a 12AT7 that is directly connected to a small output transformer. That stage is a little amplifier with it's own output transformer, seeing the reverb tank as a speaker, and thus one needs to match the impedance in a similar way as connecting a guitar amp to a speaker cabinet. Hence the 4AB tank being the best match.
Thanks for clarifying that; the page from which I quoted pertains to a Fender Blues Junior amp, but I thought the techniques for testing the input and output sides (not the IC) might be worthwhile additions to what you advised.
I understand your lesson regarding the matching of the reverb tank to the output transformer; it's just that I don't know which pair of values is the best match.
Reverb circuit test results
Recovery side:
I first disconnected the RCA cable from the output jack of the reverb tank (other end of cable connected to 'REVERB INPUT' jack on back of amp) and, with Reverb and Master volume set about halfway to full, touched the center conductor of the RCA cable while adjusting reverb volume. Although I heard a buzz that varies with the reverb volume setting, touching the center conductor has no effect.
Next, I connected the headphone out jack of a small portable radio to the 'REVERB INPUT' jack on the back of the amp. I could hear the radio playing through the amp's speakers. Increasing the reverb volume only increased the buzz, but did not increase the volume of the radio through the speakers.
Send side:
I disconnected the amp's two speakers and, with one end of RCA cable connected to the 'REVERB OUTPUT' jack at the back of the amp, connected the other end to the input jack on the front of a powered-on second amp. Strumming the guitar connected to the powered-on Guyatone amp produced no sound from the second amp's speakers.
After some phoning around, I've now located one person within driving distance from whom I can purchase a 4AB3C1B reverb tank.
