Interesting job came in as it was squealing badly. Gave it a look over cleaned reverb tank connectors and fired her up. No squealing but sounding a bit anemic. Checked the bias and that was set very cold. Corrected it for the 500V plate voltage I measured and things got a lot better on the sound front. But for big bass signals with the amp cranked it was making a not nice farty distortion… I opened her up again and checked where the distortion was coming from. As it was not changing with volume (just starting with high volume) I thought it was around the inverter. Scope probes on the output of the inverter tube and the distortion disappeared! scope probes off and back it came! The sound reminded me of a blocking distortion. Then I noted that the 6L6GC tubes had no grid resistors increasing the chance of instability and blocking distortion (early fenders did not bother with this resistor). Adding two 2K2 resistors in series with the grid cured the problem and also effected the tone making it much richer though still able to give a good Fendery tone. Reverb on full and she is giving the sound the customer wants! Lovely Amp Got this amp back again a few years later as exchange for a HH Amp. This time I carried out some major surgery to improve reliability and tone. Please look at the later pics for a description but main things carried out were to add a decent ground buss, revolve the power transformer, add a Valve Rectifier and rewire the reverb for an ECC832 so it uses all currently produced tubes.. Sounding great!