1973 Fender Quad Reverb
The Quad Reverb is a bit of a rare bird and not widely known amongst tube amp users. This is a CBS designed 100 watt amp sporting 4 6L6GC power tubes, a solid state rectifier, 4x12” speaker config, and Tremolo + Reverb. It also has the Master Volume push/pull boost circuit. Overall it is a killer sounding amp. This one had all the Tolex ripped off, the pine wood was stained, and it has a new baffle board as well. I’m not sure why that was done and I’m also not so sure about the results - It is unique though!
This amp was already somewhat restored by another tech from out of state, but not all of it was done right and some things needed correcting. The filter caps/board was not done by me and was good enough to leave be - although I would have preferred all the leads cut to the same length. The bias caps installed were 450v Radial caps and not at the right values so those were replaced with 100uf 100v MOD axial caps. 2 of the cathode bypass caps were sloppily done so they were replaced with 25uf 50v MOD caps. All of the under-rated plate load resistors were replaced with 2 Watt Metal Oxide Type. The cathode resistors in the Reverb section and Phase Inverter were replaced with 3 Watt Cement and 2 Watt Metal Oxide respectively. All screen resistors replaced with 3 Watt Cement. The owner wanted this thing to be bulletproof so this was all done for reliability - This amp will be gigged heavily and the original 1 Watt or 1/2 Watt resistors in these positions do not hold up well in the long term. They will also lower the noise floor - Win/Win. A .01 630v Mallory film cap was used to mod the Trem circuit to stop it from ticking.
The Bias Supply was given my standard modification to be an Adjustable Fixed Bias control rather then the original Balance control - this allows the power tubes to be set to idle at the correct current/voltage.
The original tubes did not test good and were all replaced: V1 = PSVANE 12AX7 V2 = EHX 12AX7, V3/V6 = JJ 12AT7, V4 = JJ 12AX7S, V5 = PSVANE 12AX7, V7-V10 = JJ 6L6GC Matched Quad. These tubes were picked for best tone, reliability, and selected for specific circuit position. The Bias was set to %68 Plate Dissipation with a B+ of 430VDC. All knobs were replaced and the chassis straps/hardware were replaced as the originals were completely worn out. For an amp that came in squealing and had almost no output volume - It is now the very loud/clean Pedal Platform amp the customer wanted it to be.
