1976 Fender Twin Reverb
This mid 1970’s Twin Reverb has the CBS designed 100 Watt circuit inside of it. This same circuit was also shared by the mid 70’s Dual Showman Reverb, Quad Reverb, Super Six Reverb, and Vibrosonic Reverb. The thing that differentiated all of these amps then was not the schematic design but the speaker cabs and output impedance - which also changes the amount of negative feedback. Each amp came with either a 2x12”, 4x12”, 6x10”, or 1x15” cab respectively. This Twin Reverb is characterized by being cleaner and more hi-fi sounding then earlier circuits and by the Master Volume Push-Pull Boost feature that was added for this series - as well as the 100 Watt power rating. The Master Volume (and the Push-Pull Boost which came into use separately and in different years) is probably the most debated and maligned feature ever added to a Fender amp. I’m not always sure why that is though, because if the MV is turned to 10 then it is essentially out of the circuit and then the preamp volume can be used normally like any other Fender amp. The Boost is the same, just leave it off and it’s like it’s not there. The other changes to the circuit that are inherent to the design though are real, and make this era of Silverface Twin Reverb’s sound noticeably different. Notable users include: Jerry Garcia, Bob Marley, East Bay Ray, Jack White, Sonic Youth and Kurt Cobain.
This one came in needing a standard restoration having had very little work done to it over it’s nearly 50 years of life. It had leaking Filter Caps, out of spec/heat damaged Power resistors, and bad tubes. It also had a broken fuse receptacle meaning the amp could not even turn on in the state it was brought to me in. Aesthetically this amp was very clean!
Amp was completely restored. All filter/bias/bypass caps were replaced with upgraded voltage and temperature rated MOD brand electrolytic’s. New 2 Watt Metal Oxide power dropping resistors were installed for better reliability and lower noise floor. The cathode resistors for V3 and V6 were replaced with 2 Watt Metal Oxide type units due to heat damage. All plate resistors were replaced with 2 Watt Reduced Mass Metal Film type units for preventative maintenance and reliability. All screen resistors were replaced with 3 Watt Cement 470 ohm type units for same reasoning. A new 3AG Fuse Receptacle was installed as the original was broken - a unit was chosen that matches the original aesthetically. A new 3 Prong AC cord was installed to replace the frayed and taped one present. A .01uf 630v Mallory film cap was used to mod the Trem circuit to stop it from ticking.
The original tubes did not test good and were all replaced - with the exception of V1/V2. This was the lineup chosen: V1/V2 = Sylvania 12AX7, V3/V6 = EHX12AT7, V4/V5 = JJ 12AX7S, V7-V10 = JJ 6L6GC Matched Quad. These tubes were picked for best tone, and reliability. The 6L6GC's Fixed Bias was set to %62 Class AB Plate Dissipation with a B+ of 444VDC. The tube sockets were all re-tensioned to stop loose tubes from potentially falling out and were then treated with De-Oxit to remove/prevent corrosion. The pots were sprayed out and the amp was cleaned inside and out. Amp now functions well.
