1966 Epiphone EA-12RVT
This is an extremely unusual and rare amp that was made by Gibson under their Epiphone brand name. Gibson had acquired Epiphone in 1957, just 9 years before this amp was made. Despite the model number printed on the back-panel, the official schematic for the EA-12RVT has absolutely nothing to do with the circuit/topology found in this particular amplifier. This amp - although it has some minor differences - is actually a Gibson GA-55RVT. The tube layout and parts inside trace out to this other amp. Why this is - I am not sure - but Gibson was infamous for making changes to their amps and not accurately documenting it. The change here though is so dramatic that it is hard to understand the continuation of the model number.
This amp (the GA-55RVT) was Gibson’s answer to the 1960’s Black-Panel Fender Super Reverb. At about 40 watts, with two 6L6GC power tubes, a 4 x10” speaker cab, spring reverb and tube/opto-isolator tremolo the similarities are obvious. But under the hood is a completely different animal with a highly unusual preamp section, tone controls, tube choices and circuit engineering. This amp uses the hum-bucking alternative to the 12AX7 - the 6EU7 - in many preamp slots. The reverb driver is a 12AU7, the phase inverter is a 6CG7, and the oddest choice is a gas-voltage regulator 0A2 for the tremolo modulator. The tone controls are a mix of Baxandall and T-Filter topologies which makes for a very different experience dialing in tones. It’s a cool amp that is super unique and the type of gear I love restoring.
The amp was completely restored. All the electrolytic’s/bias/bypass caps were replaced with MOD/Vishay brand electrolytics with upgraded voltage and temperature ratings. New 2 Watt Metal Oxide power dropping resistors were installed for better reliability and lower noise floor. The same was done for some plate and cathode resistors. Upgraded 3 Watt Cement resistors were installed for the screens of the power tubes. Upgraded 1N4007 1000v Diodes were installed for the rectifier and bias circuits. A new 3 prong AC cord was installed to replace the old AC cord and the death cap was removed. The polarity part of the power switch was also deactivated for safety. A new Fender style handle was installed to replace the broken original.
Most of the original RCA branded 6EU7’s tested good and were kept in place. The other preamp tubes and power tubes did not test good and were replaced. These were the installed tubes: V3 = EHX 12AU7A, V5 = Tungsol 6EU7, V6 = NOS 6CG7, V7/V8 JJ 5881, and V9 = NOS 0A2. These tubes were picked for best tone and reliability. I installed the JJ 5881’s to bring on earlier power tube distortion and to alter the EQ to bring out more mids and less shrill treble frequencies. The customer requested a way to make the amp less bright as it had an extraordinary amount of treble on tap. The 5881’s Fixed Bias was set to %60 Class AB Plate Dissipation with a B+ of 474VDC. The Cathode Bias on the V3A 12AU7 Tremolo Oscillator Driver is adjusted by a trimpot instead of a fixed resistor value - this was set too cold from the factory and did not produce a strong enough Tremolo signal. This was adjusted (per customer request) to provide a hotter bias and a much stronger Tremolo effect. This will wear out the 12AU7 a little earlier, but I made sure to stay at a max of 70% of the 2.75 Watt Plate Dissipation limit. The tube sockets were all re-tensioned to stop the tubes from jiggling or falling out while they hang upside down. The pots were sprayed out and the amp was cleaned inside and out.
