Tuesday, October 14, 2014

Epiphone Valve Jr Hot-Rod Modification

EVJ Hot-Rod
The Epiphone Valve Jr Hot-Rod is a 5W Class A all valve guitar amplifier with built in spring reverb, and is the less common, and slightly more expensive cousin of the original EVJ.
Although modifications for the original EVJ are found in abundance across the web, the EVJ Hot-Rod seems to have been overlooked by the modding community.

The EVJ Hot-Rod has essentially the same amplifier structure as the original model, with an additional 12AX7 to drive the spring reverb unit, a multi-tap transformer output for different speaker impedance, and a "gain" pot on the front panel (which is just a voltage divider after the first preamp stage). With this in mind, modifications for the EVJ Hot-Rod can be easily copied straight from existing documentation for the original EVJ.

Safety Note
Power supply capacitors in amplifiers retain charge for weeks. Always discharge capacitors before working on any amplifier. Valve amplifiers are especially dangerous, as the working voltage of the power supply capacitors can be in excess of 300V.
A 1K 5W resistor attached to probes can be used to discharge capacitors. Double check with a multimeter before working on the circuit.

Disassembly of the EVJ Hot-Rod head

Remove screws from the back panel, and pry off (The panel may be hard to remove due to traces of glue on the vinyl).
Label the cables to the spring reverb unit for re-assembly, and disconnect.
Remove four plastic caps from the top of the amplifier case.
Remove the four screws, and slide the amplifier chassis out of the case.
Remove valve covers.
Remove valves.
Discharge the power supply capacitors by shorting R14 and R15 to ground through a 1K 5W resistor.
Label all cables for re-assembly, and disconnect.
Remove the input socket retaining screw.
Remove the circuit board support screws.
Remove the circuit board from the chassis.
Thoroughly wash the circuit board with isopropyl alcohol to remove flux and dust.
The circuit can now be worked on.

Clean dust from chassis.

Clean all pots and sockets with de-oxidiser fluid.

I have largely copied the EVJ "marshall mod" (schematics for which can be found elsewhere). After about six hours of changing component values, I found a sound which I was happy with.

Original Circuit

Modified Circuit

The result was a brighter sounding amplifier, with a well defined low end, and higher gain. Adjusting the controls for low gain settings produces a smooth overdrive, and higher gains ramp up to a broad fuzz sound.
A word of caution; the quality of the EVJ circuit board is very poor. Excessive heat will easily lift circuit board pads and traces. Ensure that you use a good heat controlled soldering iron.
Lowering R3 will increase input level to the first triode stage. Lowering R6 increases quiescent current through the second triode stage, while lowering C8 and C9 serves to 'tighten' the bass response.
Increasing C16 improves bass reproduction of the power pentode.

The addition of a 1K 1W resistor at the pentode screen will limit current through the amplifier in the case of valve failure. This modification requires cutting a circuit trace, and drilling holes to mount the additional resistor on the top of the circuit board.
I found the reverb on the stock unit to be very thin, and so decided to modify the reverb circuit also. Lowering R22 and R25 increases quiescent current through the triode stages, while removing C21 and C22 produces a more linear frequency response.
Increasing C18 prevents low frequency rolloff (use ceramic cap due to voltage present).
Increasing R27 reduces output of reverb to a more subtle, controllable level.

Details and schematics for the EVJ Hot-Rod can be found here.