Nayyte
100+ Head-Fier
If you want to improve tube filament heating circuit before final closure the chassis there is way to do that by increasing capacity of two 1000uF/25V capacitors near power switch. Voltage for heaters are controlled by two voltage regulators LF60A with fixed voltage of 6V and according to data sheet up to 0.5A with limit of 1A. Jason Stoddard is clever guy and decide to use high PSRR for low frequency of this regulator to control also maximum current by limiting capacity of input caps. With ripples ca. 1.8 V current limit is ca.400mA. With increased capacity on input we get smaller ripples more room for voltage regulator. As usual there is no free lunch. On power on when tubes are cold and resistance is quite low heater circuit will go with 1A so for 300mA tubes which could shorten tube life.
Just bought a MJ2 and already digging around for mods I could do. I came across the 5687 tube variety. Same 9 pin mini design as 6922 but pins 6-9 are swapped around so a converter is required. Only problem is that in order to run in 6.3V, pins 4 and 5 need to be tied together between pin 8. Other wise its between 4 and 5 for 12.6V. On top of that the current draw is 900mA! I think these two things are the only differences but not sure.
Assuming I was to mod the MJ2, I would set up the heaters to be 12.6V @450mA or original 6.3V but I would have to figure out how to squeeze 900mA out of it. Really not sure if its possible without completely gutting it all.
@gumisb "current will be over 3 times higher" - so you tell me there is hope?
Thinking about this for fun of course. Also because I have a mild aesthetic issue with short plate tubes that the MJ2 takes stock.