Right, it's fairly easy to work along the signal path in this amp. There is of course a risk of shock when working on a live amp, so all the usual caveats... I'm not responsible for any harm you may do to yourself or if you set fire to your house... but honestly it's not a very dangerous amp (I did get a decent belt off one of the my big decoupling caps a while ago whilst trying to discharge it).
Carefully power up the amp with case open and the innards accessible.
First of all, stupid question do both the heaters light up?
When following these instructions if any step doesn't read like it should report it here so we can try and work out the problems.
Now let's check the power supply path. With your DMM on VDC you should see 48V at the power input to the amp and going to pin 2 of each mosfet. You should also have a CRC pi filter with about 45-47V on the other side of the 2k resistor. Then check the voltage at the top of R1 and R7, they should be the the same as the voltage you measured after the pi filter. After this measure the voltage on the tube cathodes (pins 3 and 8 on 12a_7s and pin 7 on 19J6s) it should read around 2V from memory, but the most important thing is that both tubes read the same.
You can also check the heater voltage now (between pins 4 and 5 on a 12a_7 or pins 3 and 4 on a 19J6), it should read either around 13V or 19V.
Now for a signal path check:
I recommend connecting a cheap source that you're not fussed about and then play some music or test tones at a decent volume from your source (test tones are best due to them being constant power). Set your DMM to AC 20V or similar then measure the power on both RCA input jacks (or just both channels of whatever input jack type you're using). Left and right should have the same voltage, if not you're problem is before the amp, if its the same move down the signal wires to the pot. I turn the pot to about 50% and then measure the voltage on the wiper (with the source still playing). Once again the left and right channels should measure the same (I recommend turning the pot half way because they are rather imbalanced in the first 10-20%).
Now you want to measure the AC voltage on the tubes plates (pins 1 and 6 on 12a_7 builds pins 1 and 2 on 19J6 builds) you should now see your amplified voltage and it should be very similar across both channels (allow a bit for valve differences). If it is the same continue this process if not go the note a. Next I would ask you to measure the AC voltage on the other side of the coupling capacitors C2 and C4, it should be the same as on the other side. Now, set your DMM to DC 20V and measure the voltage at the mosfet gates (in the same place as you took your last AC measurement). Once again they should be the same and about 16-17V for 12a_7s and 24V for 19J6s. If this is the case then switch back to AC and measure the output of the mosfet, where it connects to coupling capacitors C3 and C5. Once again report any difference you find here. If everything is still the same then check the voltage on the other side of these capacitors. If you still haven't found your problem then I would say to check your headphone jack for damage, beyond that I'd be at a loss.
note a: So if your not getting the same voltage out of your tubes then check the plate voltage (same place as the AC signal but in DC this time) it should read around 24V for both channels. If it's not an issue and your tube heaters are correctly powered then I'd reckon that your left channel tube is damaged. To test this swap the left and right tubes around, the faulty channel should now have switch over (only swap tubes with the amp powered down!)
Good luck with your debugging,
Chris
