- Oct 24, 2008
- Reaction score
- Oct 24, 2008
EDIT: After reading nikongod's post I needed to clarify that I'm assuming a regular IRF510 / IRF610 MOSFET in which pin 1 is the gate, pin 2 the drain and pin 3 the source; as mentioned in this post from Dec 09.
Since you're talking about C2, and refering to this schematic, I assume it's V1 which doesn't light up.
By your description of your tests it seems to me that pin 1 of MOSFET Q1 is not at it's intended voltage level, which should be ~17 volts for a 12A_7 build and ~24 V for a 19J6 build. Since bypassing C2 makes the MOSFET turn on (which in turn lights up the tube), we can assume R3 is OK. Thus I can only think of R2 and R4 as the culprits, most likely R2.
Check the values of R2 and R4 (off circuit, of course; you can unsolder one end of each to check their values). Check their solder points. Check that the 'upper' (refering to the schematic) end of R2 does indeed sit at ~48V as the schematics point out. Check that the 'lower' end of R4 (again referring to the schematic) does indeed sit at ground ( 0 volts). If those previous conditions are true then the joint of R2, R4 and R3 must be at ~17 V. If this is true and yet the MOSFET does not turn on, then most likely the MOSFET is faulty.
BTW, about C2, it's purpose is precisely to isolate the MOSFETs pin 1 from the DC voltage and current present at the tube plate. Thus if it had somehow failed and became a "completely open circuit" that wouldn't affect the DC biasing of the MOSFET; you'd see the amp light and warm up correctly but no sound would come out of the Q1 channel, since the audio signal would not be able to cross C2 to get from the tube to the MOSFET.
On the other hand, if C2 had failed in other way and became a short circuit, then you'd see the tube light up 'as a Christmas tree'as you yourself saw in your experiments. That happens because with a shorted C2 the MOSFET gate sits at the DC voltage of the tube plate, ~27 V if I remember correctly. Thus it's source pin is then forced to be at around 22 volts, which is the voltage that feeds the tube heater: 10 volts higher than it's intended voltage, making it light up brightly.
Since from your description we know that none of these last two things happen, we can pretty safely assume that C2 is OK.
I hope this helps you in understanding the circuit, but more importantly, to get your amp back up and running.
As it turns out, the resistor connected to the positive voltage was cold soldered (I never reflowed this one, it was the best looking joint on the terminal strip >_<)
Thanks again for the help.