Voice of Music 8810 catastrophe

[bluzmn]

A short while ago I got a little VOM 8810 amp head; the tube complement is 6x4, 6v6 and 12ax7. It worked, but had too much gain for harp. So I decided to make the circuit a little bit more like a Fender Champ. I replaced the 220K and 150K plate resistors on the preamps to 100K; I replaced the 1K cathode resistor on the first half of the 12ax7 with a 1.5K ( the 2nd half already had 1.5K). I also replaced the 300 ohm cathode resistor on the 6v6 to a 470 ohm, and the 10 mfd bypass cap with a 22 mfd. When I started to enlarge the hole for the strain relief, the torque of the drill threw the amp 6 feet across the room! After cussing for about 5 minutes, I picked up the amp, checked all the solder joints, and finished installing the 3 prong cord. I put the tubes back in, plugged the amp into the variac, and brought the voltage up slowly. I got nothing; no sound, no hum, no buzz and no smoke. The tubes didn't even light up. What could have been damaged by the little mishap?

[Jazz P Bass]

Little mishap!
6 feet across the room!
It is amazing how much torque you can get fron a drill.
Anyway.
Did you test the amp, after all of those mods, but before the torque demonstration?
If not, then you should check over your work.
After that, treat it as a new repair.
Remove all the tubes.
Go across the power cord L! & L2 & check for transformer continuity when the switch is turned on.
That will check a lot right there. The switch, the cord, the fuse, the primary connections.
Next, go across the B+ to chassis ground.
Always looking for shorts or low resistance.
Check the output transformer windings.
At this point, with the power supply drained, I would poke around looking for ground connections that must be.
If all is well, it " the power supply should come up.
Check all the main power points.
B+
The heaters.
If all is good, start putting in tubes.
Start with the preamp. Verify plate, cathode & heater voltages.
Send a signal to the output tube grid.
If everything looks good, load up the output.
Attach the speaker or dummy load.
Check the idle conditions of the output.
Send it a signal.

[bluzmn]

Thanks for your quick reply. Are L1 & L2 the black and white wires of the 3 prong chord? I scorched the treble pot/on-off switch checking voltages before my original post, so I'm thinking of Muntzing down the circuit to a 5f1 and just putting an on/off toggle there. Before immolating that pot, I got a reading of around 0.3 volts on pin 1 and pin 6 of the 12ax7, and 0 on pin 3 of the 6v6. What does that mean?

[Jazz P Bass]

Thanks for your quick reply. Are L1 & L2 the black and white wires of the 3 prong chord? I scorched the treble pot/on-off switch checking voltages before my original post, so I'm thinking of Muntzing down the circuit to a 5f1 and just putting an on/off toggle there. Before immolating that pot, I got a reading of around 0.3 volts on pin 1 and pin 6 of the 12ax7, and 0 on pin 3 of the 6v6. What does that mean?

Yes, L1 & L2 are the mains conductors. Black/ White.
No voltage on 12AX7 pin 1 & 6 points to a failed dropping resistor.
The plates of the output tubes run at full B+ voltage (minus the output transformer winding drop).
The preamp tubes will have a dropping resistor (2K?) coming off of the B+.

[ThermionicScott]

Did you have the chassis in a vice or clamped to your bench? It's idiotic to drill holes in metal if the piece isn't secured. You could have injured yourself.

- Scott