Trouble with C4S voltage measurements in Paramour 1

[Pfenning]

So I've had the C4S and iron upgrades for a long time, and some other upgrades for Seduction too. They have just been sitting waiting to be installed. I finally decided it was time, and tackled the C4S. I built up both boards, one right after the other, not at the same time. I installed one C4S board into the first amp, and checked the voltage like the manual says. Here are the measurements I got.

15 - .58 vdc - should be 390
left terminal of new terminal strip - .58 vdc - should be 295
B6 - .64 vdc - should be 155
B8 - .66 vdc - should be 2

I grounded out on pin 8, where the ac ground meets the chassis ground.

So I figured I had a bad solder connection, so I double checked everything, and reread the instructions looking for the problem. Nothing found. So I installed the second one in the second amp, and I get the exact same results. So I figured it wasn't a bad solder joint, since both amps were measuring the same. I rechecked my work again, and can't see any issues. The LED's light up, and all the board connections look good. Any ideas where to start? The only common item between the 2 amps was the power cord and the DVM. I check the incoming power where it attaches to the terminal strip, and it was good (114 ish)

Pfenning

[Paul Joppa]

It must be a measurement problem, either a bad meter (battery?) or a bad connection. If the LEDs glow, then there cannot be less than 3 volts at the measuring points.

T15 is the positive high voltage supply. It should connect to the plate choke (black wire), a 47uF/450v capacitor, the 270 ohm 5 watt resistor, and the new 15K resistor which feeds the left terminal of the new strip. If there is little or no voltage there, then it is likely the 270 om resistor is not connected. Soldering the new 15K resistor may have disturbed the connections - that's all I can think of off hand. You can check at B2 for plate voltage on the 2A3, and at terminal 5 (other end of the 270 ohm resistor) for the presence of any high voltage at all.

[Pfenning]

Paul, thanks for the help. I followed your advice, and couldn't find high voltage anywhere except the incoming power. Then I realized the ground plane for the ac coming in and the ground for the signal were probably different. I moved my ground wire from terminal 8 to the ground side of the input jack, and all the voltage measurements look great except B6. It's suppose to be 155 vdc, and I'm getting 95 vdc on one amp, and 204 on the other. Could this be tube related, and what does it tell me when the voltage is off that far. B8 was all but dead on 2 vdc.

Pfenning   

[Paul Joppa]

If B8 is dead on, that means the current source is providing the correct current. If the leftmost terminal is reasonably close, and specifically greater than B6 by more than 10 volts, then the tube and the C4S are correctly wired and running. So the errors at B6 are due to the tube. You can swap the 12AT7s and check that the error follows the tube.

The error is not fatal, and you will probably get sound out of the amps. But a better balanced set of tubes is in order to get the most out of them. You can expect the B6 value to be more variable than most voltages in the amp. We had some variability in LEDs back then, but if you are getting 2.0 volts at B8 then you have LEDs that are close to the correct spec.

[Pfenning]

Alright!! I'm going to put them back into my system tomorrow night and test them out. Before that, I'll swap the tubes and also try another set and measure the B6 again. Thanks so much for the help. Up next is the iron upgrade.

Pfenning