Help trouble shooting some voltages.

[pfrischmann]

Hi Guys,
I've had my metro built for a while but I've also messed with it a lot. I think I've tried every circuit, including a 2203 in it...A while ago the sound would fade in and out. I decided to start trouble shooting using Georges voltage check method for new amps.

I found a few unusual readings.

Wall voltage was 118.8 V

Pin6 on V-1 is 149.8V...It should be like 213.
pin1 on v2 is a bit high 183v. It should be 168.....(close enough?)
Heaters (pin 4-5 on v-1, v-2 and v-3 (only pin4) are measuring 2.95 volts....pin 9 is fine and the power tube heaters are fine.

I think the heaters are just a bad solder joint. I'lll re-do them.
pin 6 on V-1 puzzles me. The voltage before the 100K resistor is 272 volts. Do I just need to check that resistor and possibly drop it's value?

The other 100K that is fed at that position on the board goes to pin-1 on v-1 and measures 152volts which is within spec....

Any ideas?

Thanks,
Paul

[pfrischmann]

I lowered the 100K feeding pin 6 on v-1 to 45k. That gets me closer to 205v.
I'll listen to it and see if that sounds good. I wonder if having 16uf on the preamps would affect this?

[flemingmras]

First of all, which amp circuit are you even working on?

Second off...why do you think the plate voltage should be 213? What cathode resistors does each side of V1 have? What's the value of the V2 cathode resistor on pin 3?

Plate voltage is dependent on how hot or cold the stage is biased which is determined via the CATHODE resistor, NOT the plate resistor. Two stages using the same plate resistors but different cathode resistors will have quite a bit of difference in plate voltage because one stage is drawing more current than the other.

However, on an amp with a shared cathode circuit on V1 (where pins 3 and 8 are tied together) you will in fact see the same voltage on pin 1 and 6 since they use the same cathode resistor and as such are biased the same, which would be in the neighborhood of 150 volts. I'd change that plate resistor back to its stock value of 100K otherwise you're dropping the gain of that side of the tube way down.

Only on a split cathode where pin 3 is on a 820R resistor and pin 8 is on a 2.7K resistor will you see Pin 1=150V and Pin 6=210ish. This is because the b side of the tube is biased colder and not drawing as much current so the plate voltage won't drop as much as the other side that is biased hotter and drawing more current. But on shared cathode circuits that use plate resistors that are identical in value you will see the same plate voltage on both pin 1 and 6.