Need help with Marshal 2203l PI pin voltage specs

[Normybaby]

Hi, I have a 1980 2203 chassis 100 watt Marshall head that I did a cap job, (6) F & T cap cans. I gave it a quick test and it didn't sound right.....farty, low volume etc....I sent the tube pin voltages to a Marshall friend of mine and he said that the PI voltage on pin 3 and 8 were too high.....150 Volts. I went back and re-soldered a couple of suspect joints on the caps and now it seems to sounds pretty good.....but the voltage on the PI Cathode is still the same 150V...I can't find any info as to the pin voltages specs for a 2203 Marshall........Please help
Thanks

[J M Fahey]

Please post the schematic you used so we all talk about the same.

[Normybaby]

I checked the components i.e. 1m, 470, 10K, 82K and 100K resisters good; changed the tube and still the same reading....150V. The pin readings on V3 are :
1 - 280V
2 - 100V
3 - 150V
6 - 172V
7 - 107V
8 - 150V

[g1]

Pin 1 voltage seems way high. Should be around the same as pin 6 and from what I recall both should be somewhere around 180V. The side with the 280V does not seem to be conducting properly.
Maybe there is an open circuit somewhere between the cathode and ground? That would account for the high cathode voltage.
Don't trust those voltages on the schematic, there are missing digits and 1's that are covered by the boxes around the voltages.

[oc disorder]

I've attached a chart which applied to the original (pre master vol) Marshalls
which should give you some idea (V3) of what to expect.
What are the voltages of the plates of the previous stages ?
Did you check the 4k7 (4700 ohm ) that goes from the 10k to ground in parallel
with the presence circuit ?

[acorkos]

you might have a tube that is bad. pull the v3 tube and remeasure your voltages

[J M Fahey]

You have 2 unreliable voltages there.
1) your grid voltages look *very* odd (no way you have 50V grid-to-cathode), *but* grid circuits are very high impedance and your meter loads them heavily, rendering the measurement useless, that's why on Marshall charts grid voltages simply are not stated.
2) your cathode voltages are too high and the first triode plate voltage is too high too, it looks like the cathode to ground string is open somewhere , and the voltage you measure on the cathodes is what is dropped across the high input impedance of your voltmeter .
Just for testing solder an extra, external 10K resistor from the 470/1M/1M/10K node to ground.
Do you still have 150V across it? Doubt so.
Now to another anomaly.
Besides a bad tube, you might just have a bad contact on the first triode's socket.

[jmaf]

.....but the voltage on the PI Cathode is still the same 150V...I can't find any info as to the pin voltages specs for a 2203 Marshall........Please help
Thanks

That 150V reading isn't really there. Measure voltage from pins 1 or 6 to 3 or 8, that will give you the drop across the tube, subtract from +b for the real cathode voltage. You can't measure from cathode to ground using 10MEgohm multimeters.

[jmaf]

You have 2 unreliable voltages there.
1) your grid voltages look *very* odd (no way you have 50V grid-to-cathode), *but* grid circuits are very high impedance and your meter loads them heavily, rendering the measurement useless, that's why on Marshall charts grid voltages simply are not stated.

grid to cathode is ok to measure using a dvm - gives a 10% error max.

cathode to ground on that PI is the trick. he doesn't have anywhere near 150v on the cathode.

Y como estás, todavia in Buenos Aires? No retorna a São Paulo ?!

[Normybaby]

Sorry for not responding right away....I found the problem. The 4.7K resistor which is supposed to be parallel across the presents pot was missing/removed....probably during a previous MOD....I installed the 4.7K resister and the cathode voltage went down to around 35 volts. I don't know how the amp worked for so long without a ground reference or anyone complaining about the tone...actually, the owner thought it sounded great....go figure
Thanks to all for your responses
Norm