Marshall problem

[Valvehead]

working on a 2204 , im getting 23 Vdc at both grids of the PI tube . . Shouldnt this be 0 V ? According to my Marshall voltage chart , should be 0 .

Its not the tube and im getting about 12v at all 3 tabs of the presence pot .

the .022 caps that feed the grids are new .

[Enzo]

OK, the old 2204, not the JCM800 2204?

What Marshall voltage chart? Looking at the schematic
http://www.schematicheaven.com/marsh..._50w_2204u.pdf
I see it calls out 87v on each plate and 39v on the cathodes of the PI tube.

From that cathode to ground are a 470 ohm, a 10k, and the 4.7k resistors - 15,170 ohms total. With 39v across all that, calculating voltage division to find what is across only the 4.7k, I get 39v x 4700/15,170 = 12.08 volts. Since your presence control is connected to the top of that 4.7k resistor, the 12v you find on that control sounds about dead-on correct to me.

The two grids have their voltage taken from the bottom of the 470 ohm resistor. Using the same procedure, I get the voltage dropped across that 470 ohm as 39v x 470/15,170 = 1.2v. Which means the grids should sit at 1.2v lower than 39, or 37.8. You only get 23v. To me that is low. And no, you should not see zero volts there, by the way.

Why low? Your meter usually is fine for these readings, but in this case, that 1 meg resistor and your meter make a voltage divider. If your meter has a 9 meg input impedance - which normally is way hiogh enough to not bother anything - that and the 1 meg resistor will make the reading 10% low. Not quite 4v. Since your reading is almost 15v low, Your meter seems to have a much lower impedance.

Try this. measure the voltage at the cathode with your meter grounded as usual. 39v? Whatever it is, write it down. Now, instead of measuring the grid with your meter grounded, put your meter probes on the cathode and the grid. In other words measure the voltage between grid and cathode. SUbtract that amount from the 39v or whatever you wrote down, and that result is more like the true grid voltage.

Furthermore, you can also calculate that there is 39v across that total of 15,170 ohms. Ohm's Law tells us that there must be about 2.6ma flowing through them then. Or about 1.3ma per triode - a pretty reasonable amount.

All of which is to say it looks like your circuit is operating normally. Zero volts on those grids with 39v on the cathodes would mean the tube had 39v of bias for the signal to overcome.

[Valvehead]

its a 78 JMP .

that 87V at the plates cant be right.

i just checked several charts for these and they are all 210-220 V at the PI plates.

this amp measures 207 / 220

it does have 38v at the cath

[Enzo]

Where are these charts? I agree 87v doesn;t look right, but the cathode voltage does, as you found.

[Valvehead]

ive got several in the back of the doyle Marshal book .

I also have at least 3 or 4 charts i personally made to keep records of voltages inside Marshall some models ive worked on 2203/2204 / jmp / jcm 800 /900's


all the doyle /marshall charts show " --- " for the grids of the PI

[JerryP]

What's wrong with the amp?
Jerry