Marshall JCM800 power/ biasing

[MarkusBass]

I'm looking for some help with JCM800 (version 4210 - 2xEL34). I measured its power and it's only 20W. After this it starts distorting the signal. I'm going to check the bias (I'm looking for 1 Ohm resistors). But the manual says that it is 50 W RMS amp. Is it correct? I checked the valves (with a valve metter) and they seem to be OK.

Mark

[g1]

I would expect 45 to 50 watts output. What are your plate and screen voltages at the power tubes? Are the screen resistors ok?

[Chuck H]

Also, have you tried injecting a signal at the PI ??? There could be a problem with the preamp output. What your hearing may actually be preamp distortion and the power tubes aren't being pushed to their potential before this happens.

[MarkusBass]

Thanks for replay. I will measure voltages and currents today. I injected the signal into the Effects Return input. Changing the phase splitter valve (ECC83) did not change anything. The resistors are OK.

Mark

[Amp Kat]

If the signal you are injecting into the return isn't large enough it may not be able to amplify it to it's full potential which is what the preamp does . However the case of at least a -10 to 0 dB signal should yield full power points to the output. I have seen shorted OT's in these that reduce the volume drastically and it shows up with an ohm test on the primaries from CT to each leg. For say a 50 ohm reading if bad would yield 25 on 1 side and 8 on the other. If both sides read about 25 ohms the tranny is likely good and you either have a bad preamp,poweramp tube or bias problem. Inserting 1 ohm resistors from Cathode to ground is good way to calculate the wattage dissipation through each tube at quiescent or idle.

[MarkusBass]

If the signal you are injecting into the return isn't large enough it may not be able to amplify it to it's full potential which is what the preamp does . However the case of at least a -10 to 0 dB signal should yield full power points to the output. I have seen shorted OT's in these that reduce the volume drastically and it shows up with an ohm test on the primaries from CT to each leg. For say a 50 ohm reading if bad would yield 25 on 1 side and 8 on the other. If both sides read about 25 ohms the tranny is likely good and you either have a bad preamp,poweramp tube or bias problem. Inserting 1 ohm resistors from Cathode to ground is good way to calculate the wattage dissipation through each tube at quiescent or idle.

I use a signal generator so it is not a problem with the amplitude of the input signal. As the input I use the Effect Return input. The signal goes through 2 ECC83s but I replaced them - without any changes.
I made some tests and here are the results.
I inserted 0.33 Ohm resistors (I couldn't find 1 Ohm) from cathode of each EL34 to the ground. I found out that the currents are not equal; I've got 7.5 and 10.7mV which means 22.5 and 32.1 mA respectively. So the valves are not matched correctly. I changed the valves and the currents also changed. I assume that this means that the OT is not shorted. I also changed the 0.33 Ohms resistors just to be sure that my measurements are are not influenced by unequal resistor values.
The power supply is 425V so the static power dissipation is 13.6W. I increased it slightly (to 18W) but it did not change anything. The amp's power is about 30W and I wonder that maybe this is correct.
Attached is a picture that shows how the signal looks like when the output voltage is 12V (36W).
Do you think that I should replace the output valves?

Mark

[pdf64]

Yes, it could be weak valves.
Is your power line supply voltage close to the tap rating of the power transformer primary?
Have you confirmed that your load resistor is 4 ohms (not much less), and that the 4 ohm tap on the OT is being used?

[MarkusBass]

Yes, it could be weak valves.
Is your power line supply voltage close to the tap rating of the power transformer primary?

I have 230V.

Have you confirmed that your load resistor is 4 ohms (not much less), and that the 4 ohm tap on the OT is being used?

Yes, I have four 16 Ohm resistors connected in parallel. I selected 4 Ohm output tap with the switch on the back of the amp.
I may order a pair of new EL34 but on the valve meter they seemed to be OK.

Mark