Marshall JMP 2203 Pair of valves redplatting when cranked.

[henter2017]

Hi Folks,

I have a JMP 2203 which sounded awful and discovered that 1 pair of the el34s (which are matched and drawing the same current when idle 33mA at 475V plate voltage) is redplating when the amp is cranked.
I had to put a resistor in paralel with the original bias resistor as it did not let me crank the bias to 33mV originally, just to about 10mV.
Voltages looks ok and cant see any burnt resistors around.
Any tips?

Thanks

[Chuck H]

I had to put a resistor in paralel with the original bias resistor as it did not let me crank the bias to 33mV originally, just to about 10mV.

Do you mean mA? So you reduced the bias voltage with the parallel resistor. Are you measuring mV across a 1R resistor in the cathode circuit? And, if so, do you have individual 1R resistors on either side of the push/pull?

Are these two tubes both on one side of the push/pull circuit? Does the amp have all the same tubes in it?

If the red plating is only happening on the two left or the two right tubes you should remove the tubes and test for bias voltage at the sockets. If you do have proper bias voltage on all the sockets you can swap the left and right tubes to see if the problem follows the tubes or remains with the sockets.

Answering these questions and doing the testing will go a long way in finding the problem.

[Leo_Gnardo]

1 pair of the el34s (which are matched and drawing the same current when idle 33mA at 475V plate voltage) is redplating when the amp is cranked.
I had to put a resistor in paralel with the original bias resistor as it did not let me crank the bias to 33mV originally, just to about 10mV.

What Chuck said ^^^ and what are the bias voltages at present, pin 5 on each output tube? You may find the deficient tube is drawing grid current & forcing its bias voltage down compared to the tubes on the non redplating side.

It's possible one output tube is failing when you're calling on it to deliver power. Since they parallel output tubes, the failing one is causing its mate to also fail. Try removing one of the suspect tubes, fire up the amp & if the remaining one redplates, that's the baddie. If no redplate, the other one's headed for the bin. After having been redplated some, it's possible both are shot & need replacement.

[henter2017]

Thanks!

Unloaded on Pin 5 I got -39.55V on each socket.
490V on the plates and pin 4, 488V on pin 6.

Loaded, I got around -39.0V on each socket.
I have tried a completely different set of valves, and the same pair redplates when driving the amp, fine on idle.

[kanks]

Check the resistors and capacitors in the phase inverter circuit if you have checked the tubes why swapping sides of the push pull output.

[g1]

Do all 4 power tubes have screen resistors?
Is the screen voltage the same for all power tubes?

[henter2017]

Hi thanks, yes screen voltages after the resitors on PIN 6 are on all tubes: about 488V.
I cant see anything wrong on the PI, but will double check this and get back...

[Enzo]

Pin 6 is not the screen, pin 4 is the screen. Recheck. having B+ on pin 6 doesn;t mean the resistor to pin 4 is intact.

[Chuck H]

screen voltages after the resitors on PIN 6 are on all tubes: about 488V.

I interpret this to mean he DID measure the screens since the voltage on pin 6 is 490. Though he didn't specifically identify pin 4 on every socket.

I've read about this problem with some Marshalls when nothing is functionally wrong from the original design. It supposedly happens because of shift in the duty cycle at the PI and power tube grid loading during heavy clipping double up to cause the over dissipation. Unfortunately the 2203 already has the cursory circuit for mitigating this. That is, 150k bias feed resistors, 1k screen grids and .022u coupling caps. I've had some trouble with modern el34's at high-ish plate volts. The Ruby tested Shuguang tubes were the best performers, although both pairs I bought at different times through different vendors were somewhat microphonic.

I used a zener and resistor in series across the PI grids on an amp with a really wonky duty cycle once to clip the peaks of the PI drive and keep it from being driven asymmetric. The components need to be valued such that the clipping occurs after the PI is clipping to avoid any audible diode clipping. This WILL change the tone of the amp some because an asymmetric clipped wave form sounds different from a more symmetric clipped wave form. But since you can't crank it now I think it's worth a try.

[Pedro Vecino]

JMP 2203 are susceptible to oscillations. They even include several formulas to mitigate them although they are not always 100% functional. If you put the gain at 10 in the high input and screams or it sounds like torn cardboard (very common) the oscillation is there.

[henter2017]

Got it, I will try and check for oscilations then, Screens on all (loaded with the el34s) is about 470V on all sockets...
The sound of the amp when cranked is like having 2 distortions going on, way too much overdrive and breaks up with no definition whatsover, it must have anything to do with the pair or redplatting valves for sure...

[Chuck H]

If you could also check input grid voltage on all sockets under overdrive conditions that could help (pin 5). I have a hunch the red plate side will be different from the other side. That would implicate an uneven duty cycle. Whether this would be caused by oscillation or not is hard to say yet.

[nickb]

You can test the uneven duty cycle theory like this. First disconnect the negative feedback then transpose the connections from PI coupling caps to the EL34's. Then see if the problem switches sides.

Adding a picture as I was really struggling on how to word this clearly :

[g1]

Are these two tubes both on one side of the push/pull circuit? Does the amp have all the same tubes in it?

If the red plating is only happening on the two left or the two right tubes you should remove the tubes and test for bias voltage at the sockets. If you do have proper bias voltage on all the sockets you can swap the left and right tubes to see if the problem follows the tubes or remains with the sockets.

This was a very important question which was never answered. Most of the responses so far have been on the assumption that the problem is either on the push or pull side. If the problem is with the inner or outer pair, that changes everything.

[TimmyP1955]

Check the coupling caps between the PI and the outputs.