Jcm800 bias supply problem?

[JoeK]

I picked up a 2203 today for a friend (he bought it on the bay) and it seems to be losing bias voltage slowly after 5 minutes. One bias cap looks suspect (has a patch of wrap missing) but I think everything is original (1981 production) Tubes were idling at 35ma (I've got a four port bias meter) then one pair starts to ramp up and up, one pair was doing about 60ma each and the other side was still about 35ma each. Swapped tube pair and fault followed the tubes but possibly because they draw more current? and the bias voltage was dropping from -48vdc to -45vdc. The bias supply to the grids is split by 220k resistors between output tube pairs so the other pair does drop bias voltage but at half the rate.
So is it a bias supply problem?? I took out the tubes and monitored the bias voltage without any load to see if it was a resistor or cap leaking, going high but without a load it has gone from 48 down to 47 and back up to 48-49. Is there an expected current draw for the bias voltage that I could simulate with a resistor or two to test the supply with a load?
Thanks in Advance Joe.

[pdf64]

May be bad tubes, leastways you need to rule them out with a known good set.
1/ remove power tubes and confirm that the bias supply is stable over time (grid current draw is minimal)
2/ Inspect all components / connections between bias supply and tube pin
3/ Fit known good set of EL34 and power up via a light bulb limiter
Pete.

[JoeK]

Thanks I've got a matched pair of new EL34's so I might try swapping the impedance to suit and seeing how they go.

[Enzo]

Rather than risk damaging your new tubes, swap the old ones. (Oh, I see you did)

You had one pair of tubes that was stable, the other pair was running away. SO swap places with the pairs. Now if the same pair still does it, but now in the other sockets, then you know the tubes themselves have an issue. (And probably just one of them dragging them both down) If the other tubes now start doing it in the same sockets as before, then something about the socket is funny.


SO you determined the problem follows the tubes. How can it be a bias supply problem? The same supply feeds the stable tubes too. That supply has no way to selectively affect only some of the tubes.

Take your two runaway tubes and remove ONE of them at a time. That will let you find if one remaining alone is stable, but the other is the bad guy.


Your bias voltage drifts around because the power line voltage in your wall drifts around.

[Bruce / Mission Amps]

Adding to Enzo's good advice....
I don't think I've ever worked an old Marshall amp where it was "not" a real good time to replace the two bias filter caps when I had a chance.
In other words, for the $2-$3 it costs, take the time to replace them anyhow.