MarshallJCM800 2210 hum

[jvm]

Resistance between first two nodes-94 ohms( Choke)
between 2nd and 3rd nodes-9.95k (10k resistor)
between 3rd and 4th nodes-9.86k (10k resistor)

[Chuck H]

Grrrr !

[g1]

Just for the heck of it (and sorry if it's been done and I missed it), could you plug into FX return and short the tip to the sleeve? And same with V1 through V3 removed? Any difference with V1 to V3 in or out?

[J M Fahey]

In a similar vein, set up the amp as normal, with all tubes in, all pots on "5" except volume on "0" , listen to the hum level and then short across R34 tack soldering a small piece of wire.
Any change?
If not, lift the left ("Y") point of C25 and short it to the lower end of C30 .
Any change?

[Tom Phillips]

... lift the left ("Y") point of C25 and short it to the lower end of C30...

Please point me to C30. I might be looking right at it but I can't find it on the schematic.
Thanks,
Tom

[g1]

C30 is right above C31, lower right of phase splitter. (looks like it says C38 but it's C30).

[J M Fahey]

In computerese it's common to cross "0" (zero)with a diagonal line to differentiate it from "O" (capital o )
That schematic crosses most but not all, but I think some "kind hand" tried to make some values more readable and lightly Photoshopped it.
Problem is that they made a half hearted work, so it still creates confusion.
Oh well.

[jvm]

Ok. Shorted across R34-no change. Lifted C25, connected to C30, hum greatly reduced. Also, a random (white?) noise came up before I even touched anything. Fairly loud and then slowly faded to nothing all in about 20 seconds. Never heard THAT before! I glanced at the output tubes when it happened and there was no red plating. The noise sounded a lot like a Mesa I serviced recently that was redplating. I think we have angered the poltergeist!

[J M Fahey]

Ok. Shorted across R34-no change. Lifted C25, connected to C30, hum greatly reduced. Also, a random (white?) noise came up before I even touched anything. Fairly loud and then slowly faded to nothing all in about 20 seconds. Never heard THAT before! I glanced at the output tubes when it happened and there was no red plating. The noise sounded a lot like a Mesa I serviced recently that was redplating. I think we have angered the poltergeist!

OK, we are decreasing the search area size bit by bit to find the culprit.

Shorted across R34-no change. tells me that hum is not coming through the effects loop, or from V4B or the general Reverb area.
That's useful, removes some strong suspects from the list, including the long wire from the return jack to V4A grid.
Now continue testing with all controls on 0, since we are already past them, reconnect C25 as original and un-short R34.

Lifted C25, connected to C30, hum greatly reduced. tells me the hum can come from:

a) jumping from filament to cathode at V4A .
Test it by parallelling , say, 100uF across R25. Any change?
I mean, gain will increase in general, so I expect some extra hiss, but hum should reduce .... if it comes from there.
Pull that cathode cap after the test.

b) It may come from poorly filtered +V at V4A ; momentarily parallel a good cap with C47. Any change?

c) it may be getting into the "Y-Y" wire, sending signal from preamp to power amp.
Move it around with a chopstick and see whether hum changes.

4) some 12AX7 are hummier than others, try many different brands in the V4 socket.

5) well, no more ideas, hope some of them works .
In any case, good luck.

[jvm]

100uF across R25 greatly reduced the hum. Good 50uF cap across C47, no change. Moved all wiring around, no change. I had previously subbed an entire known good set of tubes but did so again with pre amp tubes with no change.

[Jazz P Bass]

Has the amplitude of the 'hum' ever been measured?
As in, set your meter to read Volts AC and measure across the speaker terminals.
It would be really great to have a 'number' other than 'greatly reduced'.

[J M Fahey]

100uF across R25 greatly reduced the hum. Good 50uF cap across C47, no change. Moved all wiring around, no change. I had previously subbed an entire known good set of tubes but did so again with pre amp tubes with no change.

OK, then that's it, you are getting hum from the filament.
Lowered even though you have increased the stage gain so it's doubly confirmed.

Options:

1) there are less hummy tubes.
There's talk about cheap chinese 12AX7 being better (in that aspect) than other more expensive options. Try it.
Or since there are spiral wound filament tubes, usually reserved for the V1A position, try one there.

2) do the Heroic thing and feed DC to those filaments.

3) try elevating those filaments.
Fellow Members may suggest an approppriate voltage.

4) probably somebody can suggest another idea, now that we are reasonably certain you found it.

Of course, good luck

[g1]

Just wondering if it's worth trying a new socket in that position, after all, the customer claimed the fault started sometime after the choke replacement, so it should not be a design problem, rather some component or lead dress issue.
Doesn't necessarily sound like a socket problem, but stranger things have happened.

[Joe L]

OK, then that's it, you are getting hum from the filament.
Lowered even though you have increased the stage gain so it's doubly confirmed.

Wow, that was a good suggestion. So the 100uF cap shunted the 60Hz present at the cathode to ground instead of modulating the bias of the stage. And of course, you removed negative feedback developed across the cathode resistor so the overall gain increased.

Is that correct? If so, and if the tube is good, then there is a short between pin 3 and 4 on that socket.

I'm impressed.

[jvm]

Can anyone suggest a proven schematic for elevating the heaters with the center tap? As in, where and how to derive the supply voltage. New territory for me and I want to be sure about the circuit I use.