HUM Marshall JMP 50w

[lowell]

Friend of mine bought this amp and it had a fair amount of mods. I pulled all the mods out, and rewired the input jacks per normal. The "normal" channel sounds great, but there is a LOT of hum on the bright channel. I think it's 120hz. When I pull V1 the hum remains, when I pull V2 the hum goes away. I tried 2 new ecc83 which didn't help. When I turn down the bright channel volume all the way the hum goes away too. The hum gets louder as this volume is turned up until about 9. Then it lessens from 9-10. I tried a new volume pot, didn't help. I tried disconnecting V2's grid from the circuitry altogether and connected V2's grid to a 1meg resistor to ground, didn't help. Moving filament wires around doesn't help either. Also tried a new filter cap at that node as well. any thoughts? I'm STUMPED. BTW V2 is direct coupled and the second half is a cathode follower that drives the tone stack. Oh the things is blowing the HT 500ma fuse as well, but not immediately. If I put a new fuse in, then troubleshoot the hum for a 5-10 minutes it blows.

http://www.drtube.com/schematics/marshall/78614-2.gif

[Jazz P Bass]

Does the fuse blow with V2 removed?
Edit: Well that is cute, Marshall failed to label the tubes.
So which tube is V2?

[g1]

What exact tube is in the V2 position? There are certain types of 12AX7 that will not work there (spiral filaments).

[pdf64]

I suggest that you monitor the HT current; it's difficult to envisage a failure mode of anything in the pre-amp that could blow a 500mA HT fuse.
What is the static current draw of the output tubes?
Are the power supply electrolytic caps ok (HT and bias)?

Regards the pre-amp hum, consider a new pair of tubes for V1 and V2, not Russian made, eg JJ or Chinese, as use in the cathode follower may have damaged the heater-to-cathode insulation of Russian made types (which then introduces heater supply into the signal path). The cathode voltage here is generally above the limiting value of the insulation rating.
Pete

[big_teee]

Is that the right Schematic for a JMP 50 watt amp.
Maybe this one?
2204
http://www.drtube.com/schematics/marshall/2204u.gif
or 1987
http://www.drtube.com/schematics/marshall/1987pljp.gif

[lowell]

Looks like this might be the correct schematic http://www.drtube.com/schematics/marshall/1987pljp.gif

Thanks big_teee

There is a Tung Sol 12ax7 in there. Those are made in Russia but not sure if they have spiral filament.

V2 is the direct coupled stage.

[lowell]

This can't be a problem with V2 now that I think about it though. As both channels use V2 and there is NO hum on the normal channel. Has to be something between V1 and V2. Again, when I pull V1 the hum remains. But again, I disconnected V2's grid altogether and ran it to a 1meg to ground. STILL hum. boggled.

[52 Bill]

Whenever there is a confusing hum problem like this one, I always think that it's ground related. How are the ground circuits arranged?

[lowell]

all preamp grounds, input jacks, and bias supply are grounded to ground bus along the pots. Here's a sample of the hum:

https://soundcloud.com/pete-galanis/hum

sound clip is as I turned the volume up, left it at 10, then turned it back down.

[chinrest]

Some folks have reported that moving the Presence pot ground off the buss improves this layout. I have one of these amps with the same issue (the buzz is pretty acceptable except at full power on the Treble channel). I recall another identical build with the same issue.

Keep at it. Let's see if we can get this thing 'dead quiet'. -- AKA, "acceptably noisy."

[EFK]

Whenever I hear about noise that gets louder, then goes away when the volume pot is dimed, I think of noisy crosstalk that ultimately becomes all out oscillation. Are there 1 ohm resistors between cathode and ground off the tubes, to measure bias current? If so, you might want to clip a meter on one, and then watch the cathode current as you bring up the volume pot. If the current spikes when the pot is dimed and the noise goes away, you've got an oscillation issue above your hearing range. Just something extra to try, and it could be very channel dependent.