MarshallJCM800 2210 hum

[Chuck H]

Same thing for the AC input with no tubes plugged in.

+1

[jvm]

I lifted the filament string from the diodes when I unhooked the IC supply. I have no DC at all on filaments. Now, when checking for AC on HV rail, do I test at the rectifier output referenced to ground?

[jvm]

I installed the 1 ohm resistors in series with the filaments with all tubes removed as Chuck H suggested. I measure 8mV on one and almost 10mV on the other. One question I have is this: Should the choke wires be twisted together? It came to me that way and when I subbed the original choke I also twisted them.

[Amp Kat]

I'm not an expert by any means ,but have you worked your way back from D2 ,and D3 (filament supply) to see where the voltage changes ?

This could be the actual culprit(D2/D3 and I would change them even if they test good with the meter or at least test AC to DC at that point.

[jvm]

I had already changed D2 and D3. I had hoped that they were in fact the culprits but no.

[big_teee]

I installed the 1 ohm resistors in series with the filaments with all tubes removed as Chuck H suggested. I measure 8mV on one and almost 10mV on the other. One question I have is this: Should the choke wires be twisted together? It came to me that way and when I subbed the original choke I also twisted them.

Are you reading this current with D2, and D3 disconnected from the filament string?
If so You do have a problem and need to find where the current is going.
If you are reading this with the D2, and D3 connected, then you need to take d2 and d3 loose and try again.
T

[jvm]

I just inserted my ammeter at the first socket filament connection. With D2 and D3 connected, I have a 4.5 mA draw. Unhook D2 and D3, no draw. Can I retube and with these unhooked and check for hum? I know the IC is channel switching but I don't know if I will lose my preamp signal with it disconnected.

[jvm]

Just turned on with IC supply disconnected. Hum still present.

[big_teee]

Sounds like your chasing a ghost.
I would insert the tubes and measure the heater voltage one more time with d2,d3 disconnected.
If your voltage is in range I would start chasing the hum in the ground circuit.
That is the normal culprit with marshalls and hum.
I guess you have done the chop stick lead moving while the amp is plugged up, trying to change things.
and thump everything with the chop stick.
Trying to change something.
Sometimes, if you can just get things to change, you can isolate and fix it.
Good Luck,
T
** Another thing with hum, try some low output 12AX7s, some 12AX7s won't work in marshalls.
Sovteks 12AX7WAs usually work. Not necessarily the best tone, but quiet.

[Chuck H]

I would insert the tubes and measure the heater voltage one more time with d2,d3 disconnected.

Just turned on with IC supply disconnected. Hum still present.

What big_teee said. Also... I can't know from your wording if you reconnected the diodes and then lifted the supply from the chip. We need to know that the hum was still present with the diodes OUT OF THE CIRCUIT. As in, disconnected from the filament supply. Is this what you did?

[jvm]

Yes, I lifted the filament string connection from the diodes. I have also already replaced both diodes.

[Chuck H]

Sorry. Thank you for being graceful about it. You did mention it twice and I missed it somehow.

Check for AC on the HV rail from the positive filter nodes to ground. Please report on this.

Since you have hum on all tube plates, please do the tube pull test, again. Start by pulling the first preamp tube. If the hum is still present, pull the next preamp tube... etc. Do you need to get to V4 before the hum stops?

[jvm]

No problem, there are so many posts here that I am confused myself! lol I have 3.2 VAC at the first filter node, about 1VAC at the second, and yes, I have to pull V4 before the hum stops. I have also replaced every IC, diode and transistor in the amp just to eliminate them as potential problems.

[Chuck H]

Great! Progress! 3.2VAC on the first filter node means that the filament supply and the B+ rail are connected together somehow. They shouldn't be. It could be a shared ground that isn't grounded (my gut feeling) or anything else. Your mission, "should you choose to accept it", is to find this link, wherever it is. This post will self destruct in ten seconds.

[g1]

Just to make sure we're all on the same page, please clarify what you are calling "first" filter node?