MarshallJCM800 2210 hum

[Chuck H]

You'll see a lot of examples where a divider is tacked onto the end of the HV rail. I don't see the sense in that. It just puts a small circuit in proximity between the heater and the first gain stage. It may work most of the time, but it's not ideal. Create your divider after the first filter node. Be sure to include a bypass cap on the dropper.

From the first filter node, a 220k resistor in series with a 15k to 47k resistor (this is the dropper, bypassed by a 100V/22uf cap) to the main filter ground works fine. The junction of the two resistors is your elevated node. The size of the dropping resistor depends on how much elevation you want (opinions differ on what is best and why) and your starting rail voltage.

[Enzo]

I hate adding to the B+. Just me. If I needed a small positive supply and had no power tube cathode voltage to steal, I'd add another resistor and diode to the bias winding and make a small positive supply out of that. FIltered with a cap. Divide it down if you like, just like the bias does.

The point of elevated cathodes is to avoid having the cathode being more positive than the heater and thus act like a plate to it. The voltage isn't critical. All you need is positive enough that the lowest negative peak of the heater is still more positive than th cathode. So look at all the cathodes of tubes with elevated heaters and find the most positive one. We need to elevate the 6vAC so its negative peak, about -4.5v is more positive than that. SO if all the relevant cathodes are under +5, then +10 on the heaters has it covered, or +15 or +20. I made up the example.

[g1]

I take it that you have ruled out the socket itself as being the problem? That is unfortunate as it may also account for the loss of heater voltage.

[jvm]

Sorry for the slow reply fellas. Preparing for my daughters wedding in a few weeks. I built a power supply off the B+ as suggested by Chuck H. About 22 volts. Hooked filament center tap to it and fired up the amp. It is as quiet as any of my own amps. I want to express my gratitude to EVERYONE who chimed in with suggestions. I learned more in the course of this repair than any other I have ever done. I now have expanded troubleshooting skills and a much better understanding of hum and its causes/remedies in a tube amp. Thanks to all the members here for being so gracious and willing to share their knowledge and experience. I very much appreciate you all. I hope to be able to contribute and help someone else in the future. Thanks again!

[gtrplayr1976]

These people are the best !

[Chuck H]

Yes we are. Everyone give yourself a hand job. I mean, HAND. Hand... Not hand job. Yeah.

[gtrplayr1976]

Yes we are. Everyone give yourself a hand job. I mean, HAND. Hand... Not hand job. Yeah.

I'm too old for that these days Chuck. I'd sooner have a bowl of ice cream

[Joe L]

Wouldn't that be kinda cold? Shrinkage ya know

[gtrplayr1976]

I was plannin' on eatin' mine not sittin' in it .

[J M Fahey]

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.

Well, not so much as a short, but capacitive or inductive coupling or the slightly positive cathode working as a "mini" plate.
Any of 3 mechanisms can couple some hum which in a sensitive stage gets amplified along with the guitar signal.

[hotguit]

I KNPW THIS IS A VERY OLD THREAD
I have the same problem of hum with my 1986 JCM 2210.
The hum start when i engage the reverb and get up turnig up reverb pot
Any idea?
thank you
hot

[52 Bill]

The hum start when i engage the reverb and get up turnig up reverb pot
Any idea?
thank you
hot

It sounds like the problem is with the reverb tank or wiring. Please start a new thread and we can help you figure it out.

[gtrplayr1976]

I KNPW THIS IS A VERY OLD THREAD
I have the same problem of hum with my 1986 JCM 2210.
The hum start when i engage the reverb and get up turnig up reverb pot
Any idea?
thank you
hot

You may want to start a new thread. Your problem sounds like reverb circuit issue.