MarshallJCM800 2210 hum

[jvm]

Marshall JCM 800 2210 head with a significant low level hum at all times. I recently retubed this amp and it has been working beautifully until it developed this hum. Bias checks ok at 32-34 mA per tube. I have replaced all electrolytics with no change.(They were long overdue anyway) I can pull V4 and stop the hum but this tube is used for reverb recovery and an additional gain stage feeding the PI. Any suggestion where to go from here? How can I determine which stage is actually causing the hum?

[acorkos]

have you swapped in a good working tube one-at-a-time in all the preamp sockets? new tubes can go bad...swap in a new quad of power tubes, too.

[jvm]

Thanks for replying, acorkos. I had previously subbed a good 12ax7 but I just did it again to be certain. No change. I have not swapped output tubes due to the fact that I can pull V4 and stop the hum. That would rule out the output stage would it not? I have checked the bias and they are all 32-34mA. Pulling V4 stops the hum.

[acorkos]

try plugging a guitar cord into the effects return jack...any change in the hum?
also, try a line out from the effects send jack into a second amp...is the hum audible in the second amp?
the results of these tests can help narrow down the location of the problem

[Enzo]

jvm, acorkos suggested swapping a 12AX7 into every preamp tube socket, but you said you subbed "a" good one. Did you try one in EACH socket, or just the V4? Pulling V4 kills not only noise from V4 stage itself, but also from every stage before it. SO if V1 were hummking, V4 pulled would kill that too.

Do ANY of the controls have ANY effect on the hum? In other words do any controls affect its loudness or its tone? Is the hum unchanged with ALL controls at zero?

acorkos also suggested splitting the amp at the FX jacks. Do his tests. Plug a guitar into the return jack and set the volume control on the guitar to zero. Still hum? If so, the power amp is humming, if not, the preamp is humming.

Is the hum 60Hz or 120Hz? They are the same note, but an octave apart. 60Hz is the hum you get when you touch the tip of a guitar cord. The two come from different sources in an amp. If you are not in the USA, it may be 50Hz and 100Hz. Same story.

Depending on your answers to the earlier questions, don;t forget the low voltage stuff. The channel switching is controlled by IC1, which is powered by rectified heater supply filtered by C9. It needs to be about four and a half volts and CLEAN. Otherwise it could in fact inject hum into the channels.


Look up "signal tracer" online. This is a very simple thing to use a spare amplifier for during tests. It really is nothing more than a probe connected to an amp, and that allows you to probe points in a circuit and listen to what is there. You can probe stage by stage through an amp listening for your noise.

Or use a scope.

[jvm]

Thanks, Enzo. The hum is 60Hz and is on my filament string. Filament voltage is 5.5VAC. Since my hum is present at every tube filament, why can I stop it by pulling V4? Enlighten me please!

[52 Bill]

Since my hum is present at every tube filament, why can I stop it by pulling V4? Enlighten me please!

There will always be hum on the ac filaments because they are ac. You are stopping the hum by pulling V4 because you are cutting off the audio signal from all of the stages from the input up to V4. The hum is in the audio circuit not the filament line.

I'd also try and figure out why the filament voltage is so low.

[Chuck H]

I'd also try and figure out why the filament voltage is so low.

Indeed. The two problems could be related. A systematic method must be employed to narrow down the problem. The first thing you need is a known good, not hummy tube. Take out V1 and stick the good tube in. Any change? If no then take the tube you pulled from V1 and replace V2. Any change? If no then... etc. In this way you'll find out if it's a bad tube. But, also...

You may be able to find the problem without a spare tube of known excellence. Your filament voltages are low. Why? Bad tube? Maybe. Something may be drawing excessive current from the filament winding. Pull tubes one at a time and check filament voltage in between. If pulling a particular tube brings the filament voltages up to spec, replace that tube.

[Enzo]

Or some other winding is loading down the transformer bringing the 6v with it. Or the inrush thermistor is croaking and the PT is not getting the full mains voltage.

[gtrplayr1976]

I've had transformer bolts loosen ,and make a hum /buzz sound, but I could actually feel it if I put my hand on the amp.

[jvm]

Thanks for all the advice guys. Here is where I am now: I subbed a known good 12AX7 in every position one at a time. No change. Pulling V4 ONLY stops the hum. The hum IS affected by the master volume and is lower but still audible with MV at 0. Guitar in eff return, volume at 0, no change. Eff send to another amp- hum present.
Filaments at 6.3 VAC with all tubes removed. Removing any single tube only increases filament voltage by about .1 volt. Filament string is balanced at 2.6v per side with tubes removed. Unhooking IC supply from filament string made no difference in filament voltage. Any suugestions as to what else could be pulling down the filament supply?

[g1]

6.3V unloaded still seems low, that's with all tubes out including power tubes? And with amp in standby? What is the voltage going into the primary of the power transformer (after the power and voltage selector switches, right at the primary)?

[jvm]

Hi, g-one. yes the filaments are at 6.3 with all tubes removed and in standby mode. The PT primary is at 122VAC.

[g1]

Sounds like the filament winding in the PT could have a problem, but you should disconnect the filament wires from the PT and measure without any sockets or wiring connected (to be sure it is not a socket issue or a wire with insulation worn away and touching the chassis or something).

[Chuck H]

+1

If anything. including false CT's or mistakes in wiring, is amiss it would be best to rule that out before pronouncing the PT broken.