100Hz Hum issue - hmmmm.... please help!

[jimmyflanders]

Hi everyone!
I've built my first ever amp, a JCM800 50w 2204, its awesome and sounds fab, however, I do have a hum issue.
I'd really really appreciate any help and experience anyone can give, its PtP wired, I'll pop some pictures up soon but wanted to get the ball rolling in case anybody had any initial ideas!!

I've managed to narrow it down a little by noticing the following:

its definitely 100hz - checked with an oscilloscope.
The sound is HUM FREE when the master volume is off. As you turn it up, the hum turns up inline with the master volume level setting, and is quite noticeable when playing.
the sound is effected by any tone stack pot adjustments, just gets bassier/treblier etc in line with the eq controls.
makes no difference if I plug in a jack cable or not into the front end, no increase or decrease in hum.
Turning up the preamp volume up or down makes no difference, and hum is the same with the preamp volume off.

Any ideas where to start? I'm not a technical amp person, but can solder and test stuff etc.

Any help GREATLY appreciated!!

Jimmy

[pompeiisneaks]

Pictures will help a ton, but often 100hz hum is one of:

1. bad solder joint on filter caps, so you've got extra noise on the power supply rail
2. lead dress issue. Some wire with B+ is too close to a wire with signal
3. bad grounding issue. Creating a ground loop.

there are many other possibilities, but those come to mind.

~Phil

[pdf64]

Might the 12AX7 in V2 be bad?

[alkuz1961]

Very often, the source of the hum is the heating wiring. To check this problem, it is enough to power the heater through the switch, and open the heater circuit while the amplifier is running.
By the way, I used to think that the heater wiring can only cause interference with a frequency of 50(60)Hz. But several times, I observed interference of 100 Hz and even 150 Hz, which could be reduced by changing the heating power system.

[harleytech]

Check the preamp tubes first... I have built quite a few of those amps and it was always the V1 tube. I have a stash of tubes and I just tried a different one until the hum went away... You can try and isolate it by pulling the V1 and V2 preamp tubes and turn on the amp to find out which one is causing the hum. Plug them in one at a time with standby switch on then off to listen for hum. Put every knob at halfway - volume at 0 , Then slowly turn volume up to check for hum with each tube.. It's most likely V1...

[pompeiisneaks]

Very often, the source of the hum is the heating wiring. To check this problem, it is enough to power the heater through the switch, and open the heater circuit while the amplifier is running.
By the way, I used to think that the heater wiring can only cause interference with a frequency of 50(60)Hz. But several times, I observed interference of 100 Hz and even 150 Hz, which could be reduced by changing the heating power system.

Except OP said the scope showed it as 100hz not 50hz and that's rectified hum...

Edit: except I didn't read your last line... odd I've not encountered that before in my shorter experience at this... why would 50hz somehow increase? couplling in different locations yet perfectly in sync buy just out of phase enough to basically double itself?

~Phil

[jimmyflanders]

Thanks everyone for your replies! I know its a can of worms searching for the culprit (and I'll take some chassis pics tomorrow which should help - hopefully you'll see I've done something stupid and easily rectified like multiple grounds or something!!!)

I have a set of Tung-Sol reissues too (I've got JJs in there at the mo) to test out the tubes in the different positions, as advised, thanks. I'll state on here what I find when I pull the individual tubes!

I'll also check the filter caps for cold solder joints.

In terms of the heater wiring, I've tried my best but you hopefully will be able to see from the pics whether I've done things ok or not. I've think I've done a relatively tidy job, as I say I know some pics will help!

Thanks again

Jimmy

[jimmyflanders]

Pics (hopefully!)

I pulled preamp tube 2 and it was silent (just hear the noise very very faintly), pulled tube 1 and it was silent (just the faint noise), pulled just tube 1 on its own and its still really quite with just 2 onwards.
Tried swapping it out for 3 different tubes, but still the same noise as soon as a tube is put into 1.

Once I opened it up I remembered I put in an active FX loop too, between the tone stack and the MV, could that be the issue? I could always remove that to see if that's a factor if you think it could be in that part of the circuit.

Another mod I did was put the negative feedback on the impedance selector itself along with a cap, as advised on one of the mod sites, but happy just to tap that to the 8 ohm if you think that might be worth a try, but isn't that further along the chain to where the issue could be?

I know the pictures aren't great so if you need me to draw anything diagrams or annotate anything to show where any cables go I can do that.
I tried poking inside with a wooden stick to see if moving any cables around at different angles made any difference to the noise, but nothing changes it.








Thanks again, and appreciate any advice!
Jimmy

[jimmyflanders]

The main pics didn't seem to work...
they're here:




thanks!!!

[pompeiisneaks]

I haven't done a deep dive, but looking here:

PhysicalConn.png

I see what appears to be a copper wire pushed up against a turret and soldered.

This kind of connection is notorious for not being well connected and creating dry soldered joints. You should always be sure that a strong physical connection is made via wrapping any leads around the turret and crimping them tightly against it as best you can.

Also many of the solder joints look either a: to be made of lead free solder or b: dry soldered. If it's the latter, that can be fixed with reflowing those joints that look frosty. If the former it can be really hard to tell if you've got dry joints because lead free solder always looks frosty. You might have to just reflow any ground connections to see if it helps.

~Phil

[pompeiisneaks]

Oh and your question the loop. You've already proven the problem seems around V1. The loop comes after that in the chain and with V1 out there's no noise so the loop can't be causing it, I would think. Locating the hum around V1 seems that if not a tube issue, it's the physical wiring/components that directly connect to V1 in some way. I'd use a wooden chopstick to touch wires, components, etc to see if it changes the noise. That can help isolate bad connections or bad components (Yes even new components are sometimes bad, or go bad due to taking too long to solder them etc)

~Phil

[jimmyflanders]

Thanks Phil - yeah I’m definitely no pro!

It is lead free, should have just used the leaded, it’s so much easier to work with too! Shame it’s not the best of things for your health!

I’m going to redo the connections to v1 and ergo over the whole thing to check for bad joints - hopefully that’ll help as a next step!!

Thanks again, process of elimination hopefully I’ll get there!
At least the amp is working and I got good B+ voltages etc.

Jimmy

[jimmyflanders]

One thing that did just dawn on me, like an idiot, is that the board I got was partially populated and I hadn’t considered they’d probably used lead solder - so if I’ve then used lead free to solder my wire to the turrets then that’s a sure fire way to get some serious dodgy joints
I can’t just reflow it in leaded either as I’ve done pretty much all of my side in lead free. Oh well - as you say, I’ll go over all of it bit at a time to check the joints.

Proper rookie error!

Jimmy

[alkuz1961]

I don't think the hum problem right now is related to the type of solder. Although of course this can create problems in the future.
How is grounding heating winding in this amplifier? I don't see any balancing resistors, but maybe it's the central tap of winding?
Another source of noise can be the choice of where preamp ground connect to the chassis. And as far as I know, this was a frequent problem for the old Marshalls.

[TUBEDUDE]

Just as an aside, lead bearing solder is a health concern in the environment, but not so much when soldering joints. It's a heavy molecule and is not released into the air when soldering. Make sure to wash your hands after handling it however. Now the flux on the other hand is full of halogens and bad stuff. There are solders with safe fluxes but they are very spendy. Use an evacuation pump or fan/filter to avoid inhalation.