jcm 800 2204 120 Hz hum

[LamontGrady]

I bought a local 2204 50 watt head from original owner. Undid some minor common mods. It had a 12AT7 in V1.

Mine is very very close to this schematic.
http://mhuss.com/MyJCM/JCM800_2204.gif

But this amp is missing the 10k 2w resistor on B+ between V3 and V2. This is a horizontal input factory pcb. It turns out they stopped using three resistors on B+ (some had four: two 10k in series right after the choke). Everyone seems to have this resistor. Mine literally has a blue circle dot sticker on the pcb where one would go, but no holes no nothing just a long unbroken trace between R29 and R8. This is normal and not the issue. But my preamp voltages are then obviously high, like 285v on pin 6 the first half of V1 where the "high" input goes first. The schematic shows 245v (just to the right of C3).

Started swapping tubes, including power tubes with a Weber Bias Rite. Only a couple (lower gain?) 12AX7 in V1 are tolerable. 8 out of 10 12AX7 I tried (all tubes that work fine in other amps' V1) have a 120 Hz hum. Two sets of power tubes no change, biased anywhere in the general range.

The preamp knob doesn't change the hum, the master knob directly controls the hum. The tonestack affects the hum especially the bass knob.

If V1 is pulled, the amp is silent. So it should have nothing to do with bias supply. It is 120 Hz so it is most likely B+ ripple??? Help.

Putting amp on standby the hum fades out, off standby it surges louder then quiets down a bit over 2-3 seconds.

If guitar is plugged into "low" sensitivity input, the hum does not change. When plugged into "high" it gets louder when the guitar cord (or chopstick or whatever) breaks the ground switch on the "high" input jack. In other words if you start to plug a cord into the "high" jack the hum gets louder when the cord is halfway plugged in. When it is fully in there is no change from that state. Obviously the additional noise of the guitar and cable now can be heard on high gain. The guitar/cable is NOT the issue. Guitar pot on 0 or 10 makes no difference, humbuckers used to test so far, known good guitar cable.

Poked and prodded for a WHILE with chopstick, the only thing that seemed to have any effect was around C4 and C5, the chopstick seemed to induce a completely different sort of hum when near these components. Same sort of "new different hum" can be induced by moving around the wire going from R4 (plate resistor) to pin 6 on V1. Evidently I can hear what lead dress issues in this area sound like, it's not 100% silent in this regard but the hum at issue is a completely different (120 Hz) sound.

Test continuity across input jacks, cleaned, reflowed.

Installed new F&T filter caps and new Sprague bias supply caps. No change. I can't believe this didn't fix it! There are no other electrolytics in this amp. There are no relays or anything else using power supply but B+ and heaters.

Replaced R5 (470k), R6 (10k), C4 (470p), and C5 (1n). Zero change.

Replaced one of the power supply diodes that was leaking some AC, no change.

Swapped back in "strongest" of the perfectly testing old LCR caps for the cap closest to power switch, no change. I'm still tempted to swap an old cap in for the other two new caps?

Loosened and reconnected grounds, if green wire is disconnected from iec power cord it still hums. Does not seem to be an issue w/ the house juice itself, this amp is one of many and it's the only one with any kind of 120 Hz hum issue.

Swapped choke, no change.

Added "missing" 10k resistor to B+ between V2 and V3, no change. 387v above V3 and V2, 374v above V1, 285v on pin 6 of V1 (this is the FIRST half, "high" sensitivity input goes into V1b).

Disconnected first half of V1 cathode resistor bypass cap C1, no change.

Added 100uf cap in parallel with each of the two 50uf between V1 and V2, one at a time, no change.

Any ideas? Thanks!

Here's a token picture. Everything was very clean and pristine.
slightly bigger

[big_teee]

Your probably fighting a ground noise problem.
I built a 2204 Cascade amp a couple of years ago.
The Cascade 2204 JCM800 is notorious for this.
Try the Preamp grounding in this Ceriatone layout.
Make sure there is no preamp ground loops and no double grounds.
You only want the ground bus grounded on one end.
http://ceriatone.com/images/layoutPi...4Ceriatone.jpg
Instead of grounding the preamp ground bus to star ground, I had better luck grounding the other end to the Chassis mounted Preamp Can Cap ground.
Mine is under the board.
T

[kka]

You need to go in with a scope and measure where exactly the hum occurs and what the amplitude is. Go through DC supply and also go through the AC circuits. Make sure that all coupling caps don't leak any DC signal to the other side. I had a few amps lately with issues and could locate the culprit parts with my scope easily.

[Bruce / Mission Amps]

Sorry if you said so but how do you know it is 120Hz hum?
Frequently, it is 60Hz hum injected into the cathodes of the preamp tubes.

[LamontGrady]

Thanks for the replies. I want to clarify that this is the most challenging troubleshooting I've done so far, I've learned quite a bit from this.

I actually borrowed an oscilloscope thinking this might be the project that got me to finally take the plunge to learn how to use one, but it seems like a pretty steep learning curve at this point. I can't find any documentation on it, it's an Iwatsu 57100 or 5710 with the multimeter/counter stuck on top. It says 400v max on two of the inputs and 250v on two of the other inputs, 60MHz. The ss-0011 probes say 10:1 and 600v max so I guess this should work? I don't even know if it works it probably hasn't been turned on in 10+ years. I found a pic of one similar without the extra thing on top.

Here's a pic of the hum I quickly grabbed with some random rta plugin in Reaper. Is it possible this is misleading?

[drewl]

You have a neon or flourescent lamp hanging over the amp?

If the noise is still present when plugged into the low jack, and gone with V1 removed that v1b stage is the culprit.
Are the heater wires inducing the hum or close to something picking up the hum?
Are you sure your not losing the input jack ground somehow?
Is there dc on v1b's input grid?
What happens if you ground that input grid with a cap?

[Bruce / Mission Amps]

Right.. ground the grids (lugs 2 and 7) of the first preamp tube.... won't hurt anything and it will shut audio part of the tube off.
If it is noise picked up by the input jack, that will kill it.

Another trick I use is:
Let the amp get good and warm, then shut the amp off, unplug it from the wall and unsolder one of the 6.3v filament leads from the power transformer... now, use an double alligator clip lead wire to shunt across the open wire, plug it back in and fire it back up. Let it warm up again.
Turn it up pretty loud and listen for the hum... then reach over (one hand behind your back, or in your pocket please) and disconnect the jumper lead from the filament source.
The hot tubes will still conduct 120Hz noise from the B+ rail or noise coming in the input jacks for a few seconds but 60Hz filament hum will stop instantly!
Then you'll know where to start looking.
I think it will be 60Hz filament noise or a bad input jack ground.
But who knows? You are there and will at least have a better idea of what's up.

[Enzo]

Esspecially since most hum is not sine waves, but a distorted waveform, all the overtones can make it difficult to determine if you have 60Hz or 120Hz. At this point in my career, I feel pretty confident I know them by ear, but I always check anyway.

With a scope, I touch the tip of the scope probe with my finger, and set the sweep to get one or two complete cycles on the screen. That is 60Hz. Now I clip the scope probe to the output of the amp and see if I get the same number of cycles on the scope or twice as many. Twice as many would be 120Hz. DOn't move the scope sweep setting.

[LamontGrady]

I disconnected the heaters while it was humming. It made a loud noise that obscured the hum so it was hard to tell what was going on. It looks like a bunch of harmonics of 60 Hz? I think the hum is fading out under this noise? Then it fades down to just a 60 Hz noise for a while.

58-121201_1314.mp3

[LamontGrady]

I should mention last night I swapped out the first two coupling caps with no change.

On V1 I grounded pins 2 then 7 - no change. Then pin 2 on V2 - viola! Hum went silent.

[LamontGrady]

That got me off the ground with the oscilloscope! Ha ha!! It's been a goal of mine for many decades to actually use one of those things. Very cool. I have to ask, why is my finger zinging along at 60 Hz?? I was able to clearly see the relative frequency of the hum on the output. Roughly double. There is some adjusting necessary in terms of the wave forms being too tall/short in my experimenting. There is nothing called "sweep" but I'm pretty sure I know what you mean.

So since shorting the grid pin 2 of V2a killed the hum, I figured I'd start there. I had to "zoom" WAY in (what do you call that?).

I probed various points in the vicinity and it seems the same on the plate of V1a (the second half of V1). On the schematic, V1a and V1b are reversed relative to the amp but I will just keep referring to the schematic. Then if I go anywhere back before V1a it seems to still be there but I'd have to zoom in more but it's at the limit. Whatever I'm seeing is much less amplitude and the whole thing is too fuzzy to see what's going on.

Either the second half of V1a is amplifying the hum or could that mean it's getting introduced there?

[Gtr_tech]

Redoing the gnd scheme to a star gnd at the first filter cap gnd point is a good way to quiet a hummy 800 down. The gnd scheme sucked and is a holdover from the old superlead days.

[Enzo]

"Sweep" means the same thing as horizontal frequency or horizontal time/div.

Ever touch the tip of a guitar cord and hear a loud hum come out the speaker? That hum is 60Hz, and it comes from your environment. Your house has tons of 60Hz AC running through the wiring in the walls, the lights over your head run on 60Hz, it is everywhere, and your body acts as an antenna picking it up. That is what is on your finger.

If you powered your scope from a battery and took it out in the middle of a field with no power lines nearby, you wouldn't get that 60Hz on your finger. You might pick up a local AM radio station. Seriously.

[drewl]

I think you'll need to redo the ground scheme like Gtr Tech said.
I've built several high gain 2203/2204 type amps and the old 1987/1959 style of grounding isn't sufficient for the higher gain of the cascaded stages.

Recently had a Weber Fender build where the only way to quiet down that hum was to star ground to the first filter node.

[big_teee]

I think you'll need to redo the ground scheme like Gtr Tech said.
I've built several high gain 2203/2204 type amps and the old 1987/1959 style of grounding isn't sufficient for the higher gain of the cascaded stages.

Recently had a Weber Fender build where the only way to quiet down that hum was to star ground to the first filter node.

I provided the layout in post #2.
Also if you look at his picture, I would redo the heater wiring and tight twist the heater wiring.
I had a hum I couldn't get rid of, until I redid the ground layout, and rerouted the heater wiring over V1 like in the Valve wizard website.
I couldn't get the black wire going to pin 9 of V1 routed under the other wires to work on the Cascade arrangement.
The Valve Wizard
I only wired V1 like that.
See the picture, near the bottom.
T