jcm 800 2204 120 Hz hum

[DC Bluez]

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
[ATTACH=CONFIG]21063[/ATTACH]

Hey,

I had this problem and is ground related. I dd several things to eliminate the 50/60hz hum.

1. Connect the ground on the preamp as close to the input jacks as possible using a good bolt with start washers and 1/4 inch ring terminals squeezed and soldered. This was the biggest difference in the hum and eliminated it but I took the following steps also.

2. Make sure and even replace the input jacks since the jacks can become resistive and have a poor ground contact.

3. I also had a problem with V1 when removed solved the hum issue. I soldered 2 100ohm resistors to the heater pins and connected with a bolt and ring terminals to chassis.

4. Check the center tap connections coming from the heater coil from the transformer and make sure this is connected very tight. This should go to the main star ground.

Now I can dime my master and preamp an no hum whatsoever.

5. Some crappy preamp tubes can be noisy also, these need to be run and burned in to make sure they are noise free once burned in.

6. I do not like the 22nF caps in this circuit on the preamp. I use on V1A .0047uF and V1B .01uF. Caps also need some burn in time.

7. I got rid of the cascaded input section and changed this by adding another 1 meg gain pot and eliminated the voltage divider 470K/470K. I separated the 2 gain stages and can adjust them both to blend very good tones.

8. I also on V1B used a 270K with a 500PF bypass cap and V1A is 470K with 500pF bypass cap. The 270K is same as the old bassman values and has an extremely good tone. This can also be 330K/500pF. This smoothed out the tone dramatically when using pedals. The distorsion is much more clear and I like the dynamics and is more usable in club scenes.

9. On V1A cathode I use a 4.7K/220uF setup and is very good tone. V1A plate voltage 251V and is a perfect point on tone and not too aggressive at 1-2 O-clock gain knob but can be at 3 O-clock etc.

10. Bright cap on V1B is 200pF and resistor is 220K. Not too bright and usable. Dont have to use 500pF/470K bright cap resistor values. You can start with 100pF-200pF and on to get what you like. I left the 68K resistor in place on V1, both sides and this combo works out great. Treble knob at 1 O-clock is fine and linear and more usable.

10. On V1B cathode this can be from 1.5-2.7K bypass 68uF.

[audiopete]

Make sure all the heaters are in phase if it is 60Hz hum (one side to all the 4-5 pins and the other to all the 9 pins)

[g1]

3. I also had a problem with V1 when removed solved the hum issue. I soldered 2 100ohm resistors to the heater pins and connected with a bolt and ring terminals to chassis.

4. Check the center tap connections coming from the heater coil from the transformer and make sure this is connected very tight. This should go to the main star ground.

If you are installing 100R's from heater windings to ground (virtual centre tap), then the heaters actual centre tap should be disconnected. Use one method or the other, not both.