DSL Hum on Lead Channel

[AmpFix]

I have a 1997 JCM2000 DSL50. This amp has been very reliable but it seems that the PT (power transformer) has developed an audible, mechanical hum that also comes out through the speakers. Generally, the hum level is not really bothersome, except on the lead channel. The hum can be heard ever so slightly though the speakers even with the amp on standby.

Using the Green channel, the noise level is totally acceptable and almost no hum can be heard through the speakers.

When going to the Lead channel, the hum (not the white noise) becomes very apparent, especially when the master is set at 5 and the lead gain is set at 5-8. At a Lead channel gain setting of 9-10, the hum almost dissapears. If find this behaviour odd as I would not expect the hum to go down when the lead gain is maxed out.

Removing V2 eliminates the hum and noise completely. Removing V1 doesn't affect the hum.

I have tried different tubes in the amp but none of them have made a difference. I'm wondering whether the problem could be caused by either a bad PT (heater noise), some of the LDR couplers or maybe an bad opamp.

Is this a known problem with the DSL. If not, what could be causing this issue?

Plugging right in to the effects return also gets rid of the hum... or reduces it greatly. The last time I had a problem like this, it was on a Jubilee and the problem ended up being the AC Heaters. The PT was noisy and was inducing hum that was amplified by V2 on the lead channel. I was going to try to run the heaters from a battery tonight to see if that gets rid of the problem. Until then, I was wondering if any of you have any suggestions of things to look at.

[AmpFix]

I tried power the heaters from a battery but with 4 preamp tubes and 2 el34s, the battery could not deliver enough juice.

I did notice that when the ultra gain pot is on "10", the hum dissapears. I think that maybe that pot could be bad. The knob on that pot was replaced and was forced on as the fit wasn't the best. Perhaps it was slightly damaged.

Does anyone know which part to order from Digikey as a replacement for this pot? I'd also like to order some of those LDRs while I'm at it, just in case that's the cause of the problem.

Any other parts that could be the problem here?

[AmpFix]

Anyone?

[Enzo]

I don;t have a meaningful response.

If you want to try battery power for tubes, you don;t have to power ALL of them. Try just powering the ones that run on DC already - like remove the heater bridge and connect battery there.


I am a bit confused when you say the transformer has a mechanical hum but it goes away when tubes are pulled. if the transformer buzzes, it shouldn;t care if a tube was there or not.

Transformers run on 60Hz. The noise field in your environment is 60Hz. Naturally any hum you get in your amp will also be 60Hz. That doesn't make it the transformer's fault. It is not the transformer's fault if a tube picks up 60Hz from its own heater.

Unless your volume pot is scratchy, it is probably OK. Hum won;t be the pot's fault.

Most amps have more than one source of hum in them. If your amp has a background hum, and then you turn up the volume, which increases the amplified hum, if the two are out of phase, what you are hearing is phase cancellation between the two.

That is the same thing as when you hear peo0le report that their hum is least at about 2 on the volume control. In that case 2 is where the two sources are balanced out and null out. Yours happens to do it at 10.

[AmpFix]

For the heater noise test, I'll try a DC power supply. I just got a Hewlett Packard supply so I'll be trying that. The first 3 preamp tubes are powered from DC but in reality, it is far from ripple free DC. I figured I would simply disconnect the AC heater supply from the PT and connect it to the DC supply instead for the purposes of the test.
Removing the heater bridge is an option but more time consuming. Do you think that it would be ok to use the method I described above instead of going through the bridge rectifier? I just don't want to melt the traces in the process


For the PT hum, the noise goes away from the speakers when I pull V2. The PT itself will hum regardless and it seems quite loud and IMO, abnormal.

I can live with a mechanical hum, even though I don't like it. What bothers me is that it comes through the speakers, mostly in Lead mode. When using the effects return of the amp, same thing - The hum goes away (through the speaker but the PT still hums).

The reason why I suspect the Lead gain pot is that the hum goes away from the speakers when its on 10. To me, that would mean that it is essentially bypassed. The pot was stressed with that new replacement knob that was force fitted on it. If I can find a suitable replacement, it could be worth a shot.

The LDRs are another possible cause (I'll have to see which one is for the Ultra Gain channel - LDR2? I have the Issue 1 JCM2-60-00 board).

[Enzo]

If your bench supply can handle the current then sure, use it. In your earlier post you only mentioned having a battery.

[AmpFix]

I just got a DC power supply. I'll be picking it up on Monday. I don't know how many times I've needed one but ended up being stuck without one. I figured now was good time to get one. I'll report back with the test results.

[bluesfreak]

I remember a DSL having a similar problem years back and there it was caused by the PT not having Gnd to the chassis due to the laquer/anodization on the chassis. We fixed it by scraping the laquer from the mounting points and used good star washers to reattach it. Have you tried to shield the PT from the preamp tubes by inserting a sheet metal shield between the PT and the Preamp Tubes? Might be worth a try as well but first give the Lead pot a try...

[AmpFix]

I finally got around to trying to connect the heater supply to an external DC power supply and although most of the white noise remained, the annoying background hum that was most prominent with the lead gain at 6-8 has pretty much disappeared.

It seems like maybe the PT is the source of the problem OR the electrolytic caps for the DC rectifier are old (the amp was made in 1997) and leaky.

I'll start by changing out the heater filter caps from 3300 to 4700uf and see if that improves anything. If not, I guess I'll have to switch out the PT.

One thing I noticed is that the PT doesn't hum as much with the heater tap disconnected.

[AmpFix]

The problem is not the PT. I'll be replacing & upgrading the DC filter caps when I get a chance to get some proper caps from Digikey.

[rf7]

Do you have a oscilloscope? If the hum is 120Hz, then that is a power supply issue, and then, possibly the filter caps may be suspect. If the hum is 60Hz then you have a grounding problem or possibly a heater induced hum.

[Amp Kat]

Also try disconnecting the reverb pan and also make sure the pot is turned all the way down. Take a signal out of the send or preamp out and send it to another amp. Still hum ? Then it's from the preamp send back to the front-end. No hum ? Then take a signal and inject it into the return and see if it hums then. The rectifier BR1 that changes the AC to DC on the heaters should be checked for dark burn marks and if present you should change it with a larger type. IMO it's not the caps but it's you're call.