Loud Hum

I changed the output tubes on my ED head tonight and after the change I powered it up (with a cabinet) to test. It has a very loud hum, like a 60 cycle hum or a short hum.

I put the old tubes back in, and the same thing...

Any ideas what could cause a hum like that ?

Did it ever have the hum before?

Is there a guitar plugged into the input?
Is the hum affected by the volume knob? How about the master?
Is it plugged into an outlet you've used before?
Is there other equipment plugged into it?
Is it possible you accidentally changed a setting or switch position?

elvis said:

Did it ever have the hum before?

Is there a guitar plugged into the input?
Is the hum affected by the volume knob? How about the master?
Is it plugged into an outlet you've used before?
Is there other equipment plugged into it?
Is it possible you accidentally changed a setting or switch position?

Click to expand...

No - nothing was plugged into the input....no cable or guitar
Will have to check - but had the master quite low and it was very audible hum
Tried two different outlets - actually in two different rooms with different speaker cab's
No
Checked that -nope.

I never noticed that hum at all. I had the amp playing in my man-cave last week and don't recall a hum at all. Last night when I changed tubes noticed it was really loud and thought it might be picking up interference so I hooked up my Mini Rec head and it was extremely quite. So I brought the ED head upstairs and tried it on another cabinet and it is indeed very loud. Almost sounds like a short....

Could be the fact that there is no input. Try plugging a guitar in with the volume rolled off.

Also could easily be a preamp tube that went bad as a coincidence. Try swapping those.

Did some more testing today - what I found is this:

Hum is louder with no guitar plugged in...

AND the gain trim switch - when it is in normal or hi lo the hum is loud, when it is in Clean it is very quiet.

I don't understand that gain trim at all - I just normally left it in normal and kept the trim pot low to balance the 3 levels. If I move it to clean, then I have to turn the control full up to come close to achieve that...

Maybe this is normal, not sure. The other guitarist in my band has a ED combo, I have asked if he could check his as well...

Thanks Elvis~

OK, call Mesa's customer support for sure. I'm sure they can help. I have an Electra Dyne long head with matching 1 x 12 head and I have not had that problem.

That being said, it sounds like a preamp tube problem.

**Ideas**

The gain trim switch changes the amount of gain / clean headroom between the clean mode and vintage lo / hi.
If you want more gain on Vintage Lo Hi with more clean headroom, you select clean. If you want to overdrive the clean mode and have a less saturated tone on vintage hi then you select low hi. Normal is the happy medium with the most balanced tone between modes.

SO, you are hearing the hum on vintage lo / hi? I'd suggest looking at the preamp tubes, especially v1 and v2. Look at the tube task chart and determine which tubes are driving vintage lo and hi.

Yes - again amp on and nothing plugged into the input jack...

Amp volume 3:00, master volume very low, say 6-7 oclock

Gain trim switch in Clean - very quiet, moving it to Normal and Hi/Lo the hum level increases substancially

Gain trim in clean effectively lowers the volume of clean. In other positions it either lowers the volume of the vintage modes or does nothing. So if volume is lowered on clean, it's less noisy than when volume is NOT lowered on clean. I suspect any preamp tube that affects clean more than other channels.

I would swap V4. The gain trim lowers the gain between V4A and V4B, which are the first two gain stages in clean. HI-LO modes have V1A and V2A as the front end.

As a long shot, you may have a bad ground connection in the clean circuit, and turning on the gain trim adds a ground path. It's through 100k, so unlikely.

I tried replacing V1 / V2 and V4 - no changes.

Also, I note that touching the gain trim toggle switch increases the hum. The amp really sounds like a cable is plugged in to the amp, but not to an instrument, but at low volume. And when you touch the gain trim toggle its almost like touching the end of the guitar cable.

I am thinking a short. Gonna call CS today, and take it in for service.

My Dyne returned home last night. What was wrong is the gain trim toggle switch was not properly grounded. Oddly it still hims more than I think it should, but probably always has.

Gonna check it out after work tonight carefully

Actually, the gain trim switch is no longer buzzing, but the amp was returned to the tech because it was still humming loudly. The tech has to return it to Mesa for repair.

o tube amps u

Hopefully it comes back with a clean bill of health!