Amp hum help!

stefken147

Hi all, would appreciate help on a troublesome amp hum that has recently occurred in my Two Rock Exo 15. Amp was retubed about 9 months ago and has been sounding great and super quiet up to now.

Switching the amp on from standby there is a hum, sounds like a ground hum, pretty low pitched and does not vary within change in amp controls. 

Hum is present with gain and master on zero. 

Hum is there with and without guitar plugged in.

Amp is still sounding great when being played, just the hum is annoying..

Hum is there when I try the amp in another room.

The amp has 5 tube, v1 is preamp, v2 is PI, v3 is rectifier, v4 and v5 are power tubes.

Removing v1 makes no difference to the hum. 

Removing v2 results in the hum disappearing meaning issue is upstream. Have tried various 12ax7s in v1 and v2 with no change.

Any thoughts???

Winny_Pooh

@ICBM heater wiring perhaps?

ecc83

Winny_Pooh said:

@ICBM heater wiring perhaps?

Do they perhaps elevate the heater line a few 10s of volts to give the PI h/k an easier time and the +ve DC has gorn?

Could be a failed HT filter cap but modern electro's are very reliable.

Dave.

ICBM

Be careful assuming that hum in the early preamp stages is coming from them - I just caught myself out like that with a Fender Hotrod last week... pulling V1 stopped it so I assumed it was the usual failing filter cap problem, so changed it - no difference.

It was caused by a power valve with a cathode to filament leak putting hum into the filament loom.

If the amp is fixed-bias (I think it is), trying pulling each power valve in turn and see if it stops.

stefken147

Thanks guys.

@ICBM - I'll try that later. Thanks!

DJH83004

Looking at some photos on line, it is definitively fixed bias, with an adjustment pot, so the bias may also have drifted over time, they are EH 6V6GTs, but because of the very compact design, I read the tech had tried to fit JJs and they were too tall, just something to bear in mind.    

stefken147

So tried removing one power valve at a time and hum is still there, but louder. Hum is comparable with either valve removed. 

Next step will be to rebias. 

ICBM

stefken147 said:

So tried removing one power valve at a time and hum is still there, but louder. Hum is comparable with either valve removed. 

Next step will be to rebias. 

OK, that's normal so it's not a power valve fault - even if one was out of bias and drawing too much current you would most likely not get the same amount of hum from each. It could be an overall bias setting issue, yes - but if the fault occurred suddenly it's more likely to be something else.

stefken147

Yes, was fine one evening playing at home, next evening it started humming. Amp wasn't moved or changed in anyway in between.

Time for an amp tech? Can't think of anything else I can try, could try a 12at7 instead of a 12ax7 as the PI or swap out the rectifier tube, but can't see those making a difference. 

ICBM

stefken147 said:

Yes, was fine one evening playing at home, next evening it started humming. Amp wasn't moved or changed in anyway in between.

Time for an amp tech? Can't think of anything else I can try, could try a 12at7 instead of a 12ax7 as the PI or swap out the rectifier tube, but can't see those making a difference. 

If you have a spare rectifier valve try that - if one half has failed it would cause hum. (Have a close look at the valve - if both filaments are glowing it's unlikely to be faulty although it's maybe possible.) If you don't have one, it may be best to buy a spare anyway - in fact, a solid-state plug-in would be just as good to get you by.

stefken147

So, replaced the rectifier and phase inverter, hum is still there. Checked bias and it is fine. 

Anybody know of a good amp tech in N. Ireland?