ground loop hum from left output of Stereo Power Amp

lukedlb

In the past I've used a Humdinger to eliminate ground loop hum when playing with two amps. But what would be the cause of ground loop hum coming out of the left output of a stereo power amp (powered by EL34)? I've changed the speaker cables and tried it without any input. The hum is present with volume at zero so perhaps this is not ground loop hum at all.
This is the first time I've tried this amp after buying it second-hand.
It has 3 x 12AX7 and 4 x EL34.

Should I begin by swapping over the two pairs of EL34s?

any thoughts? Here's the amp in question: http://www.advancetubetech.com/prodotti/prodotti.asp?artid=807

ICBM

Probably a failing power valve, filter cap or bias cap on that side. Swap the power valves for both channels and see if the fault follows or stays on the same channel.

ecc83

Yep, amp problem. Just to be  clear, you can ONLY get a ground loop hum if there is a ****ing loop! That is you have to have two pieces of mains powered audio kit, both with a mains earth and an audio connection between them and that cable must have a continuous screen to both.

As more and more gear is made earth free now (class ll insulation) people are getting hums, CALLING them "ground loops" but are in fact due to a LACK of an earth/ground.

Dave.

ICBM

Yes, it does drive me mad when almost every time there's any kind of hum or buzz from a piece of musical equipment, it's described as "ground hum". It *can* be, but most often is something else entirely.

Sorry, pet peeve .

ecc83

ecc83 said:

Yep, amp problem. Just to be  clear, you can ONLY get a ground loop hum if there is a ****ing loop! That is you have to have two pieces of mains powered audio kit, both with a mains earth and an audio connection between them and that cable must have a continuous screen to both.

As more and more gear is made earth free now (class ll insulation) people are getting hums, CALLING them "ground loops" but are in fact due to a LACK of an earth/ground.

Dave.

Heh! I am at the moment involved in just such a thread over at homerecording.com

Dave.

lukedlb

ecc83 said:

Yep, amp problem. Just to be  clear, you can ONLY get a ground loop hum if there is a ****ing loop! That is you have to have two pieces of mains powered audio kit, both with a mains earth and an audio connection between them and that cable must have a continuous screen to both.

As more and more gear is made earth free now (class ll insulation) people are getting hums, CALLING them "ground loops" but are in fact due to a LACK of an earth/ground.

Dave.

You're right, Dave. I'll go with ground hum although I fear it might be something else causing that low hum. I opened it up and then thought better of it and closed it up again. Nice and tidy inside though. Will post photos.

lukedlb

https://imgur.com/a/ahXp1n4