Intermittent whine / squeal from GM 40

Hey all,

Has anyone experienced anything like this with their GM40?

https://www.dropbox.com/s/fgu89mgnztssotj/MOV_4135_000.mp4?dl=0

...this start/stop noise is intermittent and seemingly random. Sometimes it's evident immediately after switching the amp on, other times I can be playing for hours before it appears. I've moved other electrical devices (not associated with the amp) away but that makes no difference. It has me a little flummoxed this one.

Cheers

Yup, had the same problem. It's most likely one of 4 EL84 power tubes. See my post https://handkusers.forumotion.com/t634-loud-buzzing-noise

Easy to check though. Carefully loosen the side panels, remove the top cover (there are videos that show you how) and gently tap on each of the large EL84. You'll know when you find the dodgy one.

I'm with Proddy on this one.  There is no change in the noise when you change patches so the preamp settings are not having any effect.  It would appear to be after that in the circuit.

Try inserting a signal from another amp by using the other amp Loop Send socket into your H&K Loop Return socket.  That way you have cut out the H&K preamp entirely.  I'd bet the noise will not change at all.  If you don't have another amp then just insert a guitar lead into the Return socket.

It could be output valves as Proddy advised.  It may also be a fault in the auto-bias circuitry.  Have you tried the "plectrum test" by inserting your plectrum into the slot on the rear and watching the dancing lights?  If you haven't done that then do it straight away and tell us what that reports back to you.  It's a starting point in testing the state of your output valves.

If that shows nothing then swap the outer pair of EL84s for the inner pair, (you MUST do this as pairs to keep them together), and see if the noise changes when you are on your 20W setting which switches one pair of valves off.

Thanks for the replies gents - the store wanted it back shortly after I posted the video so I didn't get the chance to run any of your suggested tests. I believe it's gone to H&Ks repair centre for assessment...still waiting for the verdict.