NEW AMP DAY - GM40D

New member ,
I bought used GM40D in excellent shape from a Guitar Center w/floorboard(Mkiii), WiFi/MIDI adaptor, and bag.

It came w/Chinese tube and I bought a full set of Tungsols a replacements. After replacing the tubes, the Lead/Ultra channels have an extreme amount of white noise(Clean/Crunch is fine). I restored the amp to the original
tubes and it went away.

While it probably obvious that the 4 EL84s have nothing to do with this, should I play swappies with V1 or V2 while troubleshooting. I plan on putting the new EL84s back in and swapping the 3 pre tubes one at a time.
It appears that one of the new 12ax7 is compromised.

Any advice from those in the know?


Thanks so much.


Edit: I swapped all the EL84 back 1st, no noise, then swapped 1 12AX7 at a time and now the unit is noise free(within reason). Certainly not the issue I had last night. I don't know exactly what I may have done to induce the white noise, but now it is gone!!

Welcome Marcus!

It isn't totally unusual to find that valves take a little time to settle in.  That is why some suppliers will insist on a burn in time before they test and select them into bias groupings.  Keep an eye on things for a while to be sure.  If you have a degree of noise in your amp it is worth noting that the V1 position is more susceptible to noise in the valve as it is followed by all of the other stages which means the most gain.  You often buy a "selected for low noise" valve for that slot.  In the same way some people think it beneficial to buy a "balanced" version for the PI slot in the power amp so as to have the same characteristics on both sides of the signal.

Thanks for the response, you seem to be the resident guru. It's all fine after reseating everything a second time. I believe I also vetted a bad cable on my pedalboard.

FWIW, I traded in some more stuff and got the matching cab(TM212) from the same store, same previous trade of another customer.

I can see why some people might not use drive pedals at all with this amp, but I'm using a Chase  Tone Secret PreAmp as the last stage before the pre H&K. I'll control the overall level into the amp with that. I've been able to use multiple ODs stacked in front of the amp this way w/o inducing excess
noise or squealing.

Hahaha! Thanks for what I know is intended to be a compliment but I do see myself as anything other than a guru.     I prefer to be an "experienced engineer with ears and mind open" who tries to get others to look at the problems from a sensible footing.  I know a lot of what I post is not popular with other H&K users and it stops them from taking part in conversations but it isn't because I am at war with them, only that guitarists are reluctant to think of the undisputable engineering truths and answer questions in ways which are posed based on technical facts.  Everyone without the background wants this field to be a mix of smoke, mirrors, magic and membership to a "priesthood of the knowing" to which they are then able to qualify to become something more than they are beforehand.  Well, at the last look inside my own amps, they were made of physical components strung together according to engineering principles and it is those engineering principles which make the whole lot work.  To refuse to accept that is not a sign of an "enlightened knowing mind", it is a sign of a "wannabe a member" mentality.  Yes there is a place for taste and preference of course and there are still questions which can be argued about on a technical basis but there is no place for refusal to accept the laws of physics and to work to long established accurate working practices.  The theory and background of this field exists so that any further advancement is fully accurate, repeatable and relevant in real life.  Anyway, I ramble as always...  

If you haven't seen it already, this might be of interest regarding your use of pedals before the amp.  Understanding the GM36 Input Buffer  This was based on the GM36 but it should apply in principle to the GM40 as well.  There is too much gain in this amp for most people's uses, even high gain players, which is one reason why they are often criticised for being strident or rough at the top end.  Square waves are not normally considered as the signal of choice and that is pretty much what you will end up with if you amplify, clip, amplify, clip, amplify, clip, amplify, and clip again.  Just ask my dog!    Learning to choose the areas of distortion which you will apply to get your desired sound is crucial to getting the best out of these H&K amps.

Yes sir, read all your tech posts on the 40D. I'm a 63YO Eng Tech. Been in Semiconductor, Aerospace Optics and Sensors, and currently RF electronic environmental stress testing.

My troubleshooting skills are on par with my experience

I have a decent amount of experience with gain staging pedals together with good/great results.