GM 40 Tube Related issues...

Hi guys, great thread, and I have read absolutely ALL of it!
Just purchased a second hand GM40, have dreamed about a GM for quite a few years....

I have to say I have seen every single YT demo of the GM40, at least twice.... for the past 3 years before pulling the trigger... great sound, features, and it seemed quite fit and flexible for what I play, Floyd, U2, 80's metal (yeah... I'm 51....50 lol). I can equally say I have been really disappointed by the sound I'm hearing in person, harsh, fizzy, thin, boxy, cold and sterile... a high gain tone that sounds like a bad solid state distortion of 30 years ago...in fact the shock had me almost resell it immediately... but I took a breath.... I will give it a second chance.
Fortunately I have definitely found all I needed to know here on swapping tubes, thanks to all of you.
I guess power JJs and a 5751-5751-83s combo might do it for me.
Years ago I had a 73 Fender Twin, that was at some point revised and set up with Sovteks by a tube amp repair shop in Paris, and the difference was just amazing.

Now, I have an issue/failure that I believe tube-related, hopefully someone here has a clue.

After playing for a couple of hours just on the clean channel, there was a massive drop in volume, I mean easily 80%.... the crunch channel is affected , say by 50%, lead and ultra are as loud as ever.  I would imagine it might be a preamp tube assigned to the clean/crunch channels that just failed?  I took all tubes out and put them back in, they all seem to glow normal, no klonking sound when I tap them, so.... I'm hoping it's just the one tube that decided to go south.... I messaged the HK team on FB, they were very nice but didn't seem to have an answer.

Oh, and for the record I'm playing through a beauty of a Panama 1x12 cab with an aged V30, and despite the speaker influence, I can say I have played it with a 5w class A Baroni Lab head and it sounds sweet, not Fender-ish of course (also EL84s) but warm and clean, so I can't blame the cab

Any clues appreciated, thanks all.

Damn, sorry you are having a tough time with the GM40. I can say for sure that mine does not sound harsh/fizzy/then etc. It did when I used it with a particular v90 cab though, so maybe the GM40 does not match up with your cab as well as your other head does.

I've no idea what the volume problem is, but I'm sure bordonbert will chime in with some electronics advise.

JonnyNonsense wrote:Damn, sorry you are having a tough time with the GM40. I can say for sure that mine does not sound harsh/fizzy/then etc. It did when I used it with a particular v90 cab though, so maybe the GM40 does not match up with your cab as well as your other head does.

I've no idea what the volume problem is, but I'm sure bordonbert will chime in with some electronics advise.

Thanks man. UPDATE.
I've run a few tests:
*Took a 12ax7 from my other head and swapped for each of the preamp tubes in the GM one at a time, and checking for some difference every time. Nothing.
*Swapped the outside EL84 pair to the inside. Nothing.
*Took an (the) EL84 from my other head and swapped for each of the power tubes in the GM one at a time, and checking for some difference every time. Nothing.

The clean channel is basically dead... I have to put the master AND the gain at 10 to even hear anything...
Meanwhile the crunch channel is now a pristine clean, and I have to put the gain way up and use a humbucker for it to start crunching...
From limited experience but common sense this would seem to me like a preamp gain problem, so tube failure, but replacing one at a time didn't do anything     I suppose the only way to be sure would be to change the whole set... but buying a complete set of tubes for an amp that may be faulty sucks the big one.  Damn, really not a good start.  
btw I also find the lead and ultra channels extremely loud on 1W setting.... but I suppose that is normal.... " yessss yesss..metalll... hehe  hehehe hehe"

Hi Firewind, welcome to the forum. What I can say technically has to be based on my knowledge of the GM36 which I do know well. I believe that the GM40D is a revoiced GM36 with tweaks. That revoicing may be major, it may not, I have never played with one nor have I seen the new schematics. This is not to run the GM40D down in any way, it is a natural progression in the life cycle of any electronic product. While it still does most of what people want you keep developing it without any major changes and that seems to be the entirely logical path that H&K are taking. However, in its core I am sure it is a GM36 (with aplomb and hauteur? ) so I do think what I say can be considered seriously. (Until we find out differently and I fall from grace... )

Really this point and discussion should not be in this thread. I do have a long answer with infpo and advice but can I ask you to repost it as a new thread of your own and I will pick it up there. We don't want such a standard thread as "Valve Swapping" to be hijacked with a discussion about a specific technical problem. Let's keep the continuity.

bordonbert wrote:Hi Firewind, welcome to the forum.  What I can say technically has to be based on my knowledge of the GM36 which I do know well.  I believe that the GM40D is a revoiced GM36 with tweaks.  That revoicing may be major, it may not, I have never played with one nor have I seen the new schematics.  This is not to run the GM40D down in any way, it is a natural progression in the life cycle of any electronic product.  While it still does most of what people want you keep developing it without any major changes and that seems to be the entirely logical path that H&K are taking.  However, in its core I am sure it is a GM36 (with aplomb and hauteur?   ) so I do think what I say can be considered seriously.  (Until we find out differently and I fall from grace...   )

Really this point and discussion should not be in this thread.  I do have a long answer with infpo and advice but can I ask you to repost it as a new thread of your own and I will pick it up there.  We don't want such a standard thread as "Valve Swapping" to be hijacked with a discussion about a specific technical problem.  Let's keep the continuity.

Of course you are correct, I noobed.
Plus this will make it easier for other guys who come across the issue to find relevant info.
Let me open another thread repeating my original message, thanks man.

See the thread below this one: "Extreme volume loss on the clean channel, lead and ultra unaffected, tube failure?" for the followup responses to the posts above.