GM40 odd sound with Lead and Ultra channels at high gain

Trying to figure out why my amp has a strange high-end sound on the Lead and Ultra channels at high gain. Hard to describe...a growly, vowel-like sound almost like that of a cocked wah pedal. I mostly play a Telecaster with pretty high inductance pickups, but I've noticed this playing a Strat with vintage pickups as well. Not at all desirable to me and I haven't been able to do much so far using all the controls to minimize it. Ideas anyone? Or...is that the intended voicing of the amp?

Can't say I have noticed anything like that

From reading other recent threads, perhaps what I'm hearing is related to problems others have had taming the high end on the GMD40 and the GM36 in general. I love the amp's clean sound and its versatile MIDI control, but sound comes first, and if this is a problem, at least with the high gain sounds I want to get out of my guitar and amp combo without resorting to pedals, it might be time to venture beyond the HK realm...

A battle with the top end has always been a feature of GM life, and TM too.  As you have probably read speaker choice is paramount, other factors fall a long way behind that in effectiveness.  Trying most valve substitutions to me is mostly a waste of time though others always disagree with that.  The result of changes of maker are subtle at best.  One thing you could try is to substitute 5751s for the 12AX7s in the preamp.  These are proper substitutes.  My advice is always to steer clear of the 12A*7 "equivalents", T U and Y, they plain aren't!  They screw with every aspect of the stage design and make a pig's ear out of what the designer spent long hard hours getting right.  Putting a 5751 in either or both of those slots can soften the gain profile which may help.  That isn't guaranteed and many people do not prefer them but it is a reasonably cheap thing to try and you have your 5751 available for any other 12AX7 amp you own too.

Apart from that I would say you must know the control system of the GMs well to get the best out of them.  Hitting that front end with a high level output from your guitar/pedals then matching that with high gain settings in the amp is asking for a parcel of distorted distortion.  The front end has that TS type buffer and that needs to be dialled back if the sound is getting rough.  If you haven't already done this, try rolling back on the input signal quite severely on your guitar or pedals then readjust the Gain/Volume in the amp to sound as best you can after that with the Boost OFF.  The input stage becomes a clean buffer and the job of introducing distortion is left to the valve stages after.

Thanks for those pointers. I did replace the stock tubes with "Mesa" EL84's and SPAX7's--Mesa in quotes because they're outsourced. The Mesa EL84's look identical to the stock tubes so probably no change there, no problem since I don't mind having the spares, but the SPAX7's (12AX7's with rubber collars to help minimize microphonics) were different than stock and darkened the tone a little bit which I liked, but still didn't tame that high end growl nearly as much as I wanted.

By the way, I get this same high end growl whether going through the speakers or the RedBox, whether playing a Strat with vintage pickups or a Telecaster with Zexcoil Convertibles whose inductance is off the charts even compared with humbuckers. I can experiment using the guitars' volume controls at first to see if I can dial in something better, but that may darken the tone too much. What next, treble bleed circuits for the guitars? This is getting mighty complicated.

The 5751's sound like a good idea. I like what I just read about them, lower gain factor (70 vs. 100) with a plate resistance keeping them closer to the 12AX7 than other 12A*7's. Do I replace all three, or just one or two? That's not entirely clear from the above.

I don't think you will  darken the tone too much by backing off on the guitar volume.  You are simply removing overdrive from the solid state buffer at the front end and replacing it with overdrive from the amp valve stages.  That front end buffer is a great tone tool and very few people ever get to grips with it really.  "Common knowledge" tells you that you need to slam the input stages to get a good driven sound, except in this amp it is wrong, or at least not always right.  If you had a normal design of amp with a Tubescreamer in front of it you would want to dial in just the right amount of drive to and from the TS not just drive it into maximum clipping and get just a squarewave out.  This is exactly the same as that.

This might be more technical than you are interested in but have a look here and get the .zip file I posted some time ago:  Spice Simulation of GM36.  These are plots of the outputs of the stages of the GM36 preamp with increasing input level to that stage, sometimes with parts removed to show the action of other parts.  It ties in with this long thread that I put up while I was investigating:  GM36 Spice Sim Investigation Thread.  Take a look at the Input Buffer folder and pics.

The buffer has 3 levels controlled only by the signal level going into it, by the Channel selection, and at the third level by the Boost switch.  Where it is in the pic name, the plots are for channel selection "CC" - Clean/Crunch and "LU" - Lead/Ultra.

First with lower normal playing level signals it is absolutely transparent for all channels.  It simply amplifies the input signal without overdrive or distortion of any kind as a preparation to transmitting it to the Gain control and on to the preamp valve stages.  As a protection mechanism and as a means of preventing any solid state characteristics from being imprinted on the signal by the devices in use, the input signal is limited by a pair of zener diodes.  This means that really extreme signal levels around 2.5V and above will be clipped on both sides by simple diode action, not by the internals of an opamp say which is very audible.  This has absolutely no effect on signals lower than that level at the input socket.

See pic 1 for the effect of only those input zeners on the input signal just after them.  For the buffer stage output with and without these zeners for CC and LU see pics 2, 3 and 4.  Remember the input level is increased beyond anything normal to get to the point where the effect is obvious.

Secondly, with larger signals one side then begins to limit and eventually clip.  You can see this asymmetry clearly in the plots.  This gives rise to even order harmonics which make the sound thicken up and lends it a valvey tone.  It's the sweet signing pre-Crunch stage not the high gain players dream.

Finally higher signals again cause the other side to begin to clip also adding in the odd order harder harmonics.  This is sounding more like the way that people usually use distortion pedals.  This stage is given a kick if the Boost is applied as, amongst other effects later in the amp, it alters the gain of the stage to increase the ease of clipping, still making sure that the only way the signal is affected at any time is via its diode chain.

If you can get your head around the plots you can see that that stage is a really useful tool for getting a number of approaches to "pre-overdriving" the signal before the real work of the valve stages begins.  Too much of its distortion however can give you a very hard sound.  If you get the knack of not slamming this automatically to match the level of gain you think you need and you simply add a touch more gain in later in the chain where there is plenty to spare, you will get a much more manageable and refined type of overdrive.  That may help with the hardness, I know it did for me when I found this.  Remember H&K are a German company.  The amp has a "Germanic" sound quality to it as the Germans love their metal stuff and that is, I suspect, the primary target of the GM series.  It offers what to me is extreme levels of distortion way beyond anything useful.  That is, until you discover that often less is more and learn to throttle back and pick where your distortion is coming from.  The Input Buffer gives one character and keeping the Gain low and Volumes high to clean up the amp following it gives one sound.  Lowering the Input Buffer contribution and increasing Gain later on to drive the valves into saturation is another different path and sound.  Using both is extreme and some peoples' dream but must be handled with kid gloves even for high gain players.

Sup HKers!
After gigging many shows with the H&K GM40D for over a year ... I just recently started using the Ultra channel.
Literally, I have never really tried to dial it in. The other channels worked fine for live playing, and I was satisfied.
MANGO! I think I will be using this for one of my presets. I think I have been missing out!

I don't find any extra noise or anything. And I have the boost on right now.
I always keep the noise gate dial at Noon.

Still loving this amp for live gigs.
16 lbs (7.26 kg), tube, midi ... still running on all the original tubes.

How are you using your guitar/pedal volume level and gain settings Ravensha?  I'm interested to find out what it is you have found that has made this come to life for you.  Maybe this is something we can all benefit from.  I agree with you on the Noise Gate.  I think the H&K implementation of that is one of the forgotten secrets of their amps.  It's clever to sense the signal at two different points and react to both.

bordonbert wrote:How are you using your guitar/pedal volume level and gain settings Ravensha?  I'm interested to find out what it is you have found that has made this come to life for you.  Maybe this is something we can all benefit from.  I agree with you on the Noise Gate.  I think the H&K implementation of that is one of the forgotten secrets of their amps.  It's clever to sense the signal at two different points and react to both.

Yes I've begun to incorporate the noise gate into high gain presets as well. So far so good for the most part, though it can kick in a little early sometimes and choke off long sustained notes. Too bad its setting is not programmable per preset. I wonder how its operation compares to that of using something like an ISP Decimator?

Per earlier conversation about preamp tubes I have yet to get the 5751's but it's on the list.

Problem solved I think. Had been playing with EQ in my Boss GT-100 multieffects unit, which helped some but not enough to matter, pointing to an amp issue for sure. So back to amp EQ--the bass, mid, and treble controls seem to be only fine adjustments to those dialed in via presence and resonance, so I cranked the presence and voila--the sound I disliked was gone. I had previously set it about midway, which was producing an undesirable midrange honk which I guess is coined "cocked wah" elsewhere? Makes sense since a wah pedal is really an EQ sweep. Anyway, yes my sound is now very bright, which I assume is what other posters have really been going after here. Now we're at least on the same page, so still very helpful knowledge. Thanks all!