Clean channel clipping with High output pups!!!!

Hello, new to the thread and went through all of the posts looking for this topic. I've had this amp for a bit now and can't seem to get a pristine clean sound with any high output humbuckers. I have tried 5751 preamp tubes and other combos, and it just can't seem to handle the input of my JB's. I have also tried all of the master, channel, and gain control combos. It won't take it. I am running 2 vintage30's and haven't tried other speakers. V30's are an efficient speaker so I'm lost at this point.

Running single coil guitars and even less hot humbuckers are ok, but it seems that a 36 watt tube amp should have no problem with this. That being said, my AC30 can produce a clean sound with exact setup. I mean unplug from GM36 and go right into AC30, clipped to clean.

Well, JB`s aren`t really that hot of a humbucker, so you should be getting a crisp, clean sound (I recall those being a 290mv pickup. For reference, the D Activator in my six string is a 500mv output).

Two things: Use the 36 watt setting if you aren`t already, and make sure the gain is not accidentally saved at a high setting. It should be about 30% up from minimum. You can see what it`s saved at by turning the knob and watching for the "save" light to come on. When it lights, that`s where your setting is. If it`s too high just turn it down then press and hold the save button until it flashes.

Also, if you`re using the effects loop you may be introducing a bit of signal gain, but it`s between the preamp and power amp so not as likely but not impossible.

Hi Posborne.  Welcome to the forum, glad to have you along.

I don't understand why you have to get your clean sound with your hot pickups turned full up.  Can't you just turn them down slightly?  A Volume pot doesn't alter the shape of the signal in any way really, it just trims the level down at the guitar input so you get the lack of overdrive you want within the amp's stages.  The answer is in your own post, "...and even less hot humbuckers are ok".  So for a clean sound turn your hot humbuckers down to the level of that less hot one.  Do you think you are losing tone in some way by doing that?  You aren't a member of My Les Paul forum are you?  Some pretty whacky screwed up ideas float around there and pass as gospel I can tell you!

Different amps will have different clean headroom levels.  As long as you can get clean at sensible settings then there is no right or wrong. The Vox is not better because it can take a higher input level before clipping.  The difference between your GM36 and Vox AC30 is that the AC30 simply feeds the signal in straight to the grid of the first valve stage which naturally has a high headroom, and follows that with the Volume control.  That's effective but pretty primitive!  We believe (pretty sure) the GM36 has a buffer/overdrive/distortion stage before that.  It's H&K's way of making your guitar/amp interface more predictable and ultimately reliable in tonal terms, and also of giving you a free Tubescreamer type setup for your money as well.  All you have to do is to make sure you don't pump in enough testosterone to trigger that and you have beautiful cleans.  The levels are set pretty high not stupidly low, the TM18 whose schematic I have had a chance to analyse, has the same setup and, with the Boost off, that first stage begins to clip on one side on peaks of around 1.8V.  That is easily enough headroom just at that stage to come through it absolutely clean buffered and unclipped completely. One thing the GM36 does better than most other amps I know is play chimey and clean.

It took a while for me to learn, on the advice of others, how to get full control of my sound from the guitar by setting up the Lead channel at full guitar output for the type of heavily driven sound I wanted.  Then by backing off to around 7 on the guitar I can go down to wonderful crunch sounds, and further down to 3 or 4 to get really lovely clean sounds, all at pretty usable balanced levels and without touching the amp at all.  And like that I can up the amp Master Volume to get output tube saturation without hard preamp clipping.  You can balance the three areas of distortion, input clipping circuit, valve stages, output stages, to get all sorts of overdrive tones.  Your high output from your hot pickups is only another option they offer you, not something you have to use fully all of the time.  Or am I missing something?

Thank you very much for the replies. I am definitely set where I should be as far as gain, but will mess around some more. And as far as having my volume pot all the way up, it is definitely not needed, I am just surprised that JB's could cause clipping at 36 watts. Was just curious if anyone else had a similar experience with humbuckers. Thanks again.

I think you are maybe missing something in how the input signal relates to the output power rating Posborne. They are not connected in any way at all! With a Volume control in the middle it is irrelevant how powerful the amp rating is. If the first stage of your amplifier will clip with your guitar's signal level then it can have a 1W or a 1kiloW amplifier fitted after it. Whichever you have that still clips! Forget the output power, that is only relevant to how loud you are not how you are clipping.

The idea for all amplifiers is to have a preamp stage to give the tone you want and to amplify the signal to a level which is large enough to allow the power amp section to be able to give its full power. And preamps are usually designed to go beyond that to cope with lower input devices and power amps which need higher signals to drive them to full power. A master volume control between the two allows us to adjust the level of the output to whatever we want without too much change to the quality of the sound.

You say that "I am just surprised that JB's could cause clipping at 36 watts." With the guitar volume and Gain up full the amp is still clipping even when the output is turned fully down at 0W. That's how you can play one control against another to get the distortion from whichever stage you want as they each sound different.

Ok I understand. so even with my gain and channel volume down and the output up I am still clipping. Does this happen to you as well?

Yes, it happens to everybody. It's just how amps work. That's why guitars are made with a volume control.

Ok thanks. Being that you're extremely technically proficient, could you elaborate on what the 5751 tubes did for you in v1 and v2? I have read a lot on preamp tubes, and I know the 5751 has a gain factor of about 70. What does this actually do as far as the preamp structure?

Haha!  There's an old saying, "In the country of the blind the one eyed man is king".  I seem to live on that.

Well yes, I have a good technical background but that isn't everything.  In terms of what you hear, everyone is equally equipped, we all have two ears, (at least the huge majority of us do).  The trick is to get yourself to use them honestly and trust what they tell you.  In music you will find there is a lot of the "Emperor's New Clothes"!  The Emperor was sold a suit by a conman which didn't really exist at all but the conman sold it to the Emperor by saying how he MUST be able to see how amazingly rich the material was and SURELY he could see how fantastic the cut was.  The Emperor couldn't admit to him that he could actually see nothing at all so he bought it.  When he went out in a big parade wearing it, all of the people couldn't believe their eyes, but he was the Emperor so they all applauded and pretended that they could see his magnificent new suit too so they wouldn't be the only one who didn't have such refined taste.  The bubble burst when a small boy in the crowd asked "Why is the Emperor wearing no clothes" and everyone was instantly ashamed but couldn't keep up the pretence any longer.  Yes, it's an old and long story but the moral of it here is, beware of hearing what others will tell you you should hear!

For me the swap to 5751s had a number of features.  I am not a heavy metal player, the extreme levels of distortion are not for me.  The GM36 does those really well as a German amp would.  So I was constantly using the lower settings and they were a little squashed up on the bottom end of my controls.  I wanted to take away some of the higher gain which I never use and spread out the areas where I set my controls most of the time so I had a little more control over my sound.  Everyone told me that 12AT7s were the way to go but I don't believe that sort of advice without understanding technically why.  ONE person on a technical site said he thought they made for a thinner sound and he also pointed out that that was because they weren't a good matching alternative anyway.  Everyone else either mocked him or dropped out of the discussion but I asked him to explain and I got the answer which I wanted.  I don't believe anything without proof so I then spent a bit of time researching what he had said and found that it was absolutely true, they are a very bad match, in fact, putting a 12AT7 into a 12AX7 hole is usually like putting a pickup gearbox onto a sports car engine they are so different!  Now someone out there will have done that, but it is only so they can brag about having the most unusual car in the pub.  The fact that it limps along like a three legged dog doing about 6 miles to the gallon and needs an engine rebuild evry 10,000 miles never seems to come up.

The 5751 is a totally different case.  I looked into the datasheets and found they are actually so close the 5751 is often described as a low gain 12AX7.  When I did some sums and tracked design factors on the graphs I was horrified at what was happening in the circuitry where it had been designed around a 12AX7 and was running with a 12AT7 in there.  The currents could be wildly different and voltages would be all over the place.  The 5751 upsets things much much less.  And it has gain which is almost as low as the 12AT7.  People think the 60/70 gain is a real jump, "it's like a whole 10 more dood!"  10 more?  cents, dollars, miles, chickens, what does 10 more mean?  The real truth is that the difference is not "10 more", it should really be looked at as only 1.17x as much and that's not a lot in our terms.  The gain is not the useful factor in valve swaps, it is only a calculated value from two other factors which are much more important, and they are wildly out with the 12AT7 by a factor of about 6 times as much!

Having lower gain valves means that the earlier stages drive the later stages a little less hard.  That shifts the balance of distortion a tad between them, and I find it less harsh.  But our resident professional Jeff is always around to keep us on the straight and narrow.  As a pro guitarist he has a real need to get to grips with tone in order to eat.  You would think her would be advocating Beryllium coated scratch plate screws as "they give a lot more clarity to the upper registers". But he always describes the sort of change that you get in issues like this as "at best subtle", and he's spot on.  Don't believe the hype spewed out by people on some other forum sites, (MaiLezPawl - harrummpphhh! sorry, I coughed), who don't really know they just want to be seen by others to know, (the Emperor's New Clothes Syndrome).  Music spawns myths!  Like tadpoles!!  And only because musicians actually want to believe them!!!  We all would like to believe in magic, but we don't.  So why do we believe in the mysterious "mojo" and claim it can cause things that Physics and engineering surely can't explain or disprove?  When you challenge these people to demonstrate their claims under proper conditions they generally can't or won't.  Not interested because what I say is just true, so there!

You mustn't be like that.  You must listen and only trust your own ears.  Other people's good advice on forums like this, (and mine too), should only steer you to the questions you should ask to find the truth.  And then you believe what YOU hear, not what you're supposed to hear. Jeff is right, (that's repeatedly one of the most annoying things about him ), you may just about hear a difference when you alter some of the talked about factors but they won't make a Peavey into a Fender.  They are very subtle even under home quiet conditions.  And under real life playing conditions, in a band, on stage, how many of the audience are thinking "I bet he's got a 5751 in his input stage, or is it a 12AT7"?  The guitartelligencia make out so many things are essential to be a true guitarist and a member of their club but it's all fluff.  Get those valves in and play, that's what the whole thing is about.  You can decide that you would like to experiment when you feel that something is wrong and you can be SURE it isn't how you are using the amp. And don't let anyone put worms into your brain about changing things, if it sounds alright to you as it is then it doesn't need to change at all.

Haha, thanks for the elaborate response. This is the first time I have ever actually joined a forum, and for the reasons you stated above, I need some sort of supporting evidence or information, especially on a topic that I'm not completely familiar with. I am used to this sort of thing as far as guitar posts though. No offense to anyone, but musicians aren't known for the technical, but rather subjective experiences. I have always went with the motto "take what you can get and leave the rest!" Hopefully we have the sense to know what to take and what to leave. If I don't, I just keep researching. Thanks again. I find all of your posts here extremely valuable. And of course voodoo Jeff as well!!

Now look....I step away for a few hours and the mudslinging is so quick to start (LOL. I kid).

One of the things I appreciate the most about this particular forum is the truth in advertising, so to speak. You get a few people who will post up "well, I heard that such and such would have so and so effect, so it must be gospel". Then someone else will post up "well, here`s my real world implementation of such, and I got f*ck all from it", with some convincing evidence (for the non UK folks, F*ck all is another way of saying jack sh*t).

I posted a demo video of this amp, comparing the stock tubes to some very, very expensive boutique level offerings. It was......subtle at best. I now own two GM36`s, the second has the very pleasant but FAR less expensive JJ`s tubes in it. The two have their own character, but I would not choose a $100 tube over the $13 tube knowing the difference between the two. Both are measurably better than stock and give the amp the subtle bump it needs to be remarkable.


With all that said, I did also want to mention that you may just have a tube in there that clips early, especially if they`re stock. At $10 a pop for JJ`s (and you`ll rarely find someone with a negative thing to say about them), it`s a paltry investment that will give the amp an edge over out-of-the-box regardless.

I didn't know you were multi-lingual Jeff. Oscar Wilde would be proud.

Hey, english to English is easy enough (I actually have family all over the UK, and my buddy Mat was the national champion for his age group in BMX racing there). My spanish, on the other hand (despite growing up with two of my best friends families from South America, and taking 5 years of it in school for the easy A) is pretty dismal. A dubious position to be in when I`m headed to Costa Rica in a few weeks.

Lot's of good info so far, so I'll add some anecdotal info.  My #1 right now is an SG with a JB in the bridge.  I'm assuming you're referring to the full size JB and not the strat form factor JB.  I have the JB set pretty close to the stirngs - at least 3/16 (if not closer) with the high E fretted at the 12th fret.  I play fairly lightly...
I have all my Clean channel patches set with the Gain at 255 aka "10"  That way if I dig in just a little I have some hair.  If I play lightly or back off my volume pedal just a little it's sparkly clean.  I don't have any tone bypass circuits on my guitar's vol pot, so if I roll back the guitar vol is does go clean but darkens a bit.  I'm currently using  EH 12AX7 in V1.  In V2 and V3 I have EH and JJ 12AX7s, but I can't remember the order right now.  But I found V1 to have the most dramatic effect on the Clean Channel (and Crunch too).  It's possible you have a hot 12ax7 in there, but the gain (is Gm the right parameter here?) per stage would have to above 115 to really be hot sounding?

On a side note, I just got two Tung-Sol 12AX7s from Doug - one with a gain of 90 and the other with the gain of 115 (both stages per tube).  I'm not going to swap until after tonight's gig - but the plan is to put the 115 in V1 and the 90 in V2, and play some more with my variety pack 12AX7s in V3. I like the Clean to have a bit of hair with the Gain on 255. I'm using the Clean channel with Gain at 255 for leads more lately and hitting with either a Tube Screamer or over drive or EQ with gain - very pick responsive!   On a side side note, I love the "brown" of the JJs, but I play in an Allman Brothers tribute band (two drummers, two guitarists, keys, harp), and I just don't cut thru using JJ 12AX7s (or 5751 or ...). I am trying to tame the Lead and Ultra without making it darker, that's why I'm trying a lower gain Tung-Sol in V2.

All anecdotal - take it for the price of admission.
Good Luck!
-BobK

Hey hey Namklak.  Interesting info there.  Everything you say makes perfect sense.  The only bit I'm curious about and which I think has a question mark over it is the effect of using different gains in V3.  It can depend a bit on whether that phase inverter stage is set up around a single triode or the pair in one valve.

I can tell you that the TM18 uses a single triode 'concertina' type.  It works by acting as a cathode follower at its bottom end.  This means that the voltage output from the cathode is exactly (or very nearly), the same as the input signal on the grid.  The cathode resistor also effectively turns this signal into the current going through the valve and its anode load resistor.  The anode load resistor is made the same value as the cathode resistor, which means that because the current through them is the same the voltage across them is the same.  But because one is coming up from ground and the other is coming down from the HT, they are 180 degrees out of phase.  Hence we have our two equal and opposite drive voltages for each of the output valves.

Now, the circuitry in the TM18 has feedback applied in the cathode of that stage to compensate for varying gain in the triode!  It effectively detects the higher or lower gain and adjusts the stage to behave the same as before.  So I really wonder if there is anything to be gained (urrgh! ) by swapping it out for differing gain types.  I'm not really sure on that one so I'm not going to do my usual "YOU'RE WASTING YOUR TIME" job in this case.  I'll be interested in whether you really think you can hear a difference if only that valve is changed.  It's no good doing it as a package with other changes, you can't tell which is having an effect and which may be doing nothing under those circumstances.

It's even more compensatory if they have gone for the long tailed pair approach which uses both triodes in one valve to each drive an output valve, with the triodes sharing a common cathode resistor to give exact opposite outputs on their individual anodes.  The feedback in the stage, (not added around it as most people seem to fear, actually naturally in the stage itself), works well at stabilising the conditions against changes in gain.  It isn't exact of course but it is pretty effective.

EDIT:  Just thought, a good way of thinking of this is to realise that the circuitry is deliberately set up so that the stage gain is defined by the values of resistors and actually excludes the gain of the amplifying devices themselves.  If the stage has a gain of say Ra/Rk, only those two resistors, (within reason), will have an effect and the gain of the valve has none, (or really very little).

Please let us know what you find.

Hmm, weird thing - that guy sent me that schematic, but I can't seem to open the message now in my Inbox on this forum. I can see the msg, how do I open it?

Anyway, not surprised about the single triode inverter to drive the other phase. And from your description, I agree with your analysis. But, what does the other triode in V3 do? Is it a gain stage for one or more channels? I doubt they are driving effects or the Effects Loop with it... If it is another gain stage, then swapping in different gain tubes might have an audible effect. And feedback would reduce the differences between different 12ax7. But again, what does the other triode do?

Interesting about the silicon clipping at 1.8V - I thought they'd only do that in Boost... So when I'm hitting the front of the gm with gain, I'm getting some silicon clipping eh? I'm not whole-ey against that, but I do kind of like the idea of doing that when I want to, say with my own TS er whatever. Or having it midi controllable. but it sounds good, so... I've found I like OD into the front of the amp Mostly on the Clean channel - extra silicon clipping might explain why.

My guess is that the buffers for driving the loop and accepting the signal back in are solid state.  That's what I would do.  There is absolutely no benefit whatsoever in using valves to do that job.  It just wants to be completely neutral and do its matching task without changing the signal in any way once the preamp valves have done their shaping work.

There's a limit to how close the GM36 will get to the TM18's configuration as the TM only has two preamp valves to play with compared to the GM's 3.  The TM has only two channels, Clean and Lead, so it sets up the same valves with different gain for each channel.  It is entirely possible that the GM dedicates some triodes to particular channels.  Maybe one valve, (two triodes), are used for the Clean and Crunch and perhaps Lead channels and an additional triode gets added in only for Ultra or maybe Lead and Ultra to give the extra gain.  It's entirely possible they use 2 triodes for the Clean/Crunch, 1 extra for the Lead and a different extra for the Ultra to voice them differently.  That leaves us a triode to drive the tonestack, or what does that tone shaping job, and maybe two for the phase inverter, or just one with a possible single triode left over for.....?

There are a lot of different ways you could set up the 4 channels with those valves as long as you were pragmatic and used SS for the transparent tasks.  Then of course there is the addition of the effects into the circuit....

I should have said as well that the 1.8V clipping level is set within that solid state input stage.  First there is a catch all pair of back to back zeners to limit the input to about 3V peaks just for safety.  (People do daft things with boost pedals!)  Then it is based around an opamp with zeners in the feedback loop, 3 in one direction one in the other so the clipping is assymetric for more even harmonics.  They start clipping on the first side of the signal at around 5.7V at the output and the gain of the stage is set to 3x.  5.7V/3 = 1.9V so the onset of clipping will be around 1.8V signal level, maybe a little lower.  When the Boost is kicked in the gain of the stage is lifted to about 7.5x dropping that level to about 0.75V.  Most humbuckers achieve that easily.  In truth the secondary point where both sides start to clip will be very difficult to reach, so this is mainly a mild overdrive to add richness rather than full out distortion.  The gain is also set to roll off at about 48kHz, a little over an octave above audible and about 2-3 octaves above our signal.

I really think it's quite a clever setup as it leaves you with the option of just turning down on the guitar to below that threshold and still having enough oomph to drive the later stages properly.  When you just go over that lower threshold you are only putting in additional single sided clipping which is rich in even harmonics, the musical kind.  The higher level of clipping is set just below the supply line level which prevents the opamp from clipping internally.  Now that SHOULD satisfy the purist buffs who always want to roll the opamp for their own "more musical" one, (but I suspect it won't ).  All opamps sound exactly alike in our application as long as they are used well within their designed parameters.  There must be no internal clipping, no low impedance loading on the output, no driven hard enough to make any internal safety circuitry kick in, no out of band frequencies allowed in the input signal.  You can't tell any of them apart under those circumstances, "sonic signature" is to do with them dealing with bad behaviour.  Under those conditions even the humble little TL071/2/4 family achieves over -150dB distortion levels, and creating any sort of a sonic signature is the equivalent of adding distortion to the signal.  For those not used to thinking in dB that is 0.0000032% distortion!  Reasonable hifi amps nowadays sit at about 0.005% and we actually want distortion.  What price the opamp being identifiable there?

We have another option in our distortion palette here.  When the GM's Boost is kicked in it ups the gain of that solid state front end.  It also affects the gain and signal levels in the valve stages too increasing their distortion contribution and we can't really prevent this.  If we use an external clean boost pedal rather than the internal boost, we can kick the solid state input stage into overdrive but turn the signal down internally to not drive the valves so hard, and we get the "Tubescreamer into a cleanish amp" sound.  Clever guys these H&K fellas and boy is their little baby versatile!!!

Did the OP get a satisfactory answer - or come up with his own?

I think by about post 10 Posborne had had his original question answered and also a secondary thing he moved to Namklak. His responses seemd to say that he was satisfied with the replies. After that it was taken over by us and expanded as these threads often are. (Did anyone over there actually get the Jack S**t reference in my pic? )

I know I whitter on a lot in my posts but I am aware that part of the value is to to lay down easily accessible information for others who may come across the thread in passing. It's too easy to just focus on the guy who started it in isolation and we often forget here that once the thread has died it isn't always easy to pick out the useful stuff when you come into the forum cold. When I get it wrong with my long posts it can kill a thread dead I know, but if the thread is already cold it sometimes puts the relevant info in that place where a casual searcher will always limit themselves to looking, in the last few posts.