Issues with Clean and Crunch channels (low volume, Crunch is clean)

Hi guys, I just received my GM40 from a dealer as a used unit. It looks to be in pristine aesthetic shape however I am having an issue.

Firstly, there is a drastic difference in volume between the Clean/Crunch channels and the Lead/Ultra channels. I mean DRASTIC. Lead and Ultra is like 5-10x louder than Clean/Crunch. If I have all gain/volume knobs at 12 o clock using Clean, I would have to reduce the MV to the 7 o clock position when playing Ultra in order to get a similar decibel output. This is the same using all powersoak settings or 40W setting. I have also performed a factory reset and the problem persists.

Secondly, the crunch channel is clean. Pretty much as clean as the Clean channel, maybe just a touch brighter in tone.

Performing a bias check does not reveal any issues, all lights blinking within 1 count of one another.

Help would be much appreciated. Thank you

Leo

Hi Opnoob.  The different channels in the GM40 use different combinations of the preamp valves.  If one of them has a triode in it which is going soft then it could affect your volumes in that way.  Have you a couple of spare valves that you could swap in for them.  If not, don't worry, just try swapping over the preamp valves to simply see if the situation changes.

Looking at the amp from in front, the preamp valves are the two smaller ones on the right of the three.  That's the one under the aluminium can and the one next to it in the middle.  The can is a bayonet type fit like a light bulb and just pushes down and gently twists then lifts off.  One important point to keep in mind here, remember which way around you took them out so you can put them back in the same slots after the test.  Sometimes valves may be specially selected for low noise or balanced operation and the first position under the can, the input stage, is the one where you want your quietest valve with the last one, the phase inverter, where you would want balanced specs.  Don't mix them up unnecessarily.

Let us know what you find with that simple test and we can go from there.


EDIT: I should also advise here that there is a procedure for removing the valves though it is only to preserve the warranty which won't apply in your case. To get at the valves, remove only the top two screws and the middle rear one on each end cheek. Ignore the front middle one as it only serves to hold the handle on the end cheek. Then simply slacken the two bottom screws a few turns, enough to let the end cheeks flop out so you can remove the top and front cover. When refitting make sure to dress the LED strip and its cable carefully so it does not get pinched anywhere. It will all be pretty much obvious once you get the lid off.

bordonbert wrote:Hi Opnoob.  The different channels in the GM40 use different combinations of the preamp valves.  If one of them has a triode in it which is going soft then it could affect your volumes in that way.  Have you a couple of spare valves that you could swap in for them.  If not, don't worry, just try swapping over the preamp valves to simply see if the situation changes.

Looking at the amp from in front, the preamp valves are the two smaller ones on the right of the three.  That's the one under the aluminium can and the one next to it in the middle.  The can is a bayonet type fit like a light bulb and just pushes down and gently twists then lifts off.  One important point to keep in mind here, remember which way around you took them out so you can put them back in the same slots after the test.  Sometimes valves may be specially selected for low noise or balanced operation and the first position under the can, the input stage, is the one where you want your quietest valve with the last one, the phase inverter, where you would want balanced specs.  Don't mix them up unnecessarily.

Let us know what you find with that simple test and we can go from there.


EDIT:  I should also advise here that there is a procedure for removing the valves though it is only to preserve the warranty which won't apply in your case.  To get at the valves, remove only the top two screws and the middle rear one on each end cheek.  Ignore the front middle one as it only serves to hold the handle on the end cheek.  Then simply slacken the two bottom screws a few turns, enough to let the end cheeks flop out so you can remove the top and front cover.  When refitting make sure to dress the LED strip and its cable carefully so it does not get pinched anywhere.  It will all be pretty much obvious once you get the lid off.

Thanks! I ordered some ECC803S's. Will replace all three pre-amp valves and report back.

Just wanted to provide an update for the record. I changed out all 3 pre-amp tubes and the problem was fixed! Thank you

First port of call with any volume or lack of sound problem is to swap out the valves in turn for a known good one. You don't even need a full set and often it is enough to swap the existing ones around in the amp and hear a big change. They don't have to be new just working so you know that if the problem is still there with them in place it isn't the original valves causing it. It's not only the logical option, people often miss that it's the quickest and cheapest starting point too, as long as you don't just knee jerk and fork out for a full set of valves just to test the idea! You would be surprised how many people just do that immediately and find they didn't need them. You can buy new valves once the swap with an old but working one shows that is the problem.

Glad it sorted things out. Keep an eye on your sound and, in particular, use the TSC Bias test on the back. If the preamp valves were iffy, the power valves might be too as they are likely the same age.

Thanks for letting us know you got it fixed. Your solution might just help someone else with a similar problem.

bordonbert wrote:First port of call with any volume or lack of sound problem is to swap out the valves in turn for a known good one.  You don't even need a full set and often it is enough to swap the existing ones around in the amp and hear a big change.  They don't have to be new just working so you know that if the problem is still there with them in place it isn't the original valves causing it.  It's not only the logical option, people often miss that it's the quickest and cheapest starting point too, as long as you don't just knee jerk and fork out for a full set of valves just to test the idea!  You would be surprised how many people just do that immediately and find they didn't need them.  You can buy new valves once the swap with an old but working one shows that is the problem.

Glad it sorted things out.  Keep an eye on your sound and, in particular, use the TSC Bias test on the back.  If the preamp valves were iffy, the power valves might be too as they are likely the same age.

Thanks for letting us know you got it fixed.  Your solution might just help someone else with a similar problem.

Thanks for your help, I didn't really want to experiment too much so I just full-swapped all the pre-amp valves especially since I was swapping to a different model number (803s as opposed to 83s). Its likely that 1 or even 2 of them were fine. I could not visibly see any damage to any of the tubes. I did notice that after swapping, I no longer saw the faint firefly glow that I assumed was normal (not red plating, just a small glow). I'm new to this but I would assume it could be due to the seemingly larger heat plate inside the tube able to dissipate heat more efficiently.

As for the power tubes, I still see the firefly-esque glow from all 4 of them (again, not red-plating). If these were on their way out, what would be the symptoms?

You shouldn't ever see any red plating at all with preamp valves they don't dissipate anything like their maximum wattage and there is no way to increase that other than a fault in the stage circuitry or a serious fault in the valve.  Their stages are designed to tie down the current and hence the dissipation to a fixed value which only varies by an  insignificant amount as they work, not the same as the output valves.  (That's one reason to NEVER swap out for a 12AT7, 12AU7, 12AY7 which are NOT equivalents to be swapped at random.)  You may see a little blue corona glow around the cathode when they are emitting electrons.  When they go South they are usually either obviously not working as in not heating up, or they gradually go soft and decrease in their characteristics and you hear the amp going flat.  You do get valves which blow when things inside warp and short out but it's rare.

Do read up on both sides of the "valve swapping" debate.  As a professional engineer for a full lifetime I can tell you the idea that valves all sound different is a myth.  It's put out there and stoked in order to get you to buy more valves.  There are very subtle differences under some conditions but they are of the same magnitude as the production spread differences between valves from the same maker so are meaningless.   And they get lost entirely when you play with other musicians.  For a bedroom only solo player you can afford to obsess over the slightest detectable aspect of the sound.  What else have you to do with your life?  But when you come down to it, the idea of chasing "the magical sound in my head" is NOT the way to go about this business.  As Bobby McFerrin said, "don't worry, be happy".  I play a variety of amps, H&K, Marshall, and my own designs both valve and SS.  I'm happy playing all of them.  They all sound very different but I use them as they are for the time I am playing them.  I put JJs in all of them because they sound as good as any others in my opinion, are cheap and easy to get, and last well in my experience.  It's ok for the megastars to throw a tantrum because a valve is not the specific make, model, date and comes in the right colour of box that they want but most decent guitarists have my unfussy attitude not theirs.  You will burn up and waste a lot of energy, time and money chasing a valve contribution that doesn't really exist in any significant quantities.

Modern test equipment doesn't lie.  I can put valves on the bench and immediately show you there are no differences.  Ears do!  Especially other people's ears which can hear elfin bells, spring water clear mids and underground cathedral chasm-like bass in their own chosen monster valve - of this week.  Only rely on your own and have the courage to say "I can't hear it" if you can't hear it, instead of trying to make yourself hear something someone is telling you is there which you can't get.  Waver long enough and - - - you and your wallet will most definitely hear it.  Until you get home and it's gone, along with your money.

Remember, the wise child asked "why is the king wearing no clothes?" then everyone wondered the same thing.

bordonbert wrote:You shouldn't ever see any red plating at all with preamp valves they don't dissipate anything like their maximum wattage and there is no way to increase that other than a fault in the stage circuitry or a serious fault in the valve.  Their stages are designed to tie down the current and hence the dissipation to a fixed value which only varies by an  insignificant amount as they work, not the same as the output valves.  (That's one reason to NEVER swap out for a 12AT7, 12AU7, 12AY7 which are NOT equivalents to be swapped at random.)  You may see a little blue corona glow around the cathode when they are emitting electrons.  When they go South they are usually either obviously not working as in not heating up, or they gradually go soft and decrease in their characteristics and you hear the amp going flat.  You do get valves which blow when things inside warp and short out but it's rare.

Do read up on both sides of the "valve swapping" debate.  As a professional engineer for a full lifetime I can tell you the idea that valves all sound different is a myth.  It's put out there and stoked in order to get you to buy more valves.  There are very subtle differences under some conditions but they are of the same magnitude as the production spread differences between valves from the same maker so are meaningless.   And they get lost entirely when you play with other musicians.  For a bedroom only solo player you can afford to obsess over the slightest detectable aspect of the sound.  What else have you to do with your life?  But when you come down to it, the idea of chasing "the magical sound in my head" is NOT the way to go about this business.  As Bobby McFerrin said, "don't worry, be happy".  I play a variety of amps, H&K, Marshall, and my own designs both valve and SS.  I'm happy playing all of them.  They all sound very different but I use them as they are for the time I am playing them.  I put JJs in all of them because they sound as good as any others in my opinion, are cheap and easy to get, and last well in my experience.  It's ok for the megastars to throw a tantrum because a valve is not the specific make, model, date and comes in the right colour of box that they want but most decent guitarists have my unfussy attitude not theirs.  You will burn up and waste a lot of energy, time and money chasing a valve contribution that doesn't really exist in any significant quantities.

Modern test equipment doesn't lie.  I can put valves on the bench and immediately show you there are no differences.  Ears do!  Especially other people's ears which can hear elfin bells, spring water clear mids and underground cathedral chasm-like bass in their own chosen monster valve - of this week.  Only rely on your own and have the courage to say "I can't hear it" if you can't hear it, instead of trying to make yourself hear something someone is telling you is there which you can't get.  Waver long enough and - - - you and your wallet will most definitely hear it.  Until you get home and it's gone, along with your money.

Remember, the wise child asked "why is the king wearing no clothes?" then everyone wondered the same thing.

Thats interesting, I wonder if I was mistaken when I "saw" a bit of red glow from my pre-amp valves. I obviously see none now with the new valves inside. Its great to get your engineering expertise on it, I'm not one myself but I do come from a family of engineering psychos and consider myself a man of science
I am also of the opinion that there is WAY too much snake oil in the guitar space. I'm relatively new to it but the amount of stuff I read and see online about this and that changing tone has me rolling. I think people still have this inclination to view electric guitars as similar to acoustic guitars. The amount of weight the electric guitar industry places on "tonewoods" is so laughable. While I don't think the effect of wood type is zero, I believe its negligible. I assume the same about tubes. I assume as long as the tubes are performing within spec, it is indiscernible from another tube of similar spec.

Well said that man! Keep to that mindset and you won't go far wrong. Anything which is claimed should be capable of being demonstrated. That's the killer step.

In the meantime, you can now get to grips with making your kind of music with that amp. They really are great for some genres. I have a host of other amps that I use for some things but I will never part with my own H&K GM36 along with two other amps I own, and your GM40 seems to be noticeably more flexible than that.

bordonbert wrote:   Well said that man!  Keep to that mindset and you won't go far wrong.  Anything which is claimed should be capable of being demonstrated.  That's the killer step.

In the meantime, you can now get to grips with making your kind of music with that amp.  They really are great for some genres.  I have a host of other amps that I use for some things but I will never part with my own H&K GM36 along with two other amps I own, and your GM40 seems to be noticeably more flexible than that.

One question: is there any tangible difference between the mods on the GM40 compared to standalone effect pedals? ie. is there any meaningful difference between the Reverb feature on the GM40 compared to a Boss Reverb pedal? I did order a sus/comp pedal since it's not included in the amp. And also a couple of tone/overdrive pedals to use in conjuction.

Good questions!

I have to admit, I do own a selection of pedals collected over the years, (mostly cheap as I don't see any difference between them and the ridiculously priced ones which are just clones of the cheaper version, and I can design and mod pedals in my sleep), but when I use the GM36 I don't use a single one of them. I stick to what is in the amp and, in a live situation, it sounds perfectly acceptable to me. I can't really see the point of once again obsessing about whether a reverb truly represents a plate or a room. It's like playing live in a large high venue and complaining it doesn't sound quite like the Albert Hall to me. Reverb is reverb is reverb. Of course you might disagree with that, that is an area where I am prepared to accept it is only my opinion as people's expectations are different and you can hear differences between some pedals. That said, not too many overdrive/distortion pedals sound different to my ears. There are genuinely only a few distinct circuit approaches and most are clones of each other with a little tweaking in cheap passive components. (And a 400% markup added on for that of course.)

However, overdrive pedals! Have a look here: H&K Input Buffer The H&K amps already have an overdrive pedal capability built in if you just get to grips with using it carefully. I only have access to the schematics of the TM18 and the GM36 but the TM/GM range do share some "family" design aspects across the board. The GM40 should be the same in these aspects otherwise it would have become the "NA40" or something such. In my opinion it is a common mistake nowadays to want to use too many generators of distortion at the same time. They all have their place, but, a hard driven overdrive pedal driving hard into the GM40 input buffer with the Gain of the Ultra channel turned high and the Power Soak set low so the Master Volume is up pushing the power amp to within an inch of its life is not the way to get the best sound out of the system. There are a few thoughts in there about being a little more selective and sparse with that.

bordonbert wrote:Good questions!

I have to admit, I do own a selection of pedals collected over the years, (mostly cheap as I don't see any difference between them and the ridiculously priced ones which are just clones of the cheaper version, and I can design and mod pedals in my sleep), but when I use the GM36 I don't use a single one of them.  I stick to what is in the amp and, in a live situation, it sounds perfectly acceptable to me.  I can't really see the point of once again obsessing about whether a reverb truly represents a plate or a room.  It's like playing live in a large high venue and complaining it doesn't sound quite like the Albert Hall to me.  Reverb is reverb is reverb.  Of course you might disagree with that, that is an area where I am prepared to accept it is only my opinion as people's expectations are different and you can hear differences between some pedals.  That said, not too many overdrive/distortion pedals sound different to my ears.  There are genuinely only a few distinct circuit approaches and most are clones of each other with a little tweaking in cheap passive components.  (And a 400% markup added on for that of course.)

However, overdrive pedals!  Have a look here: H&K Input Buffer   The H&K amps already have an overdrive pedal capability built in if you just get to grips with using it carefully.  I only have access to the schematics of the TM18 and the GM36 but the TM/GM range do share some "family" design aspects across the board.  The GM40 should be the same in these aspects otherwise it would have become the "NA40" or something such.  In my opinion it is a common mistake nowadays to want to use too many generators of distortion at the same time.  They all have their place, but, a hard driven overdrive pedal driving hard into the GM40 input buffer with the Gain of the Ultra channel turned high and the Power Soak set low so the Master Volume is up pushing the power amp to within an inch of its life is not the way to get the best sound out of the system.  There are a few thoughts in there about being a little more selective and sparse with that.

Yeah I purchased everything used at a good price so I won't be too disappointed lol. I heard good things about the Keeley compressors though and after some research decided on that plus a Fender Santa Ana and Dude Screamer pedal. I also still have a Multi Effects Boss ME50 if it doesnt tickle my fancy. It sounds decent enough.

The compressor I use is the one single pedal I paid a high price for. It's an Origin Effects Sliderig. I play some numbers as slide in the band and having a decent compressor really helps with that long sustain. Lowelll George of Little Feat is a slide player I really admire and he used two studio compressors in series to get his super long clean sustain. The Sliderig does that using basically the same compression circuitry as his original versions in a single pedal. For the price I get a single compressor setup which I can turn into a series double. It gives me some great slide sounds and I'm not a true natural slide player, just a guitarist who can add a little of that to the band.

I follow a guy on YouTube called Paul Stafford Cook who is a wonderful guitarist and has a great interesting YouTube channel. He also gives out some of his own thoughts on good gear. He recently did a comparison of valve v solid state overdrive pedals and included expensive as well as ultra cheap ones. Paul Stafford Cook Overdrives. (Make sure to listen to him from about 9:15 onwards without any pedals at all to see his scope.) Thanks to him including the ultra cheap Nux Steel Singer I bit the bullet and bought one. I have overdrives of my own design using a few different techniques and they work well, (two back to back diodes is done tech and all the new boutique versions are simply rehashes of things gone before, save your money!), I also have an old Marshall Bluesbreaker pedal but the Steel Singer is the one I currently use live, though not heavily. Like PSC I think that there is a lot to be said for learning how to get tone out of just an amp, a guitar and a basic cable (self made of course, why pay a fortune for a piece of copper wire?)