GrandMeister 36 Poweramp Issue, pls help!

Howdy y'all! You all seem very friendly and knowledgeable, I come to you in dire straights as I've been trying to fix this amp for a month now

So the basic problem is that the amp is saturating the power tubes super early, has low volume, and is constantly shutting tubes off. I bought new tubes but the amp continues to behave this way, I've checked all voltages and I can't find any inconsistencies tube to tube, is there something I may be overlooking? It tends to oversaturate tubes 1 and 4, which may help someone reading this.

Thanks for reading

You say " I've checked all voltages and I can't find any inconsistencies tube to tube". I am reading from that that you have been inside and measured voltages at points in the circuit. Is that right? I don't want to get into advising someone to get up to their elbows in 500V when they really shouldn't be doing that. It bites. It kills. What experience do you have in working inside high voltage areas like your amp, and please be honest and pragmatic here?

How much of the circuit design would you understand? This is not a simple Marshall/Fender point to point classic style amp. It is a complex modern surface mount PCB based design and it is much more complex, very compact and at times awkward in there.

That said, what facilities do you have to read voltages inside the amp? Do you have a decent voltmeter with slim probes? I don't mean a Fluke or anything exotic or expensive, just do you have something reliable and reasonably accurate with probes which will precisely measure voltages on sometimes small points?

Hey! Thanks for the reply

I appreciate the caution, as it can get dangerous in there, but I'm pretty familiar with working with high voltage environments. As for my knowledge, I'd say I'm learning but have a general understanding of how things work, enough to fix an old Fender SS amp but realizing the rectifier caps had fried or fixing a compressor on a Series Lab 5 by reading the schematic and inferring the IC had gone bad.

Because of my background in pedals, tubes are actually a little newer for me as far as concepts go, it took a minute to figure out how that phase inverter was working in this one! The voltmeter I have is from home depot, not the most accurate but strong enough to handle the high voltages and accurate enough to infer if something is significantly off, I also have an oscilloscope and a signal generator for troubleshooting.

I'll try to keep this short, but this is how it's gone so far..

The amp seems to bias fine enough, but is constantly hitting the power tubes with too much current, I've been able to isolate it as a power amp issue as the preamp works just fine from the FX loop, which means at least the problem is from the phase inverter up, I've also tried running preamps into the power amp section and the symptoms are the same. Some in this forum have described similar, a significant drop in volume and when you bring the master volume beyond two it starts shutting tubes off.

I initially suspected maybe something in how it's feeding power to the tubes, but everything seems fine there, with (as I understand it) the typical voltage drop upon engaging the standby switch. I've also checked all the usual suspects in resistors for a short/failure and nothing comes up.

Thanksfully I found this to explain the phase inverter a little bit, seems ok there. I can't post links but searching 12ax7 guild phase inverter it comes right up


actually... I'm pretty sure it was hitting all the tubes with -41ma, this is I'm now realizing way too much (or is it ok in this design?). Still learning >.>

Is there a component of the self biasing that would effect all four tubes in the same way?

Hopefully in this was something you could latch onto, thank you so much for taking the time to read this.

Good news on your experience front then.  Ok.

The PI is a simple long tailed pair differential amp configuration.  It's pretty much the standard for the majority of valve amps around.  I wouldn't think there would be a problem with that but I would test the valve just to rule that out by simply swapping V2 with V3.  It only takes one side of that LTP to go off and odd things can happen.  Playing another amp through the Fx Return is a good way of just getting the MV circuitry and the PI/Output stages tested in isolation as you know.

You may know most of this already but just in case...  The GM36 has an unusual biasing arrangement.  The bias is controlled via a microprocessor setup, the TSC, which reads the current level in each valve level by sensing the voltage drop across a 1R5/1W resistor in its cathode.  It adjusts this to be correct by controlling a MOSFET in each cathode line.  It has been known for those MOSFETs to get damaged.  If one of them is kaput then your amp would show symptoms similar to what you describe.  These are STP10NK60ZFP and they are readily available and pretty cheap at £2-£3 each here in the UK.

There is another step I would recommend here.  The TSC can actually be reset to relearn its necessary bias level for each valve.  Turn the amp off.  Set the Standby switch to Play.  Turn the amp on.  This may take some time to kick the TSC into action.  When people have tried this procedure they report that the bias doesn't really settle down for a few On/Play/Off steps.  The LEDs can show some odd results for the first couple of times you use it.  It seems to get nearer the optimum each time you switch the amp on as normal and use it.  Could be worth trying.

You say you have measured voltages around the valves and they are pretty much the same.  Did you check out the voltages in the cathode circuitry?  You could measure the steady state quiescent voltages around that part of the circuit and compare each one to see if there is a rogue setting or one which varies.  Looking at the voltage across each cathode resistor on your scope with no signal applied should give you an idea of how they are behaving too.  Don't be surprised if there are differences, no valves are exactly the same.  What you don't want is one which is way off or which flickers and jumps around.

Have a think about what I've said and see where you think you can go with it.  I'll also PM you!

This is great info! Is there any chance I would notice anything off testing voltages across the mosfets? I'm going to check again today but I remember them being the same DC current in each (although tbh I wouldn't even know what to look for)

or is this a case where I'd have to unsolder them to check for any irregularities? Thank you again for all the help so far! I'll be trying to reset the TSC today as well, ty for the knowledge


I've checked the cathodes as well, they're all within 2mv of eachother (which makes sense, the tubes were supposedly matched) I'll look around the resistors again as well

update, I tried the reset and did actually get a small boost in volume to the speakers, however it's still way below what a 36w amp should be putting out.

No, don't desolder anything yet, there is nothing we know to suggest you have a problem there.  All you need to do is measure the voltages across a few points in the circuit, if you are able to safely!

Firstly there are the four 1R5 (1.5ohm) cathode resistors.  These are labelled R89, R91, R132, R4.  (R4?  Yes that's really correct.  Go figure!)  You should find these alongside each of the 4 MOSFETs on the Mainboard at the opposite end to the fuses.  Each of them has a PCB test point at one end which should be there to make it easy to do this sort of checking.

Now the practical problems start.  It looks from the pics I have found online and from what I know of my own GM36 as though the area we want to work in is totally hidden by the Rearboard.  You may find that this sort of work is just not possible without jigs and other pieces of kit.  I think testing anything like this would definitely mean removing the Rearboard but keeping it attached and running the amp up with the board out of its mounting.  BEWARE!!!  That is very delicate, I could say dangerous.  There is your life at stake but that can be mitigated by just keeping your hands in your pockets.  There is also your amp to consider if something goes wrong and slips.  You can do a lot of damage here, lots and lots.  This is not just a Fender with a few valves and a lot of old worn out carbon resistors working on steam we are talking about.  It is a modern precision piece of equipment with a lot of solid state circuitry which will fry in an instant.  It would get much more costly to replace a Rearboard and Mainboard and who knows what else.  I know the idea of fixing this yourself is very enticing, I think that way in a lot of things myself too, but it may be safer to leave this sort of thing to a professional with a full workshop at his disposal.  Seriously consider it before you go ahead and pull things to bits!

Considered it?  Well here is a bit more info which might interest you.

There is one other thing I would point you at.  The MOSFETs are there for Standby/Muting purposes controlled via 4 basic transistors from the TSC so it can kill a pair of valves if they have a problem.  The bias is really controlled via some other simple resistor circuitry on each of the grids.  The TSC has a tiny ATMEGA88 microcontroller which continuously monitors the four cathode voltages and outputs four PWM (pulse width modulation) signals in response to those voltages.  The width of the pulse is set by the voltage level at the cathodes.  There are four ultra simple 3 transistor converters to turn these signals into DC currents to feed to each valve grid to set its bias.  Each grid has a pair of resistors, 39k and 27k, across a -63V DC line which would set the DC bias voltage to around -25.8V, and a 22uF capacitor to smooth out the remnants of the pulses.  The TSC circuitry then adjusts this by drawing its own current through the 39k resistor which increases its voltage drop.  If you draw out that setup with the resistors you will catch the drift of it.

I have set this up in LTspice, (the industry standard circuit simulation software), and it seems to work very well.  With a 50:50 pulse chain out of the microprocessor into the 3 transistor converter circuit the voltage settles at around -16.4V after about 2 seconds.  With 55:45 it settles at -17.5V so you can see the bias voltage altering according to the pulse chain mark space ratio. Of course my simulation has no feedback from the valve cathode into the microprocessor to alter the pulse width for each valve, so the voltages are simply to show how the system works.  In reality I would expect the voltages to be just somewhere around this.  I've attached a couple of plots of the various voltages in the system from my Spice simulation.  Notice the differences in the timescale across the bottom.

If your valves are reasonably matched you should find they all have similar voltage levels both on their grids and across the cathode resistors.  As they age they will drift and begin to vary but that isn't disastrous.  As long as the voltage across the cathode resistor is the same(ish) the grid voltage needed to produce that will not matter.  That is the point of the TSC system, even unmatched valves can be made to match up in terms of bias currents.  Of course you reach a point where the TSC can no longer adjust and correct the drift and that is when you need to replace your valves.  The TSC LEDs will show that level of drift.

Now to the work you can probably do.  Make a sensible decision as to what you can access and measure given the complex compact build of the amp internally.  You don't have test jigs available to you to hold things when they are being proved.

I'm back, and ty for the detailed analysis! From what I've gathered the four transistors are putting out evenish current. But it's all around 29-30mv at each testing point. This isn't anywhere near any of the values you discussed. This is approaching going over my head, but it seems to be the self biases system is essentially working in matching to the tubes, but as a macro it's off. Is there some overarching component that could cause all four to drift in unison?

Ty again for your time and patience, I feel like I'm getting very close to the problem at hand, or at least determining it's the microcontroller that needs replacing :/

This is going to be long I'm afraid but read through it as there are suggestions in there among the background info.

Well done on checking those test points.  That sounds pretty much ok doesn't it?  At 29/30mV that means about 19mA bias current (across 1R5 resistor) which is well in the ballpark for an EL84.  The HT is 390V and maximum anode dissipation for the EL84 is 12W.  That means a maximum current of 30.7mA.  As a rough guide 70% of that is 21.5mA.  H&K's setting of 19mA is 63% and is pretty much spot on for sensible reliable valve life.

Incidentally as a general rule for your future valve amp work, the "70% rule" for "best" bias is followed far too slavishly by a lot of people who don't really understand how variable everything is in this area.  Accuracy of 1% is ridiculous in the context of any guitar amp output stage.  As long as you don't exceed that or drop too low below say 60% things will be fine and you will hear very little difference except at low levels.  Crossover distortion, which bias is there to minimise, stays roughly constant as signal level changes so it actually increases as a percentage as signal drops.  That is why amps often sound VERY rough when you bring them down to very very low levels.  You can often actually hear the really grainy quality of the crossover distortion coming through.  If you are ever working on a manually tweakable amp just use your ears and be pragmatic, drop the bias until you can hear a difference at playable levels and tweak a tiny bit above that.  Playing around with it with a meter to fine tune it to 70% to the Nth degree is a sure way to put yourself into the mindset that you SHOULD hear a difference between 30mA and 30.5mA - so you DO hear a difference even though it's not there.  And then you are right on the edge if other factors are leaning the wrong way.  Remember, many things in a valve amp drift over time.  There is a huge amount of this problem in many areas of guitar amp work.  "Perfection is the enemy of good!"  Just go for what sounds good without looking for what could - just - might - possibly - "someone said online" - be a shade better.

If the bias setup isn't the problem which seems the case then let's put this back to looking at other things.  Another very real possibility which fits the symptoms is that you have a problem in the Power Soak circuitry.  This comes after the power amp of course and, if it is not switching and selecting correctly, it could cause all of the symptoms you described in your first post.  Check out the state of all 10 of the Power Soak resistors and the 4 inductor coils.  They are the large power resistors mounted off the rearboard board in cutouts of the PCB for heat dispersal.  As long as they look fresh and not heavily scorched they should be fine.  I would have thought that the most likely culprit here is the switching relays.  There are 3 of them labelled RL1-3 on either side of the Soak switches on the pcb.  They can be accessed on the back side of the PCB without removing any of the boards from the chassis.

Each relay will have 6 pins in a square close to each other and 2 pins further away from them.  This is a 24V relay with what seems to be a standard relay footprint.  BEWARE:  These relays are classed as 12 pin packages - BY H&K.  Other relay manufacturers like Omron number their pins differently with 16 pins.  There may be differences between similar relays which mean they won't fit on the board.  As far as H&K's numbering, imagine a row of 6 evenly spaced pins down each side.  Pins 2, 6, 7 and 11 are not used in this footprint.  They use pins 1 and 12 for the coil.  The wipers of the two switches are on pins 4 and 9.  These switch between pins 3 and 5 on one side and 8 and 10 on the other.  Here is an example of the type, there are lots of manufacturers who make these, Chinese are reasonably good for general purpose use, and they shouldn't be expensive items as you can see.  Omcron Relay  I have only posted this relay example to show what they look like as ours are very similar, the pin spacing is wrong for the H&K pcb but I can't find the correct type offhand.  I'll keep looking. You can see the positions of those on the pcb in the picture I've posted.  The resistors are of course the large ones immediately above their position.

The control system for the relays is not as you would expect, it isn't a straight "switch and activate" action on each one.  The settings of the 4 switches generate an increasing DC voltage level which is then converted into the different control states via an ADC at the microprocessor.  This is digitally processed (in some way which is hidden from us) and is then passed back out as a PWM voltage which is converted back to a DC voltage which in turn drives a ladder of opamp comparators each flipping high at a higher voltage.  As the voltage climbs up to 5V each one switches in turn at 1V, 2V, 3V or 4V, (the lowest of those is a line back to the microprocessor).  There is diode steering setup in there too so the relays don't just switch one after the other, the action is more complex as they will switch on and off in combinations.  If you can I would just measure the voltage across their coils in turn for different selections of the switches and check that you can see these swap between something like +22V and -3V.  Don't worry when they change.

Now is the rub.  Even if you get the correct (or sensibly believable) voltages across those relay coils that doesn't prove the relays are working.  It just rules out the control circuitry as the problem.  The voltages would still be there even if a coil was burned out.  Feed a small signal through the amp from your generator keeping the level low as it is only there to give you a visible indication on your scope at the output.  You can then check whether each relay is passing it through its terminals as it should.

We are now looking at the other 6 pins of each relay which are the internal switches.  The centre pin of each line of 3 is the wiper connected to either of the 2 outer pins depending on the state of the relay.  I can tell you that RL1 and RL3 are just paralleled up, both of their internal switches are connected together across the lines of 3 pins to double the current capability.  This makes them easier to check.

RL1 has the output of the amp fed to its wiper so that voltage should be passed through to the outer pins in turn, only one at a time of course.

RL2 is more complex.  One switch wafer will have a signal on its wiper in only one state and nothing (it's grounded) the other.  The other wiper will show signals in both states but they should be distinctly different levels.

RL3 is a bit simpler.  It may show voltages on all 3 pins.  One of the outer pins will show the same voltage as the wiper or no voltage, while the other pin will show the same voltage or a larger voltage.

That isn't as clear as I had hoped I might have made it but the action is quite involved.  It isn't just normal Switch On/Switch Off relay action.  I would start with the Power Soak as all of the symptoms could be caused by that being faulty.  It should be easier than what you have already done as you can access what you need with the boards all in place.  I hope this is going to give us something as the other possibility that comes to mind is - the Output Transformer.  H&K only and they will only supply to their network of repair centres.

Anyway, how does all of this sound to you, is checking out those relays doable?  Remember, THERE ARE STILL BIG VOLTAGES IN THERE.  (I can't stress that enough.)