GM40 v3 tube failing releatedly

Hi All,
New forum user, and owner of a new GM40 since December. Nice amplifier. I have had this issue occur twice now, and am hoping I can get some advice as to whether to send the amp back for servicing. After receiving the amp, playing it for some time, and reading carefully the tube swap thread, I changed the preamp tubes as follows: v1 ECC803s v2 5751, v3 5751. That quieted things down and warmed up the tone in a way that matches what I wanted to hear. I understand the 5751s are lower output, resulting in tamed volume. Before covid, I gigged this thing a good dozen times. No issues.

After maybe three months, the 5751 in v3 failed. Upon pulling it, it was clearly hazed at the top, and upon extra inspection I even found a tiny crack. Hmf... figured I had bad luck. Replaced it with the stock 12AX7, and back in business. I then replaced that with a new 5751, and after that it sounded quite good again. Well, after about 6 weeks, and only maybe 10 hrs playing, it did it again. No hairline crack, but definitely hazed light grey at the top. Put the old stock 12ax7 back in again, and back in business. What gives? Is there a reason why that tube configuration would blow up that third tube? Or if there is something independent going on. It is not related to impact or knocking the head around. power tubes are matched Genelex gold lions, in case that matters.

I am curious if anyone else has had this issue. I know there is a certain @bordonbert on this forum whom is likely to have a thought, and I would greatly appreciate your opinion!

All thoughts welcomed...

Hi neilkamman, welcome to the forum.  I can only advise with 100% certainty on the amps for which I have seen the schematic.  The GM36 is a well known piece of kit in that way but the GM40D remains a bit of a mystery regarding its documentation.  The general assumption we have to make is that it is in the same "family" as the GM36 and shares most of the common sections with at most minor tweaks.  That's the best level of technical advice I will be able to give but your question should fall into the area where we can assume common ground pretty much for sure.  In the GM40D, like the GM36, the V3 slot is the Phase Inverter.  There is no reason that I am aware of why that stage should be under more stress than in any other amp from its day to day working.  The power amp design is pretty much standard with the exception of the auto-bais setup which is nothing to do with the PI.  But after your substitution things aren't as they were!!!

Right, to the bit you don't want to hear but you knew was coming!  The 5751 you are putting in there is indeed a very close match to the 12AX7.  As you may have read in our valve thread, the other valves in the 12A*7 "set" are actually nothing like a set or matches or substitutes at all.  No decent design engineer would contemplate just sticking one in for the other on more than a "fix it to get home" basis.  If they do they aren't a design engineer, they're a tech!    The substitution of say a 12AT7 for a 12AX7 is something like swapping your flat bed truck for your sports racer and using them to perform the other's role.  Your sports car is not going to like running off road up rocky terrain and your truck is not going to last long at full revs sliding round a race track!  The 12A*7 numbers are NOT RELATED IN ANY WAY and there is absolutely no sense of a family with common resemblance.  They are distinct individual valves designed to perform totally different jobs and they share no common aspect in their parameters.  However the myth persists despite my own best efforts at making people think with their heads and not their fear of the dark.  Other engineers have simply given up trying to advise on things like this as people want to believe it so much they deny all engineering evidence and advice.  Ever wonder why there are loads of techs posting but no genuine trained design engineers in forum discussions on guitar stuff?  Techs are mojo gurus engineers are just killjoys.

The 5751 is, by chance, a very close match to the 12AX7 and it represents the only permitted direct substitution that any engineering type would contemplate.  If you put that into most common 12AX7 stage circuits you will find the design conditions of the stage are pretty much maintained with the exception of the stage's voltage gain.  So is the valve's perceived stress level and the amp's safety as long as the datasheet is adhered to, more of that later!  The idea that it is a 12AX7 with less "signal gain" is a bit misleading.  Valves don't have voltage gain at all.  It is the circuit design which they get put into which determines the voltage gain.  You can make the same 12AX7 give 2x, 20x or even more voltage gain at its anode in a stage by simple changes in external component values.  The valve only has a capability to produce a current which follows the input signal voltage.  It is the job of the stage circuitry to convert this current into a voltage which remains constant across a range of conditions like frequency and DC voltage level which the valve will see.  In many signal stages this voltage gain is set to be the maximum the valve can be coaxed to give so a swap of valve types does give a change in stage voltage gain, but it screws up every other aspect of the stage design at the same time.  If I can use my former analogy, you are paying a lot of money for a Porsche designer to come up with a phenomenal car then saying that using a Nissan flatbed gearbox bodged straight into the Porsche gives it a better drive around town.  And, as I said, there are a few other parameters which dictate aspects like how wide the frequency range of the stage will be (NOT of the valve which is a MHz radio receiving class not restricted to 10kHz, another internet myth).

So to the solution - I think.  There is a potential area of danger in the PI slot.  In the GM36 at least, the supply voltage for the PI stage is stated as 370V.  The maximum permitted plate voltage for a 12AX7 is 300V from the datasheet.  The 5751 is the same but has an additional "ABSOLUTE MAXIMUM" level quoted at 330V.  They didn't tell you that at "mojo school" did they?  In their respective datasheets the Average Plate Characteristics graph for the 5751 extends to 350V while the 12AX7 actually goes up to 450V.  I suspect the 5751 is much more finnicky about this due to its internal structure and could damage pretty much at that 330V level.  If you are using your amp to specifically generate as much power amp distortion as you can you may be overdriving the PI and putting too high a plate voltage on it for the 5751 to cope with.  The answer - stick to the designer's choice of a valve in that position.  I have talked in the past to the H&K designers about this whole valve substitution thing to get a picture of whether there was a safety risk in using other 12A*7 valves.  They came back with a "No danger" recommendation but were absolutely mystified as to why anyone would want to.  They worked hard to get the best conditions to be able to offer the tonal range that their amps do and the change of valve type was only narrowing that, and not in a good way to them.

For the record, the V1 slot has 350V and the V2 slot has 280V.  These are for the GM36 of course but would most likely be close if not the same in your own amp.  With the V1's similar high voltage that could be a problem too but the restricted signal voltage range and anode voltage swing in those earliest two stages mean it shouldn't be as obvious a problem there.  The designers seem to have set V2 as the primary overdrive mechanism.  It is set up as a switchable standard/hot biased gain stage feeding a Direct Coupled Cathode Follower.  The DCCF will clip on one side leaving a fair bit of headroom in the earlier valves.  The DCCF has the hardest life of all the preamp valves.  That stage is often said to benefit from being a Chinese choice which seem to withstand the abuse better than others.  There is good engineering evidence for this being true, (not as for why but that's a worry for us curious types ).

So, V1 & V2 = 5751 if you must.  V3 looks like a nono on maximum voltage limit grounds. But then, it is only the PI and plays a less important role in distortion generation than the preceding ones. (yes, there is global feedback around it and the output stage so that is true!)

Super helpful! Thanks so much for giving this some thought. What you suggest makes sense. I have the stock tube back in there now, and will report back if it blows. Much appreciated. N

It may not be what you need or want Neil (?) but I'll give a few thoughts with some info here regarding PIs.  (It's a slow day at the Covid office.)  Their job is not done in the way most people think.  This may even help others in this sort of area or at least make them think.

The power amp is usually comprised of two stages.  The second is the well known output stages of EL84s and Tx.   The first is the Phase Inverter.  In simple diagrams these are represented by the generic amplifier symbols, the large triangles.  The PI has the first task of taking a single input signal and generating two amplified output signals, one the same and one inverted, which are each used to drive an output valve of the pair during different +ve and -ve sections of the signal.  The Tx then takes the currents from both sides of the output stage and knits them back together again into a single output signal, the same as the input but much larger and with higher current capability, (that's the same as saying at a lower impedance).  The power amp has global feedback applied around it!  This means a bit of theory if you are interested in getting to the core of how your power amp really works.  Let's make a very simple example.  (That gives me the chance to draw pretty pictures and I always love that.)  You can skip anything that you can't be bothered with but you won't get the full picture and completely follow what I am saying.

Have a look at the attached pic.  It shows a power amp made up of a PI and an Output Stage hooked up exactly like our own power amps are in guitar work.  This is pretty much universal for us, (more of that later).  For simplicity in our example the PI has a gain of 10x and the Output stage also has a gain of 10x.  Two feedback resistors R1 and R2 are connected so the PI sees a voltage picked from the output and dropped down to 1/20 of that level.  This is attached at the -ve input of the PI, (the grid of the other triode).  Pretty simple so far.

The first interesting fact is that the input to the PI is NOT the 1V which the preamp feeds it.  The true input signal which is what the PI actually sees and acts on is (1V - 1/20*Vout) and Vout is a bit bigger so that second part is very significant.  We need to do a little bit of maths to generate some figures.  Don't worry it is only arithmetic!

The feedback equation for this sort of configuration is: GAIN = A / (1 + A*β) where A is the gain of the entire power amplifier without the feedback in place, 10x (PI) and 10x (OS) = 100x, and β is the "feedback factor" set by the two resistors of 1/20 (= 0.05).

This means that the gain of the amplifier with feedback in place is not (10 x 10 = 100x), it is actually 100/(1 + 100/20) = 100/6 = 16.67x.  That's some drop!  So with 1V from the preamp we end up with only 16.67V at the output even though the two stages both have gains of 10x.  That's what feedback does!  And in doing that it also decreases distortion, (not that from the preamp), decreases noise and increases bandwidth all for free.

Now we can see that the voltage at the PI -ve input fed back by the resistors is actually 1/20 of that 16.67V.  That makes it 0.834V.  Here is the crunch...

YOUR PI IS NOT SEEING THE VOLTAGE FROM THE PREAMP AS ITS INPUT, IT IS SEEING (THE PREAMP VOLTAGE - THE FEEDBACK VOLTAGE).  In our case this is (1V - 0.834V) = 0.166V.  This must be true because if we now multiply this by the real gains of the stages we get 0.166V x 10 x 10 = 16.6V.  That matches up with our original equation's predictions.

The point is that the PI is only amplifying a signal of 0.166V not 1V from the preamp as most people assume.  If you drop the PI gain by substituting a different valve this is another bigger system you are changing and it may change in ways you don't expect soundwise.  In reality, what you are doing with this substitution is to drop the gain of the PI stage while leaving the output stage gain the same.  The output stage requires the same input voltage as before to generate the same output level but the PI now has to supply that from the same input signals but with less gain.  It can't do that.  In reality the PI output voltage will drop, the output stage will see less voltage so its output level will drop.  This will decrease the feedback signal to the PI -ve input which will in turn increase the PI input signal, (remember this is actually (1V - feedback signal) ).  So the voltages will self adjust until the system does reset to a new equilibrium but with a different perspective from the PI's view of what its own distortion contribution will be.

One area this affects is crossover distortion.  Guitar amps are very bad at eliminating it and it is not good sounding distortion.  Its contribution is pretty much a constant level across the 0V line no matter what the signal level is so it also has the property of INCREASING ITS PROPORTIONATE LEVEL AS THE SIGNAL LEVEL DROPS.  That's another reason why low level playing can sound rough.  So we set bias in the output valves so as to minimise it - to a degree, but we can't get it down to anything like hifi standards, that's valves for you.  It isn't really necessary when you actually want to generate >5%, (sometimes as much as square wave level depending on your genre) distortion in the rest of the circuit.  However, because it is generated inside the feedback loop the negative feedback does the job of actually reducing that level of crossover distortion just as it reduces the gain of the power amp overall.  Reducing the gain of your PI will reduce the overall level of feedback you are applying and will increase the level of crossover distortion.

Ok, so this is what people seem to want to explore.  Fine, but I have always been curious to get accurate figures of how many people try out new stuff like valve swaps, RAVE about how fantastic they are when they first do them, then a little further down the line get dissatisfied and go back to their original setup BUT SAY NOTHING ABOUT THAT REVERSAL!  I know that this does go on as I talk to many people about their amps and some are honest.  In my own case, I use a JCM800 2210.  You know, the ones that sound shit because they have clipping diodes built in?  I have read posts from and talked to a lot of people who have removed those diodes completely or replaced them with their own little diode networks to get away from the simple "rectifier bridge around a 1N4007" that Marshall use.  They are all on record as saying how great the amp then sounds with their changes in place.  I have found a significant number of people who are honest enough to admit they simply reverted back after a time as the change got wearing.  And I strongly suspect it is the same with valve swaps, both model changes and changes of manufacturer.  If the differences are so great and the improvements people find are so huge why do they keep on looking for the next "ultimate" valve choice.  Looking for improvements in that direction is a genuine dead end as it is only giving in to your FOMO, fear of missing out, and of course your overriding desire to be seen other musos as a fully paid up and licenced member of The Priesthood   !  It isn't The Illuminati we should be fearing, it is the mojo fuelled Cork Sniffers.

...Oh!  I did say "more of that later".  If you want to see the difference correctly applied "new technology" can make, just have a listen to the Musicman range of amps.  Here is a link to a fix of one of these where you can see one opened up. Fix of Johnn Hiland's Musicman amp  Beware, it may make some people suffer uncontrollable vomiting as there are opamps and transistors in there.

These are not high gain metal designs but they can still be appreciated as great sounding amps.  The guy is a country player but you can hear it pushed a little harder around 44:00. I would point out two things.

A) Look at the schematic at 22:30.  In the centre you will see IC-3.  These are the main distortion introducing elements.  They are opamps with a well designed clipping mechanism around their feedback loops.  That is a superb configuration to use for that and it beats the simple "diode clipping" which people use into a cocked hat.  I have used it a number of times in SS pedal designs and it is easily tweakable to give very valve sounding distortion.  Notice how the opamp itself is prevented from having any contribution via internal clipping.  And no "magic opamps" in there at all, anything will do.

B) Secondly, look at the power amp output stage on the right of that schematic.  That is a wonderfully innovative output stage.  This is valve.  Or is it?  If you look you will find that there is a driver transistor sitting in the cathode of each output stage valve.  It is actually the transistor which does the controlling of the signal.  The valve is pretty much working as a slave and only adds "grunt" and valvey distortion.  There are more great ways to do the power amp job than most people are even aware of.

I do find this sounds extraordinarily good and a great justification for the H&K basic philosophy of mixing valve stages for tone and solid state stages for more accurate and flexible control with no contribution to overall sound.  It isn't the solid state which gives the sometimes H&K hardness, I fear it is a deliberate feature of the voicing aided at least by speaker choice.