Replacing just two power tubes

Hi All,

First post here.

I got the grandmeister 40 in Feb of this year (2021) and the TSC has indicated overcurrent in one of the two outer matched pairs of power tubes. I do run it pretty loud and use it about 3-4 days a week for about 4 hours each day. I run the master volume at about 12 oclock or 3 oclock depending on the band. The cab i was using during the summer had a faulty jack and was cutting in and out also. The first time the TSC engaged was when the cab cut out then i ver quickly turned on standby and plugged it back in. It was working but now at 20W of course. I turned it all the way off for a bit then back on and everything was working fine until now; the TSC keeps turning off those power tubes. So a few questions for yall:

1. I know i need to replace the tubes, but can I just replace the two outer tubes with a matched pair and leave the other ones?

2. Could the faulty jack in the cab cause any other issues that im not able to notice?

3. I do need to use it for the next few days while I wait for the tubes, will I do any permanent damage by running it at 20W with the bad power tubes or even 40W and have the TSC engage?

Thanks a ton!

Hi there.  Yes it is perfectly feasible to change out only one pair.  In an ideal world they would all 4 match but as they are used exclusively in outer/inner pairs, having the pairs not matching is no real big deal.

When you are in 40W mode all of the valves are in use as you already know.  When you drop down to 20W mode or the TSC pulls you down then you are using only the outer pair, the inner pair are switched off and the TSC LEDs will reflect this status with the inner pair lit up.  If the TSC diagnoses your problem is in the outer pair then that means they will need swapping out of course but there is something else to consider.  If you drop down to 20W then you are using that outer pair exclusively.  The good inner pair are out of circuit.  If I were you I would swap the outer and inner pairs so at least the known good pair are in use for 20W.

Fix that cabinet socket immediately!  A fault in the speaker wiring is one of the biggest killers of amps there is.  The worst situation any valve amp can face is when there is NO load on the output.  Everyone imagines a short is the worst case and that would be correct for solid state circuitry, it is not for valves.  With no load there is effectively an infinite impedance which draws no current at all and the output transformer reflects this back to the output valves as massive high voltage "Flyback Spikes" which damage the valves and even puncture the insulation on the output transformer winding wire.  Output transformers are not cheap! H&K have the TSC setup so it detects this situation and protects the amp by kicking in the power soak to add a load.  But it has to happen very briefly of course for the TSC to detect it.  That doesn't seem to have happened in your case yet but it is well to be aware that that is what can happen.  So NEVER EVER EVER run a valve amp without a load on it.

Yes, there are other issues you could experience.  The TSC circuitry itself has been known to be damaged.  There is a limit as to how much protection any safety circuitry can offer before it gives up the ghost itself.  In the case of the TSC there is a MOSFET in the cathode of each output valve.  It plays no part in the amplification of the signal but it is used to automatically set the bias current when the amp is idling. These can be blown. This is VERY unlikely but we have had cases of it happening here in the past.  That would mean a repair by a qualified tech as it means dismantling, desoldering and replacing the MOSFETs on the mainboard.  Anyway, let's not worry about that unless it crops up later when you have your new valves.

To use it over the next few days I would swap the valves to be sure of using the good pair in the outer slots, then stick to 20W on the Power Soak.  That will keep the faulty pair from being switched on.  My advice would be, don't be a dick like many guitarists would be and say "I'm just going to use 40W until it tells me there is still a problem, it will probably be alright".  Make your brain rule your bollocks! You know there is a problem, don't make the simple problem cause an even bigger (more expensive) one.  Play on only 20W until the valves show they are all good.

And get a replacement socket for that cab right away.  They are easy to source and cheap enough.  Swap that out before you use your cabinet again.  Bad cabs eat amps.

Thanks for the quick reply!

Just to clarify: So in my situation when the TSC engages, one of the outer two leds is flashing and the other one is solid. Does that mean it is engaging the power soak and only running one of the two outer power tubes? Im gunna try swapping the inner and outer ones like you said, thats a great idea thanks again!

It can't run only one of the two outer valves, that is impossible.  The output stage is what is called Push Pull.  One valve amplifies the positive half of the signal then swaps to the other valve which amplifies the negative half.  It is not possible to amplify with only one of the pair.  That is a feature of the design, (and all other major guitar amps which are almost universally push pull in their design too), there is no way around.  Look at the user manual in section 7.1 and you will find this:

H&K GM40D User Manual wrote:One LED flashes, another lights up continuously.

The tube assigned to the flashing LED is generating too much current and therefore has been shut down. This tube has to be replaced. In this type of power amp, it takes a pair of tubes working together to produce the best sound. Therefore TSC also switches the defective tube’s counterpart off (indicated by the continuously lighting up LED), so it doesn’t degrade the other pair’s tone. There is no need to replace this tube, because there’s nothing wrong with it.  Now this is where TSC’s intelligence saves your gig: if a tube failure like this occurs in a conventional amp, its fuse usually trips and you can’t operate it until you replace the tube and fuse. Thanks to TSC, you can continue playing for as long as the LED remains illuminated. However, please note that shutting down one pair of tubes will halve the amp’s output power from 40 to 20 watts. And don’t forget to replace the defective tube after your show.

If you have selected 40W mode then your amp can only be running at 20W with the outer pair turned off by the TSC.  What happens to it both soundwise and on the LEDs when you select 20W or 5W or 1W?  I have never tried that setup myself as I have never had a faulty pair of valves in there so I don't know for sure but you may not get any output at all because the reduced power selections rely on the outer pair as their valves.  If it were me, I would prefer to swap the valve pairs around so the good pair are in those outer slots and work for the moment with 20W actually manually selected.  Now I can take in what you are seeing I think either way should be safe but setting things up so it isn't even an issue is the better solution.

I should also add that, when you get your replacement you have two different approaches.  Buy a matched pair and keep the non-faulty one as an emergency spare.  Buy a single valve which matches the one in the faulty pair which is still good.  You would need to run the "TSC flashing LED" test and count the flashes for that valve.  From the table in the user manual you then know what spec of valve to buy.  The matched pair is the better option if you can afford it.

(As a joking aside, are you aware of the electronic engineer's acronym "RTFM"? )