GM36 TSC Manual Tube Check Lights Interpretation and FU1 400 Mal 250V fuse pop Issue

I have a GM36 which recently popped a FU1 (400mal 250V) fuse.   I discovered that at least two of the Chinese tubes had brown burn  (see Bad Chinese Tubes 20220810.jpg below) so I replaced all 4 with GT tubes with EL84 mediums.   I sourced these tubes (took 3 months to get them because of the war in Russia) at one location and stressed the criticality of them to be matching but received two separate sets from two different locations because the original source only had 2 instead of the 4 matching they told me.    I installed them and the TSC lights remained on even after replacing the Chinese tubes and the Anode fuse (FU1).    Since they were biased at medium 4 for one pair and the other was biased at medium 7, I decided to return those tubes to get a quad set of matching Electro-Harmonix EL84 mediums (Green).  I contacted Hughes and Kettner and get nothing but crickets from them; their service/technical support sucks!    I received the new EH tubes yesterday, installed them (without touching them with my hands), and the TSC lights in Play mode still light up; all 4 of them.   The FU1 fuse blew again too so I replaced it, yet again, and tried to power up and still can't get anything to come out of my GM36; not even in Red Box mode.    I did a manual tube test with a guitar pick and got 10 flashes of all 4 tubes, 1 Flash of # 1,3,4 tubes, and then 2 flashes of # 3,4 tubes.   I have no idea what this means since the manual doesn't really explain how the manual TSC tube check performs or how to interpret the results.    I still have the original Chinese preamp tubes (3 - 12AX7's) with one that has a metal cap over it which I'm assuming is like a rectifier circuit.   I'm not an electronics engineer so I don't know really and I have not seen a schematic on this either.   Anyway, the preamp tubes looked brand new with no film or discoloration; but how do I know if they are good really?   All I know is FU1 popped again after I replaced the power tubes.  Technically, I have replaced them twice in the past 3 weeks and still cannot get my GM36 up and running.    This re-tube issue has been going on for over 3 months now with the Russian tube shortage and now that I can get the tubes, I still can't seem to get my GM36 to work properly.     So I guess my questions are:

1. What are the flashing TSC manual tube check lights telling me. See attached video TSC_Manual_Tube_Check.mov for more info. This was performed in Play mode!  There were 13 flashes in all (10 flashes all 4 lights, 1 Flash of # 1,3,4 tubes (right to left on the panel) and then 2 flashes of 3,4 tubes).   I realize that the 4 flashing lights are telling me the rating of the tubes (according to HnK's User's Guide) and they are rated at a 2 @ 15.4 Bias Level, but what are the 11-13th flashes telling me?
2. is the FU1 fuse (400Mal 250V) fuse a slow burn or fast burn?   I can't remember for sure but I think I replaced them with fast burn to ensure it pops quickly so as not to burn up the circuit board in the event the biasing is not correct.
3. Why are all of the TSC lights lit up in 18W, 5W and 1W powersoak mode too (and 36W)?  It doesn't seem to matter which powersoak mode I'm in; they all light up regardless in Play mode.
4. There are no Hughes & Kettner repair shops in my area (based on many phone calls to local music stores).   I found one that said they could look at it, but the complexity in this GM36 has me concerned about their capabilities to service this amp, which they are admittedly not familiar with so the confidence is not there.   I am reluctant to ship it anywhere for service too due to cost and opening myself up to be ripped off.  Are there any shops in the Shiloh / O'Fallon IL area that can services this amp and meet my service with success without costing me half of what it would cost to buy a new GM40?  At this time, I doubt I would purchase a new GM40 since it's a nightmare just to re-tube, the TSC circuit and FU1 don't appear to do what they are designed to do either.   Anyway, it's been a long 3 months and I have already been fired from a new band I was trying to get off the ground because it has taken so long to re-tube this amp so I am a bit miffed at this point; and all in the name of good tone too...Pooey!   My backup bit the dust too and it would not have been acceptable to play it live regardless.   When my GM36 amp was up and running, I really had it dialed in and it really did sound great!  Now, that I've replaced the tubes....I can't get any sound...LOL!   How ironic.   I told Hughes & Kettner I'm about to just toss this into the dumpster because it is far too complex to get serviced compared to other amps I've owned (Mesa, Vox, Fender, and Marshall).  I never had to deal with any tube issues with any of them!    I'm still wondering if I should just pitch it but that's just the frustration talking!

I tried to attached several files most of the files had limitations from my iphone13 which far exceeded the requirements for posting (without necessitating a lot of work to be done to add these files) so I didn't include all of them.  I did include a screen shot of the bad Chinese tubes which were brown-burnt and a video of me doing a TSC manual tube check so someone can see the flashing order of lights, etc.

Any information to get me up and running again would be wonderful. Hughes & Kettner already got my $3500 and apparently feel it is not necessary to support their customer-base after warranties have expired. They have yet to respond to any inquiry since I purchased the unit years ago.

Hi Cary.  Thanks for the info on the problem you are having.  Doing it your way (and mine) and giving as much info as possible straight up front is far better than asking "Doods my amp stopped working what could be wrong?" then wading through 10 following posts asking for data on what the problem really is and getting it a line at a time by drawing teeth. I wish everyone would take time to actually put together a clear readable report of what their problem is and what they have tried.  Anyway...  I'll deal with a simple aspect of your problem first which you can address easily then talk about the deeper level of issue. I'm not sure of your understanding of technical aspects so I will assume you know nothing. That isn't meant to be insulting, it just means what is going on should be clearer in any case.

The flashing TSC lights are telling you the state of each individual output valve's bias current. The automatic bias control circuitry in the GM36 should trim each valve in order to match them up fairly accurately. That said, there are limits to what it can achieve so it doesn't mean you aren't better off sticking to matched valves when you replace them. Always go for at least two matched pairs and put one of each on the opposite sides of the amp layout. A matched quad means you don't have to worry about where you put each individual valve of course. The TSC lights match up with the position of the valves in the chassis. The left light means the leftmost valve and so on. Yours are telling you that the left valve is sitting on 10 flashes, the next 11 and the two rightmost are on 13. Your valves are all in the middle of the bias range which is looking good. They are a very reasonable match, H&K recommend staying within 3 flashes, but you can improve your setup really easily - (and cheaply)!

You can see that the two rightmost valves match perfectly and the two leftmost are only one flash out. They look like they are your matched pairs, left two and right two. You need to know that the valves in the TM/GM amps work in pairs affected by the power soak setting. When you are in 36W you have all 4 valves in use. When you switch from 36W to 18W and on down to 1W you remove one pair of the output valves from the circuit thus immediately halving the power output to 18W. The power soak resistors then drop that 18W down to 5W and 1W.

In the class of amplifier to which the GM36 belongs, the signal is split into an upper half (positive voltage) and a lower half (negative voltage) by the phase inverter valve. The two signal halves are amplified separately by the output valves and the larger signal halves are then stitched back together in the output transformer. One valve (or one pair of valves for 36W) amplifies the +ve half and swaps over to the other valve (or pair) to amplify the negative half. To achieve your matching you want to have one of each matched pair on each side of the signal. This means seeing slots 1 and 4 as a necessary pair and 2 and 3 as the other. That is fairly universal in amplifier layouts.

Valves have a "dead spot" as the signal applied to them increases from 0V until a threshold level where they begin to seriously amplify. If we just ignored this it would distort the signal every time it switches from +ve to -ve and back again in a very unpleasant sounding way, particularly at lower levels. We would effectively lose a slice of the middle of the signal every time it switched sides and crossed that "dead zone". So we set up the valves to have a "bias" or idling current.

The bias current is the level at which the valves sit waiting when they are not amplifying the signal as the other valve is active or there is plain no sound to amplify. They are effectively set up to be slightly turned on and right on the verge of amplifying. To perform this swapping over in our GM36, the valves work either as the two left valves amplifying together then swapping over to the two right valves working together, or at lower levels as just the outer pair swapping from one to the other. At 18W only the outer two valves are working. You need to put your current rightmost pair of valves, that is the perfectly matched ones with 13 flashes, into the outer pair slots and the current leftmost pair into the inner ones. You now have the signal swapping from each valve to its matched partner as it should.

Just to make that super clear, imagine your valves are currently labelled 1, 2, 3, and 4 from left to right. (I would actually label them in some way once you have them in their optimum place, more of that later.) As they stand, the left pair 1 and 2 are only 1 flash apart with 10 and 11 flashes, while the right pair 3 and 4 are a perfect match with 13 flashes. Make sure 13 flash valves 3 and 4 are in the outer pair slots 1 and 4 making them a "swapping" pair. Make sure 10/11 flash valves 1 and 2 are in the inner slots 2 and 3 making them a swapping pair. Now on the lower power soak settings where you are only using the single pair of valves you always have the best match. In short, swap valves 1 (10 flash) and 3 (13 flash). You now have optimum balance.

Playing with 36W selected and thinking in terms of "flashes", you currently have two valves working together at any time, (I mean at the same time amplifying the same half of the signal), with 10+11 flashes. They swap over to two valves with 13+13 flashes. 21 plays 26, that isn't the optimum use of your matched valves. After simply swapping 1 and 3 over you would be using two valves with 13+11 flashes as the left half and two valves with 10+13 flashes as the other. 24 plays 23 is a much better match of the two halves of the setup. The difference of 5 flashes overall down to 1 is a considerable improvement. At 18W and below you now have a perfect match as opposed to your current 10 plays 13.

Now a second point which may be relevant. The TSC automatically gradually reads, resets and remembers the bias setting for each valve when a new set are fitted. It starts with the memorised setting of the previous valves. That has been known to take some time to relearn. It seems to be related to the number of times the amp is switched on, left to warm up, then turned off again. I would reposition your valves as we have discussed, check the TSC settings again and make sure you have them optimised, then work with the amp for a while checking the TSC each time you use it. Your valves may not need that relearning period if the old and new valves were very close to each other but it may happen in your case.

Next point. Let me, as a fully degree qualified electronics design engineer with many years of working in the industry make one thing clear to you. Valves are NOT the same as halogen bulbs in any way. There is absolutely no reason whatsoever to not touch them with your hands, (clean hands that is). That is an internet myth spread by people who know nothing about electronics. Yes, keep them clean. No, do not bother putting on gloves or using cloths to handle them or swabbing them down with alcohol. They are extremely thick hard glass cylinders with no coating on them to damage. The fact that they work at high temperatures does not mean that a grease spot will cause them to crack. This has been the truth since the golden days of valve technology and it still holds now. So - - - I always mark my matched pairs with a number using a felt marker. Technicians and engineers working with very demanding valve applications have always done this. In EL34 power valves, like Marshalls, you can always put it on the base. For EL84 and 12AX7 valves you can put it on the glass envelope without any problem. Put a unique number for each pair, the same for both, say "7" and "7" or "2 and "2 so you can see unambiguously that they are a pair. The number means nothing other than "7" matches "7", it is not a measure of anything to do with the valve's performance. For any preamp valve which has a special selection, like a low noise or a balanced 12AX7, I make an indication of that directly on a clear area of the glass bottle, usually at the top. "LN" or "B" is enough. Forget anything other than normal cleanliness and call out anyone who insists that their valves benefit in any way from special treatment with kid gloves. They are just tough light bulbs and we don't have any need to fuss over them!

Now to the final point. It sounds to me as though you could have a problem with one of your preamp valves. Start by sorting out your EL84s as we have suggested and then leave them alone. Plug a lead into the Fx Return socket and try the amp by simply touching the tip.  You won't get much volume but hopefully you will get some sound out showing that the power amp is working. That means that both the output valves and the 12AX7 closest to them are ok. You can move on to testing the other two 12AX7s. If there is no sound at all it means that the PI 12AX7 next to the output valves is possibly faulty.

Assuming it is faulty, swap it with another 12AX7, (the one next to it will do at a pinch but make sure to replace them in their original slots afterwards). If the amp then comes to life through the Fx Return socket you have your faulty valve.

Assuming it is fine and the power amp stages work, swap out the first two 12AX7s one at a time for a known good one, (one at a time is important). If you have a faulty 12AX7 the amp should come to life at some point and you know which is the faulty one. It is possible you may have more than one faulty 12AX7 but that would be unlikely. If that is the case you would need to swap out both of them of course.

The first valve is under the metal cap in order to prevent it from picking up hum and noise. It is the first valve in the chain so any noise here is amplified by every other valve after it, that can mean a lot of noise by the output. Often you can pay a little more to have a first valve which is specially selected for low noise. Mark your first valve with a felt marker with "LN" near its top and always keep it in the first valve slot. There is a similar situation with the PI valve next to the output valves. This has the job of splitting the signal into its +ve and -ve halves and passing each half to the relevant output valves. It benefits from being selected as "balanced" when you buy it. Each 12AX7 actually has two triodes inside, which means it can be used for two stages of amplification. Your GM36 uses a particular type of circuit for its PI which employs both of the triodes in one valve. If the 12AX7 is selected so its triodes give the same amplification, which they will not generally do within permissible limits, it helps with the symmetry of the output signal. Mark your PI valve with a "B" to identify it as selected for good balance and keep that one in the PI slot.

That is a LOT to take in but hopefully you will know a fair bit of this already.  Read it through and sift out the actual practical advice, it is simple to follow, and see what you can find.




(Incidentally, I've moved this to the GM36 forum where it should be. The "Lounge" was created as an area where there could be general talk about no particular amp model or just social interaction.)

Hi Cary. Further to bordonbert's useful information, I'd like to add this regarding the FU1 anode fuse. In your post you state - "I replaced them with fast burn to ensure it pops quickly so as not to burn up the circuit board in the event the biasing is not correct". If this is the case this is the incorrect fuse. Fast blow fuses have a letter F on them and slow blow ones have the letter T.
The GM36 has a FU1 anode fuse T400mA/250v and a FU2 main fuse of T800mA/250v.

Excellent spot Guitardar, thanks for coming in, I had missed that. I've actually just had an episode with a valve going volcanic in my Marshall JMP SL100 and it took out the fuse. I replaced the fuse and the valves with known good ones but when I powered up it went again. When I checked, I had done just what you say, put in a fast blow by mistake. There are quite large switch on surges with the transformers in our power amps particularly if you hit your switch at a point which matches the worst point in the mains cycle. The fuses are selected and specified so a healthy amp can cope with that. The very basic '60s design of my classic amp meant that that F/T selection was pretty much essential.

I would add that changing out your fuses in your amps every couple of years as a matter of maintenance is a good move that no one ever does. (Why don't I ever remember when I actually have fuses lying about? ) They age and become brittle and more susceptible to breaking for no particular reason. It's cheap to keep a pack of 10 in your bag nowadays and its good policy. That is, unless you believe that those old fuses from the '60s lend a much sweeter elfin-bell quality to your top end! And fuses very often blow so the break is impossible to see with the naked eye. Metering them or simply changing out if they are suspect is the only reliable way to check them, you can't just take a look against the light and say "that one's fine".

The idea of frying your amp because you have a slow blow fuse in there can be real with solid state circuitry. Transistors make far faster fuses than fuses do! With valve stuff the problem tends to go away somewhat as they are much more forgiving of short term current overloads. Decent power supply design should protect the SS circuitry and the H&K amps are very good designs in aspects like that. It's not impossible to burn up a board, but it is very very unlikely because of the slow speed of a T designated fuse.