TSC Middle Lights on but all is well?

I finally made a speaker cabinet that sounded so good with the GM40 that I was inspired to noodle away for hours…and then a weird noise showed up (like a high pitched fly buzzing). I shut down, played the next day and it came back after about a half hour. I popped the top and tapped the tubes and it crackled when tapping the shielded tube 12ax7 and the two right hand power tubes when facing the amp. Of those two EL84s, the second one from the right was dim. Also the two center TSC lights were lit. So I bought a new set of four matched power tubes and put them in. Tapped them, no crackle! Played for a while and no weird noise has shown up. The problem is the two center TSC lights are still on. I tried swapping the pairs around with no change. I tried a master reset, no change. I also swapped out all 3 preamp tubes with no change.
 The amp seems to work absolutely fine. The volume drops from 40w to 20w on the power soak, it seems as loud as it did before it went wonky. I am guessing all the power tubes are working? I did the pick trick and all the tubes are within a flash of each other.
Any suspicions as to if there could be an underlying problem or if anybody has experienced this?
 Thanks all!
(PS this amp is out of warranty so return is not a solution. I have used it sparingly because it never had a tone that inspired me but I loved the midi features. This forum got me to try different speakers and cutting the presence control as mentioned in thread “GM40 High End and Treble”)

Hi Midiman.  You seem to have done all of the usual things I would have recommended.  The only other piece of simple advice I can give at the moment is - wait and use the amp for a while longer.  I was told by H&K a while ago that the TSC setup takes time to recalibrate itself and this may be the simple solution.  Yes, you can force a reset but it may just be that it requires a few more switch on/switch off cycles to work itself in.  We have had people in the past who have reported that that can be the case.

The symptoms suggest you had a duff output valve, second from the right, and this meant a bad mismatch between that and the second from the left which are a matched pair, so the TSC kicked in and shut them down as it should.  When the new valves were fitted it is then trying to compare those with the reference it stored for the old valves when they were working.  I suspect this is not showing a fault condition but it is showing that the bias is off for that pair.  As I'm sure you know, the middle TSC lights should be telling you that those EL84s are now not in circuit.  If you are noticing a correct increase in volume when you select 40W from 20W then this is obviously not the case.

If the amp seems to sound as it should on 40W then there is no valve problem.  It is most likely only that mismatch between what the TSC thinks the bias current of the valves should be and what it is reading.  It reads what the bias current value currently is constantly and alters that against the reference it stores for its dynamic adjustments.  Obviously the required value may change regularly with a change of valves so it has to allow that reference value to increase or decrease when new valves are fitted, but this must be made gradual by reading the current at repeated switch on until it stabilises.  It may still be going through the process of the reference being adjusted.  As I understand it, this gradual reference reset happens at switch on, not all the time the amp is running.  That way it can also adjust very slowly over time as the valves age but not change too quickly in response to some sort of fault.

I know it's cold comfort but get back to us after a few more playing sessions and update us as to whether it has cleared.  If not we will have to look into things like the state of the TSC MOSFETs though I don't think anything as serious as that is an issue with your amp as both valves of the middle section pair are showing odd yet all valves are flashing correctly.  The bias setting of the outer pair may be very near what your old pair were so the amp thinks they are the same, but the inner pair are further away and flagging as "oddly biased".  It is worth noting that, if you play your amp on the lower power soak setting a lot the outer pair may age a bit quicker than the inner pair so even a matched quad may drift between pairs over time.

Thanks Bordonbert for the quick reply. I might have read one of your previous replies somewhere here that it can clear over time, so that was my inclination. I have enjoyed many of your helpful posts. I almost gave up on this amp but I was playing it through a V30 single cab. After reading so much here I took a chance with a Weber Silver Bell 4ohm speaker from long ago and bought another for an 8 ohm configuration. I figured buying a speaker was going to cost a lot less than selling the GM40 at a loss and then starting the dizzying task of searching for the next “holy grail”of amplifiers. I made a 2x12 vertical slant cabinet, and like I said above, I followed the advice of dialing the presence to zero and then slowly turning it up to just below the ‘fizz’. Suddenly it sounds great on every setting! I find myself stepping through my presets and being inspired through each one, which hasn’t happened in a long time for me. I play mostly classic rock, and I was hoping for acceptable but I think I may be getting much more. Time will tell.

I'm really glad this helped.  There is another post I have just had time to make in this section in "GM40 High End and Treble" to do with the Presence/Resonance controls and simulating their response.  I think it goes some way to explaining the "fizz" you quite rightly report.   Give it a look see.

Incidentally my first use of my GM36 was with a pair of 1x12" with V30s in as a "mini stack".  It looked great!!!  It sounded dire!!!  It was the same thing of the strident hard top end.  Like you I play classic rock, (probably more classic than your own at my age ).  I just could not dial in those creamy Free and Bad Company tones.  And I wouldn't even touch my own best band of all time, Humble Pie, it was too sensitive to me to play with that hard a sound.  Changing to my current home produced, (just like yourself), 2x12" slant cab with a pair of original 1960s G12H greenbacks in made such a difference.  Now I know the effect of the Presence control I can get a much better rendition of the sound I hear, (or at least "a" sound I hear).

Just to follow up if anyone reads this. I have put quite a few hours in on the amp now. The two center lights are still lit, but the amp is working perfectly. With the Silver Bell speakers it just sounds great. I might eventually retube the whole amp, mostly because I am curious if it will change the light status, but it is working so well I probably won’t monkey with it. Originally I needed to fix it because I was looking to move on due to the tone fizz, and I didn’t want to take the money hit of selling it with a known problem. But using it the last few months, integrating it with my midi effects, it has most likely become my forever rig. If I ever do fix it, or if something goes wrong connected to the lights, I will post here for anyone who experiences this problem down the road.

That is now odd Midiman. Can I just make sure, you are still talking about when running the amp at its full 40W setting? Is there a noticeable difference in volume if you swap the Power Soak between 40W and 20W?

After a couple of months any TSC reset or recalibration procedure should be long completed. I know there is a "force TSC reset" process which is not openly documented but which H&K Support sometimes recommend. It's one of those "hold the '$' and '%' buttons while you switch on the amp" type of things but for the life of me I can't remember what it should be nor can I find it by searching. It is somewhere in the forum as I have recommended it in the past. If we can find it it makes the TSC drop its current reference settings and begin to recalibrate itself from scratch. It does no harm and may just sort out your problem.

Failing that all I can now think is that you may actually have a problem with the TSC control components in the valve cathodes. Each cathode has a MOSFET in series with it. The TSC digital and analogue control circuitry sets the bias current by using each MOSFET to increase or decrease the current through it while its valve is not conducting signal, (that's half the time). It has been known for those MOSFETs to suffer damage, occasionally due to a valve problem when new ones are fitted. Valves are pretty much tough forgiving devices but semiconductors are not. They make better fuses than fuses do!

Does anyone else have knowledge of that TSC Reset procedure? It is NOT the same process as a Factory Reset as described in the manual. I'm totally useless with the forum search facility, I can never get it to find anything I want. Can anyone else find threads with something like "TSC bias reset" or "TSC bias recalibration" in them?

Bordonbert,
Oh my goodness. This is so embarrassing part of me wanted to delete my account and disappear into the ether, but I need to fess up.
So I just went in the practice room to video the TSC and Wattage Lights to show what was happening. Start videoing, wait, why does it say 20W and 40W under it with a bunch more words? What do they say? Put on the glasses and behold, I realized that what I thought was 40W all this time was actually 20W. I have been practicing at 5W and playing with the band at 20W thinking it was 20W and 40W respectively. I've probably only ever looked over the top of the amp, saw the last light on and assumed it was 40W. I have owned this amp since Feb 2017 and have quite possibly never had it on the 40W. I pressed all the wattage lights out, heard the big volume bump up, and voila the all TSC lights extinguished.

So the follow up and the story ends with User Error. Thanks so much for the help, and my apologies for wasting bandwidth here.

In my opinion - there is absolutley no need to delete your account and disappear.
You are not the only one making this or similar user errors, I know that by own experience ...
Enjoy your "extra" 20 Watts of power.

Concerning the reset of the TSC-circuit - I think know what you mean Bordonbert.
If I remember right it goes like that:
- power switch off, standby switch to position standby
- plug in power cord
- standby switch to position play
- power switch on
That's it I guess.

Hahahahahaha! That's the best outcome to any thread for a long time!

Don't be at all embarrassed by this, just as Watersnake says, it's something we have ALL done at some time or other. One of my favourite Chinese proverbs is "A moment of embarrassment is better than a lifetime of ignorance." You dared the embarrassment and scorn and asked the question anyway. You got the answer and you are now aware that this mistake CAN be made, and in future, you know what will happen because you asked? Naaa-a-a-aah, you'll make it again, just as we all will from time to time.

Don't worry. One of the real positives is that, silly simple operational problems like this and their solutions are put down in black and white for all to see. There will be others who will see your post and realise that this happens and they may just be warned off making the same mistake by your openness. There is no waste of time in this, we got to the solution and you were honest enough to post it too, so well done you for that, that takes confidence and character. Now get that amp blasting out.



Oh, and here is a thought. You actually do know now that you can play with your band at 20W. My advice is, don't immediately switch to 40W if the 20W setting was sounding great! I use my own GM36 and also a baby Marshall SC20H for gigging nowadays and I use them both mostly on their 5W settings. Even then it is still too easy to get too loud in smaller venues. We aren't a quiet band, we like to think just a clear one that relies on correct balance.



Watersnake, thanks for the confirmation on the TSC reset procedure. I felt it involved the Standby switch somehow but wasn't too sure of any other details. What you described seems to be just about on the money. I'll keep on searching to confirm it absolutely, it is on the forum somewhere, but I'm sure it is as simple as you describe. Thanks again for the help.