TubeMeister 20 plays fine at home but low volume/cuts out at venues

Hello, first off apologies if this question has already been asked in this forum - I've been googling for a while but couldn't quite find my answer.

At two gigs (one with my own cab, and another with the venue's cab) I've had an issue where my amp would go really quiet and at my show last night - cut out completely (mid solo too!)

However when playing/testing at home it works perfectly fine with zero issues, and I can't work out why! I have quite a few shows coming up and don't want to constantly feeling paranoid of another volume drop/cut out during my set.

Here's what I run:

Guitar (active pickups) -> pedalboard which has a noise gate and reverb going through the fx loop, then a wah going through the front of the amp. I also have the H&K footswitch which has been quite tempermental recently too, with the boost switch sometimes not working.

I play on the 1w setting at home, and usually bumping up to 5w for live shows. However testing the other wattage at home I've still found no issues.

This is what I've tried at home already to try replicate the problem but to no avail:

- using different kettle leads
- plugging these into the wall and then into a plug extender (I think both times my amp failed at shows, it was plugged into an extender)

When putting my guitar pick into the manual reader at the back, I get 10 flashes on both lights.
When going into play mode, the lights turn off.

Hopefully this is enough context to find a solution, please let me know if anyone would like anymore information from me!

Many thanks

Hi Goozer, welcome to the forum.  It's odd that you get it at venues but not at home.  Assuming the levels are much the same then heating should not be a problem, unless the time you spend playing at a gig is significantly longer than when you practice at home.  Despite internet rumours about them H&K amps are very well sorted out thermally.  The metal case approach is a good one with the case acting as a heatsink and quickly drawing heat out to the outside world.  They run cooler than my classic Marshalls and their wooden cases.  The fact that they feel hot to the touch is not a problem, your hand has a very low threshold of pain compared to electronic components.  60degC is too hot to hold comfortably but is nothing for modern electronics.  The case may feel hot but is actually below any level which would cause stress to anything inside.  We did some work here a few years back on measuring the temperature of the lower compartment and it did not raise significantly at all under any conditions other than blocking the vents (which is stupid!).  Anyway, on to the info you have given.

When the amp cuts out do you mean it dies completely and all lights and the valves turn off?  That would point to some power supply problem as you have assumed.  However, if you mean that it simply loses sound but the lights and valves stay lit up that points to a different problem either with the circuitry power supply or within the signal chain.  Can you fill us in on that aspect?

Hi, thanks so much for the quick response!

The lights all remained turned on

Good news.  This means you have no problem in your mains supply or in the primary side of your PSU.  Your mains cable and fuse are fine.  When you said a "plug extender" do you mean like the top one in the picture I have attached?  The top one is what I would assume you mean but it isn't the simpler multiple outlet block below it is it?

An aside here but I will just pass on some safety information. I always recommend using the top setup myself regardless of whether the extra length is actually needed at a venue.  I replace the plug on the extender with a RCD type breaker and I add onto the board an "outlet tester" type of plug.  The RCD Breaker sensitively detects any fault condition on the line.  If there is any fault which results in you getting a shock you draw some current through your body to earth and that leakage is detected and used to instantly disconnect the mains supply.  The RCD breaker is the black unit and takes the place of the mains plug on the end of the cable which is wired into it like any other mains plug.  They are available in UK and US plug formats.  The outlet tester is the red unit below and it shows up any faults in the mains supply you are using.  Mine has identified a couple of venues with faulty mains installations in its time.

We are using valve gear.  Valve gear has lethally high voltages involved.  I don't like being bitten by lethally high voltages, it hurts!  I am old enough to remember Les Harvey, Keith Relf, John Rostill, all killed by electrocution from their gear.  Keith Richards, George Harrison, Ace Frehley and many others have received severe shocks on stage and luckily survived.  It's something to avoid at all costs and, for only a little outlay, these devices make playing a safe experience.  Guitarists often shake off electric shocks from their gear as though it is a macho rite of passage and to be expected.  That is plain stupid!  It should never happen, ever, and everything should be done to prevent it.  The DC in the supply line of our amps is much more deadly than the AC of the mains.  They are not the same at all.  AC, turning on and off 100/120 times a second, will throw you off a live item while DC makes your muscles spasm hard permanently and you lock onto it.  Holding your strings and grabbing a mic stand means you lock onto both and simply can't let go with the current directly through your heart from hand to hand.  Anyway, 'nuff said, that advice is now out there to consider.

The odd thing about this is that it only seems to show up when you are gigging, i.e. out of your home environment.  There must be something which is different between the two situations.  Yes the mains supply is a variable but we would expect it should not be a factor as the amp is staying on in its fault condition.  You could start by ruling out the valves as a source of issue.  You would need a known working pair of the EL84s and a single known good 12AX7.  They don't need to be new, just known to be working correctly.  Simply swap them out, one type at a time, and try the amp under stress conditions as you get at a gig.  The awkward thing is that this is an intermittent class of problem.  It is difficult to know definitely that it has been solved, only that it has not shown up yet.

The internal fuses for the LT and HT supplies are a possibility though I just don't think they are the problem.  You need to remove the bottom cover to get at them.  If you are going to check them you MUST follow the correct procedure.  1) Remove the mains plug from the supply.  2) Wait at least 5 minutes before going in.  The HT line has drain resistors fitted which will pull it down close to 0V very quickly but, without a meter to measure with, you should give it a significant length of time to make sure it can complete this.  3) Never ever ever replace with the wrong value of fuse, or worse, with something "temporary" like a paper clip!!!  At the very least you risk doing even more damage to your amp and at worst you risk your life.  Unfortunately, just looking at a fuse to see if it is still in one piece is no guarantee that it is actually working, even with a magnifying glass.  They often break inside the metal caps where it can't be seen and they occasionally even fracture and still touch so they fail only temporarily when they get hot.  Checking them with a meter for continuity is better but just plain replacing them is the only fully reliable answer.  You could easily try that.  The FU1 HT fuse should be about T200mA and the FU2 LT about T315mA.  These are the values for the TM18 which is about the same requirement as your TM20D. The correct value will be on the end cap of the fuse and you should confiirm that if you are going to replace them.

Other than that, this problem is often caused on many amps by the effects loop sockets.  There are switches on the sockets which are used to steer the signal either through the loop when plugs are inserted or past the sockets when there are none.  These can easily get a thin corrosion layer built up by smoke or damp which can make for intermittent contact.  This should not apply to you as you are using the loop and hence not relying on the switch wafers to pass any signal.  However, the other side of this coin is, are you sure your pedals and their linking cables are all working as they should?  The corrosion problem is also there for the pedal sockets and switches.  Does the problem occur with the pedalboard removed from the loop completely?  Are there tracks you could play with the pedalboard removed?  If so you could just unplug it before you play them and see.  It's a fag but it might give useful info.  You could also try switching each of the pedals on/off when the problem occurs.  This would identify whether any pedal switch is involved in the problem.  If it clears you may have the culprit.  Keep in mind that, with the current obsession with "true bypass", a fault in their sockets and switching can cause a problem even when they are not in circuit.

Anything there ring any bells?

Thanks so much for all that info! I was using the extender like the one at the top you mentioned.

In terms of replacing valves, I think I'll play it safe and take it to a local guitar tech to help with that one.

Trying to replicate the problem I was playing around with pedalboard cables etc like you mentioned. And I think it is the FS-2 causing some of the issue... When jiggling the cable I could've sworn it caused the volume issue I was having, could this be right? Plus the boost function wasn't working at all via the footswitch. Note that when I got the footswitch, it was modded to have a jack input in the unit itself, meaning you can change cables.

Wooo-a-a-a-ahh! The FS2 has been "modded" to include a jack? Which cable is that supposed to allow you to change? The FS2 should only be sending two DC levels to the amp which the amp then interprets as "switch between the channels" or "turn the boost on or off". What is the exact purpose of that additional jack? It isn't meaning you can plug the guitar into the footswitch for some reason is it?

Haha I purchased it second-hand, no clue why the previous owner made that change! I've just been using the cable he supplied, but using any regular instrument cable doesn't work.

Could this be a possible issue with why the amp goes quiet? I could've sworn it affected it when I was messing with the cable from the footswitch, but no luck since...

I don't see how it could affect the volume other than changing the channels or dropping the Boost off.  It should have a TRS type cable.  That's a two core and ground type designated "Tip, Ring, Sleeve".  It allows two signals to be carried on their own inner cables on the Tip and Ring with the Sleeve carrying the ground they share.  In our case that will be the control signal for the Channel selection and the On/Off signal for the Boost.  Without a TRS setup you won't be getting correct switching.  With an ordinary signal cable, (a TS type), the Channel should switch but you can't control the Boost.  It will stay stuck in one mode, either on or off.  (Without the schematic I couldn't definitely say which.)  You need to start with a TRS type cable if that is not what he supplied you with.  It doesn't need to be anything except a piece of three core wire with two TRS jack plugs as it is not carrying or affecting the signal in any way.  Have a look at the picture if you aren't familiar with this.

I've been thinking about this one Goozer.  That extra socket is a puzzler.  I'm assuming it is a second socket added alongside the original one?  Or is it that the footswitch originally had a fixed cable which needed no socket and he added one so the cable could be removed? (I've done that myself in the past.)

The only reason I can think of as to why the other guy would have added a second jack socket was to split the twin line TRS output between two separate cables, maybe so he could control two different functions on two different units.  These FS2 type footswitch units are intended to be pretty generic and flexible.  If that is the case he would have removed one line from the original jack socket you need to use and put it onto the added socket.  You would need to change that back and that would mean soldering.

Do you currently have control of both the Channel Selection and the Boost On/Off control on the two switches when you use it?

Yes I believe the footswitch originally had a fixed cable, and then he added the jack so a cable could be removed.

It used to work fine! But now it only switches channels, it doesn't engage the boost.

I've also changed a few cables in my pedalboard and still haven't been able to replicate the problem since, I'm just about to leave my house for another gig so fingers crossed!

Good luck with the gig.  I hope you are using a TRS type cable with the footswitch.  You can't get any Boost switching action at all without that.  Can you confirm whether you have the correct cable type, a TRS with two separate cores and a screen or third core inside?  I have attached a pic of someone soldering up a TRS plug so you can see what I mean about the number of cores.  There are three connections, the red, the black and the screen of the cable which is connected to the long silver tongue behind them which nips around the cable insulation. That screen could in fact be just a third core of the cable. Ordinary mains flex works really well in this application.

I've looked into the switching action of both the TM18 and GM36 which I have schematics for, (remember these are the same "family" as your TM20D so it will work the same way too).  If you use the footswitch with a normal TS single core cable such as a normal guitar signal lead, you will be permanently shorting the Boost connection to ground.  Effectively you will be shorting the Ring and Sleeve terminals in the amp footswitch socket across the Sleeve of the plug.  This causes the Boost to be engaged constantly.  I wonder if your volume drop could have been caused by you having that setup, a TS single core cable, and the Sleeve connection losing contact maybe through a dirty contact in the amp socket?  That would make the Boost drop out lowering your sound level fairly significantly.  From what you say, "It used to work fine! But now it only switches channels, it doesn't engage the boost.", it looks like the footswitch area is at least part of the picture.

Firstly check that the Boost function works correctly when the footswitch is not plugged into the amp, i.e. from the front panel Boost switch.  This will prove that the amp internal switching and boosting circuitry is all fine.  Assuming it does work that way then check, does the Boost LED light up when you press the Boost switch on the FS-2?

Another possibility, I wonder if you have a broken connection in one of your footswitch cable jack plugs or even a break in the cable itself which is making contact intermittently.  Are they moulded types which are sealed permanently or can you open the jack plugs up to check their connections?