Trilogy Amp volume drop

Help ! I recently bought a Trilogy head and 4x12 as a back up rig, for my own Trilogy Amp
The clean channel works, ( but it is noisy)  but crunch and lead channels are both way down in volume, lead channel is pretty much gone, crunch is very low.
The amp was stored for a long while in a flight case in a dry room, it was dusty and I thought maybe a clean and service would sort the issue, but it didnt.
I reseated the valves, they are all working, all lighting up at least. The amp section underneath was spotless, no obvious signs of any burnt out components or anything odd.
Visually, it all looks identical to my own, when running.  
I noticed the clean channel button was sticking so I cleaned all that so its free again.
Seemingly the previous owner never used the dash buttons, just the foot switch, so i plugged that in, and just hovering my finger on the switch causes the channel button to click on and off.
It doesnt have an electrical hum when I do this, but the whole amp is noisy,
Clicking the switch causes the buttons to stutter for a second before settling down on whatever channel you clicked on, so it feels like a short somewhere.

I will swap out the valves in my own rig just to rule them out when i get a chance,
but Im thinking the guy used mostly the clean channel and his pedal set up in through the loop, so maybe I need to reset the channels somehow, like its stuck on a midi setting maybe
Any ideas welcome

Hi BC.  I have never seen a Trilogy in real life so I'm working from just your descriptions and logic here not experience.  By the way, top man for being so clear on what the problem is and what you have done, we don't always have so much accurate info to go on.

The Clean channel works but is noisy and Crunch and Lead channels are down in volume.  My first guess would be that you have a valve going down somewhere.  Full marks for setting that as your first test to swap out with a known good set.  That should at least rule out that possibility.  The fact that the valves light up is no indication that they are electrically in good shape I'm afraid. It is very common to find you buy an amp which is being sold with a very old "throw away" set of valves fitted. Many guitarists take out their original good set to use as a spare and insert some potentially dodgy ones that they have lying around just to sell it.

In reality it might be better to just take a single known good 12AX7, (I'm assuming the Trilogy uses 12AX7s exclusively in its preamp/PS slots), and replace each of the preamp valves with that one one by one.  When (if) the amp jumps to life you then know immediately which is the dodgy valve.  Make sure to keep the original valves in the same spots you pull them out of too.  That's a small point but it is sometimes helpful if you have a valve or two which has been selected for low noise or balance.  You want them back doing the same job they were selected for.

As to the power valves, I would replace them for testing as a matching pair. Usually the inner pair and outer pair are matched, use the pair of either of those from your donor amp unless you know it is an oddity, and test either the inner pair or the outer pair in your Trilogy. If you find a dodgy one you are probably going to replace the pair anyway unless you want to get a single one which matches the remaining set and even then you have one or three old valves and a brand new one, not the best situation as they age. You can get away with swapping a single one out for a test but always swap in pairs or sets.

Contrary to popular belief (internet "common knowledge" is anything but common and certainly rarely knowledge) it absolutely does no harm to a valve to handle it with your hands (clean hands that is) or to mark it to show its location by numbering with a felt marker.  Engineers and technicians have been treating them this way for the best part of a century, even in the days when they actually really "knew" valves, without any problems at all.  If you could have seen the state of some of the valves in old TV equipment after a few years you would realise they are INCREDIBLY forgiving of dust and dirt.  Manufacturers print the valve type and their details onto the glass without any problem when you buy it.  Valves are NOT halogen bulbs, they are just a thick hard glass envelope which is externally inert in every way.  Keep them clean but don't worry about handling with rubber gloves on or using solvents to remove fingerprints or polishing them with a special lint free cloth or any of that crap advice.  If they look clean to your eye they are absolutely fine to use.

Good that you fixed the Clean panel button.  I'm assuming it was only the external parts of the switch which caused the sticking and nothing inside it?  How did you clean it?  It is a very bad thing to use anything which contains silicon in any electrical equipment.  It forms an insulating barrier over surfaces only a few molecules thick and it creeps over surfaces after you have applied it.  Mum constantly polishing the old wooden TV case with silicon polish was a recipe for eventual disaster.  Proper specific contact cleaner is the only thing you should use on your components really.  Definitely NO WD40 original anywhere near your amps and guitars!!!

Hey Bordonbert

Thanks for replying, I havent had free time to sit down with the two amps and start swapping valves yet, I seem to be working every day lately.
When Im not working Im gigging, so I just have the problem amp at hand, my own rig is in the wagon.

I cleaned the panel switch with contact cleaner, and worked it until it loosened, wasnt anything extraordinary about the connection, no dry joints apparent, was a bit grimy.
But I would nearly swear the volume on the crunch channel got worse since I did that, it seems lower than I first thought at least.

Im very dubious about the way the switches click when hovering my finger over the footswitch button, obviously there has to be some kind of short or ground issue there,
Im thinking a dodgy valve wouldn't cause that, but Ive been wrong before, ofc I could have multiple issues, God knows people do some strange things to their gear !

The back panel had been removed before I noticed, when I asked the previous owner he claimed he thought it was overheating one night and took the panel off to help it,
but there are multiple vents and Ive been using that amp for years and never even suspected this as an issue.
Makes me wonder if he was running it on the wrong speaker set up or something odd like that.

I may well need to get it looked at, will try the valve swap first though soon as I have an hour or two spare to sit with them both.
Also thinking of swapping some fuses just in case, could be as simple as an odd fuse.

Anyways, wont give up just yet !

Looking into this it seems that this is a common problem with the footswitches stopping working with age. I don't know the Trilogy line very well at all so I'll see if I can come up with any tech info to help us look into that side. Hang around for a while...

Ok.  I can't come up with a schematic for anything Trilogy, (I'm not an official service centre of course).  A couple of observations re the footswitch.

Are the switches "click action" or a soft "momentary contact" types?  This would indicate a different potential for faults in the switches themselves.

It has a single TS (2 pin) jack plug carrying the info from 4 switches.  That just 'aint possible if each switch is a plain old "short the amp's signal to ground" setup.  I've come across something like this with my Marshall JVM205H which has a microprocessor inside the footswitch.  It sends out a couple of data packets to the amp containing the settings for all of the switches every time a switch changes.  I had to reverse engineer that and hack their data protocol before I could do anything.  It is actually very basic and I would suggest H&K maybe have done something very similar.

The control line normally sits at a high voltage and the footswitch charges up its power line inside from that.  When any change is made to its footswitch it resets its data and sets up a new packet then fires it down the line with a first couple of low bits pulling the line down for recognition.  The amp recognises this and receives the data packet within a short time window then sticks the line high again recharging the footswitch's supply.  So it sort of switches between "power the footswitch most of the time" and "transmit data very quickly then swap back to recharge" states. The same thing happens in reverse if a change is made at the amp, data is sent to the footswitch to update its status.  This takes milliseconds of course so there is no problem in doing that.  Ok, tech lecture over, (somebody somewhere might be interested!    )

How does this apply to us?  Well if there is a genuine fault inside you are stuffed!  Without electronics knowledge and the right equipment you won't be able to work on it.  But...  If I am correct, the setup is pretty much bombproof in terms of damage in correct use.  (Do something daft like putting 50V on that line and its a different matter of course.)  The downside is that we have much less power available for the circuitry in the footswitch, remember it is being powered by the data line and not a dedicated power cable.  We have to run with higher impedances meaning lower currents passing through and this system becomes much more susceptible to errors due to dirty contacts between the plug and socket.  I would do the usual thing and clean thoroughly the footswitch socket on the back of the amp.  I haven't a schematic but it could very easily use the switch wafers on the socket to control how the amp responds to a footswitch or no footswitch.  If they are dirty and intermittent, and that isn't always obvious to the naked eye, then the whole action of the footswitch cannot be powered reliably and data passage can easily be corrupted.  Clean the footswitch jack plug and flood the amp's socket thoroughly with proper cleaner spray then quickly insert the jack a number of times fast and furious.

See if that helps to steady it down.

The other problem you may have is simply that the switches inside the footswitch themselves are dirty or worn. It is common for devices like this to use a tiny "tactile" switch inside and a simple mechanical push device to activate it. The tactile switches have to be PCB mounted and the activators commonly look exactly like the usual stompswitch without the guts. They are often missing their "click" action when you press them though not always. If you have the time you could open up the footswitch and take a couple of pics of what is in there. The switches may well be replaceable if they just use plain old free mounting stompswitch types. If they are using the tactiles then you would need soldering skills to do that.

Hey Bordonbert

Some excellent info there, lots for me to process. I appreciate your help and understand I need to get my finger out as it hard to diagnose things without all the info. i plan on bring the map home after this weekend.

Even without the footswitch, just using the panel switches isnt a direct single click swap to the next channel, I press it and it fast clicks a few times then settles on the channel I selected.

As I have two the same amp cab and foots switches I should be able to do direct comparisons which will give me more precise data of whats going on.
I will have to work out a list of things to check in particular before I swap the valves, its probably worth my time swapping things like the cab even, just to eliminate wiring issues there.
Happily my footswitch is like new as I rarely use it live. My own rig is in excellent condition too so a few hours of methodically swapping things one by one should yield a better idea of whats happening.

You're on top of the diagnostic work. Great to help someone who wants to do the footwork himself and just uses the advice given to help him. You'll learn quickly like that.

I have to say that, if the amp is misbehaving from use of its own switches as you describe, then the footswitch is unlikely to be the issue. Chyanging it out for the other one should eliminate that. That "clicks a few times then settles" is very odd but, thinking about it, I actually have to wonder if that is perhaps normal? Could it be that, as the amp has to cycle through 3 channels, it sometimes has to do two separate selections before it gets to the one it wants? Failing that it sounds to me as though you could have a fault or faults in the tiny relays inside, or if perhaps the switch you are pressing is faulty.

One heads up here, don't assume because your footswitch is not used very often its plug and other connections will remain pristine. In terms of plugs and sockets, it is actually better to have them in normal regular use being removed and fitted occasionally as that keeps them wiping their surfaces clean against each other. The level of contamination that can upset a good connection is often too fine for the eye to see and is just down to atmospheric corrosion of a metal part.

Make sure to keep us up to date with what you find when you can get it home.

Result ! Have found the issue. We tested all the valves and found them all working perfectly which was worryingly pointing at deeper issues. So I left it sit until I got my own head back home. My own head has had the tubes changed a few times now, its got TAD valves and the newer head has the originals still in it. So I began by swapping the pre amp valves first, and found no difference. Then I noted the power valve in the newer one was a 12ax7, same as its 3 pre amp valves, and on my own the power amp its a 7025 TAD as opposed to the 3 EC883S. They all look identical. Once that set was swapped in it fired right up ! This also sorted the switching issue. It must have recognised the power was down so it wanted to cycle for a working channel ! Oh relieved to say the least. However, this means the chap I bought it from new it wasnt working, because it couldnt have worked after he did that, buts thats a different challenge.
Anyways, your support was much appreciated Sir. I thought I was done for there for a bit haha

Rock N roll shall be resumed with vigor !

Many thanks

Well whaddyaknow? Good work you BC!!!

I never miss an opportunity to preach the word. This whole thing of "there is a magic valve combination out there which will make my new cheap Chinese head sound exactly like a vintage Fender" is such a crock! It doesn't stand up at all on the test bench where the test equipment is decades more discerning than our ears, and it doesn't stand up in terms of the electronics that goes into the design of these technical marvels, (well - sometimes).

Not only do I pooh-pooh the idea that valves sound markedly and predictably different between manufacturers, and I can show the evidence that that just 'aint so, I always speak out passionately against the idea of "12AX7/12AT7/12AU7/12AY7 - all in the same family so swap 'em at will" advice. THEY JUST PLAIN ARE NOT AND THAT IS ENGINEERING FACT! Would the people who give out this advice swap a carburettor from a Peugeot onto a Ford because they are "all carburettors"? The X/T/U/Y valve types are individual, deliberately very different valves, each designed to perform a different task. They are not a "family" in any way. Their similar designator numbers were given because they are the same capsule and pinout not because they were a group sharing characteristics. Putting any one into a slot in place of any other is a disaster inside the original design for the stage, a complete lottery in terms of how the change will perform electrically and in sound, and it could even lead to circuit damage if the amp has not been designed very robustly, and not all have been. That view isn't popular - but it can't be argued against.

Now, on paper. your 7025 really should have been a full equivalent of a 12AX7 although it should have lower noise. You have found out that maybe that isn't true in all cases - or maybe there is something else at work here. I suspect that 7025 may possibly have problems other than just not matching somehow. Best check out what you have found. Which valve slot was this valve in? If you look into the amp from the front V1 is on the right with the screening cap over it and V4 is on the left. I would expect it to be in the V1 "under the can" slot as it should have the lowest noise of all the 12AX7s. How does the newly working amp perform if you swap that 7025 into one of the other slots?

I think you would be wise to investigate and check out that 7025 and either clear it or bin it, otherwise you may assume it is good and reuse it again later when it may be faulty.

The 7025 is the one that works ! it was in the v1 slot on my own head, along with 3 ecc83s in. Part of the latest set I bought from Tube doctor.
Its the 12ax7 that wasnt working in the V1 slot in the troubled amp, Which is odd because I seem to remember a 12ax7 in there originally on mine too.
I would have sworn that blind if you asked.
Just for clarification, First I swapped over the 3 ecc83s first into V2 V3 & V4, and the amp was still down, then I swapped in the 7025 in the V1 slot and off it went.
So now Im thinking ok, maybe the 12ax7 in the V1 slot was faulty, so I start swapping back in the other 3 12ax7s along with the 7025 and its still working fine, confirming the v1 slot was the issue.
But then I swapped in that 12ax7 into V2 and it still working fine, now we checked that valve and it tested fine as well, which is why I figured,
Ok Well then the 12ax7 is the wrong valve for the V1 slot then !

Ah, gotcha BC. Sorry, I got the wrong end of the stick there. So basically, the 12AX7 just won't work in the V1 slot. You would have got a 12AX7 (that just the US equivalent of the UK ECC83) in the V1 slot originally. All of the H&K amps are supplied with basic 12AX7 preamp valves as far as I am aware. It's the common or garden workhorse. There are tiny wrinkles in some of the flavours of 12AX7 available, such as the long plate or special grid designs, but they all have to have the same overall characteristics of the 12AX7/ECC83 designation.

That is extremely odd I have to say but it is ringing bells in my memory now. I do remember having had a similar experience when trying a JJ long anode type ECC803S in my own V1 slot of the GM36. The amp just did nothing! It just stayed silent. My supplier was mystified and immediately swapped it for a different type. I sent the ECC803S back and he tested it and, just like your own valve, it tested absolutely fine. There are some situations where you just have to accept there is something at work that isn't immediately clear and move on. At least you have proved that it can still be used anywhere but in the V1 slot so it isn't a total bust.

Anyway, that sounds like you have made a good buy now. I'm very envious of your Trilogy setup, I would love to try one myself. Sounds to me like it's time for you to get some serious rocking done!

Hey Bordonbert, Disaster has stuck ! The two amps went down at the same time ! Trying out a new guitar player for the band, the guy decided he wanted to use one of my amps and BOOM, dont know what he did, but it was just crackling no output. Right after that, while Im trying to fix problem A, our bass player, decides to flip the switch on the gang plug I was using and BOOM my own amp went down too ! Couldnt script it. This was right ahead of a European tour a few weeks ago !

Both V1 valves were blown, so I had to wait for Amp Doctor to send me more, and that resolved problem A, however Problem B is another thing entirely.
(It took Amp Doctor a full week beyond the advertised schedule to send the replacements and I ended up renting some amps in the end as I couldn't rely on just one, considering I personally didnt blow any of them and this was now getting expensive, I wasnt pleased !) Anyways to the problem at hand

My own amp was turned on, when the BP flipped the switch on the gang plug, It just flashed when he flipped it back on, it still lights up, but has zero output. I have replaced ALL the valves, the anode fuse and the mains fuse on the back with the ones I know are working on the sister amp. and there is no change. The valves aren't glowing or flashing on power up, So I'm thinking its the fuse for the tube heater or possibly the standby switch is faulty, my understanding is the standby mode just sends power to the valves bypassing the pre amp section, being stuck in that mode could therefore give me no output ! Im also told H&K also have a TSC system that might need resetting. I believe thats some kind of auto bias system, but there is nothing in the manual to suggest the Trilogy has that installed.

I cant find any schematics ofc, so I have no idea of the fuse rating for the Heater or even where it is, likely more than one. Im assuming its ( or they ) got to be attached to the main transformer.

Any info would help before I take a deep dive !

Bluecharvel

Hi again BC.  Sorry to hear about your ongoing woes with the Trilogys.  I thought we had this tied down but then, there is no accounting for bass players is there?
A few of the H&K models have their schematics and even the full service manuals available online.  The Trilogy doesn't seem to be one of those.  However, a quick dip into the User manual found this:

H&K Trilogy User Manual wrote:6 Troubleshooting.
Trilogy won't power up when you switch it on.
Trilogy is connected properly, but no sound is audible.
• The fuse for the tube heating tripped (the tubes don't glow).  Ensure that it is replaced with a fuse bearing the same rating.  Always have a service technician replace fuses.

You seem to have been spot on.  The other H&K amps like the GM36 don't have a heater fuse, they just rely on the mains fuse blowing in the event of a heavy current draw via the heaters.  You have the perfect set of symptoms for that so my guess would be that that is more than likely what has happened to your amp.  It lights up so the mains voltage must be there, the valves don't light up so there is no heater current.  The common denominator is that heater fuse which your Trilogy does have reading from the manual.

Looking further down to the technical specs I can see this:

H&K Trilogy User Manual wrote:Internal fuses: 1 x TT 10 A Slo Blo, 2 x T 630 mA

That gives us the correct info.  630mA is far too low for a heater fuse and must be the HT fuse, they are usually about that value.  For a 4x12AX7 and 4xEL34 valve set you would expect to see a heater current of about 4x0.3A+4x1.5A=7.2A.  The 10A is the heater fuse.  [EDIT:  Thought it might be useful to add this as we are talking about fuse specs.  Mains fuses should be:  Europe: (variable: 220V-230V or 240V) 2x250V/T1.6A (5x20mm).  That is half the US/Canada spec of T4A due to us having the more sensible voltage .  Everything after the mains transformer is the same for both sides of the Atlantic of course.]

Now as to fitting it I would guess that you should simply open up the amp and it would be easily visible.  Do make sure you have no residual voltage left on the caps when you go in.  H&K usually fit drain resistors on all of their PSU caps which should take care of it.  Personally I just use a suitable plastic item as a lever and plastic tweezers to winkle the fuse out without touching anything with my hands.  Just do it taking into account that you don't need to touch anything else.  The heater circuitry is only sitting at about 6.3V when it is on so there should be no problem with high voltages but it never hurts to be extra careful.

I have attached a picture of what I found online for the layout of the Trilogy internally.  It isn't closeup high def I'm afraid but it should help us.  You can see the fuse area in there ringed in red.  It looks like there may be three, the lower one is odd but could be a fuse.  I'm guessing you swapped out one (or two) of these and maybe left the heater one untouched.  Is this what you see inside your own amp?

Hey Bordonbert,

Mighty stuff, Thats everything I need to know, the mains fuse and the anode fuse are rated on the back, the anode fuse is 630mA, So its safe to say the Heater fuse is 10A,

The quickest thing to do is open both amps, swap out which ever fuse is blown in there, that way my original amp is back cooking, order up a few slow blow 10amp fuses and Im off to the races

While Im at it, I should take detailed photos of the insides just in case we need to reference anything again since H&K make it so hard to get any detailed info of these amps.

Thanks once again for your sterling work Sir. Very much appreciated, If I ever get over your way with the band, I will make sure you have tickets

I will let you know how it goes during the week.

Bluecharvel

Good to hear you are still up about it.  These things are irritating and at times infuriating but they are part of life with a valve amp!

That's a great idea about taking pics while you have the amp open.  Try to take an overall clear one and maybe a couple of part shots in more closeup.  It really does help to have a reference where you can almost read every component's value.  Hope to hear it is back in working condition again p d q.

Just one quick bit of advice. Don't completely trust your eyes with fuses. Sometimes they look fine to the eye but have actually broken inside the cap. A continuity test with a meter or a battery and bulb or something similar may be needed to be completely sure a fuse is ok. Even then it can be dodgy. I have just fixed an Egnater which lit up and played for a few minutes before descending into a series of loud explosions through the speakers. Turned out to be nothing more than the mains fuse wire detached in the cap and sagging as it warmed up making for an intermittent connection flickering in and out.

Hey BB,
Problem solved ! It was indeed the heater fuse, although you couldnt tell that from looking at it, as per your last reply !
No discolouration, no burn or scorch marks.
I stripped both amps which takes ages and is extremely finicky btw, must be 16 screws holding it all in on every surface ! I totally forgot how much of a chore it was !
Wouldnt want to be doing this at the side of the stage. Im glad I have a back up !
Even have to take off the shoulder knuckles because the perspex dash wont pass them, things are that tight.
The insides are still pristine, Have to to acknowledge the work of art H&K did in there.
Inside are two 630ma fuses, No idea what they do ! They seem to be on a circuit of their own, so it could be the LEDs maybe.
There is a separate one just below that, it is a single slow blo 10amp, which they were kind enough to mark on the PC board

I took detailed pics, and close ups for future reference. I can upload them if you would like, maybe help some other pour soul out of a fix.
I have the second amp still apart awaiting a new fuse, so if there is anything in particular you need in there I can do it easily

Many thanks again

Rock n roll has resumed !

Bluecharvel

Excellent news BC.  I know that sometimes the modern style amps can be a pain to disassemble.  My own Marshalls are a few screws for the back grille and four underneath on the chassis and voila.  My current Fender HRDV is even better, lay the amp on its face, take off a handful of screws on the back and it pops off leaving the chassis still mounted inside but with the circuitry fully accessible.  But that's the price you pay for all that flexibility.

Your two 630mA fuses inside are pretty much undocumented as far as I can see.  It seems odd but the manual describes them as:  External fuses: 1 x T 630 mA (anode); Internal fuses: 1 x TT 10 A Slo Blo, 2 x T 630 mA.  I can't find any info on the Trilogy fuses strangely, but the Coreblade uses one fuse for the preamp and the other for the power amp valves.  If we take their word for it and accept that the rear panel 630mA fuse is for the anode HT then I wonder if the two internal 630mA fuses are for the two sides of the mains transformer secondary.  Anyway its fixed, and you didn't even resort to crumpled tin foil or a paper clip to do it as most guitarists do!     (...did you?    )

Posting your pics of the internals, PCBs and general layout etc, would be great.  It would certainly help to fill in a little of the picture with an amp which currently has virtually no technical info online.  Others would benefit from that I'm sure.

Hey BB

Update : Today I tested the amp again ahead of this weekends gigs, and the lead channel is suddenly making all kinds of noise, so I swapped out the V1 again, and its working away no problem ??
The tube has died I assume, and Ive no spare fuse to try it in the second amp. Amp hasnt moved or been touched since last week by the way. ( I will wait for the fuse, no tinfoil for me ! )
I ran it today for a few hours and it seems fine, will try it again tomorrow.
I dont know if I can trust it now and this annoys me no end, so I came up with an cunning plan (ala Baldrick). Ive got an old H&K attax 200w combo with 2x V30 speakers in it,
I wired two lines from the speakers into an input and mounted it in the chassis, so I can run my head through those speakers, and if it goes down I can revert to the combo !
Funnily enough that entire lot including the trilogy head mounted on top fits inch perfect into one 4x12 flight case. It looks weird but it works a treat !
Upstairs for thinking, downstairs for dancing as my old man would say ! Should have done that years ago, save me carrying the head on its own in a case that weights a ton !
Oh well, hopefully the amp has settled down.

Bluecharvel

Hi BC. Just back from a Tokyo trip without too much modern internet stuff at hand, sumimasen, sorry for the delay. Getting back into western mode with food and sleep differences and after a 14+ hour flight is not too easy. One good thing is, while I was there, I got my hands on a fantastic Yamaha SG-1000 in Tokyo at their prices. Even with the import duty and taxes it is still a great buy for me. I have wanted one of these amazing guitars for years now but they don't seem to turn up too often in really good condition in the UK. My new baby is home now and purring - loudly! It plays like a dream.

It sounds as though you have just been unlucky with your valves again. They are temperamental bits of kit and will do just what you have experienced just when you least want the problem. If it is now working with a replacement valve in as you describe, the only way is to give it all you have got and hope the fault is gone with fitting the new valve. What condition is the valve socket in? Could it maybe be in need of a cleaning spray into the pin grips? You could try putting that suspect V1 valve into one of the other slots to see if it gives similar issues there. You could be more certain that it was just the valve that way.

With this coming down to just valve issues there should be no knock on effect into more complicated (and costly?) problems. If a valve goes out in a stage and you have your fusing correctly set up it rarely causes damage to the more expensive components like transformers. That isn't guaranteed of course but in your case I would say it is very unlikely. The main problem with output transformers are caused when you lose the load through cabling issues. A total short is actually less dangerous than an open circuit, as long as you catch it quickly of course. Without any load it is possible for flyback spikes to be generated around the output valve/transformer primary area which can punch through the transformer winding insulation and voila, no more transformer. A 12AX7 issue which would be ahead of the transformer shouldn't cause anything like that. ...as long as your fuses are correct, (which we know they are)!

Your proposed solution seems to be a really good workaround for me. Having the ability to swap between the head and the combo amps more or less on the fly should give you the security you need at a gig. You are right when you say the only other thing you can do is to wait for your fuses to get your backup head in the setup. That is just a question of time. And maybe try to score a couple of spare 12AX7s for backup.

Keep on keeping us up to date, it is great to know when the problems we talk about here are genuinely fixed. Just knowing others have a bit of an odyssey to get there is often a bit of a relief for anyone suffering similar issues.