Hi Zaknafein76 (Zak?) I have no idea how experienced you are with this sort of thing so I'll do my best to keep things in words of one syllable. If you are experienced with amps then I apologise but others reading this may also be inexperienced and benefit from clear simple instructions.
This could be quite a few things but we can do a couple of simple tests to try to tie down where the problem is. Start with a simple one.
Check 1 - The Fx Loop Socket Switches: Put a cable, short as you like, into the Fx Send and Return sockets. The signal path is routed through a few basic switches which are opened when you put a plug into the socket. You are effectively taking those socket switches out of the loop. The surfaces of the switches on the back of the sockets corrode in time and this is the sort of issue that causes. It is common to all amps with a simple jack socket switch based Fx Loop which many have. They only cause a problem when there is nothing inserted, when a plug is pushed in they open and are effectively out of the circuit. If this fixes the problem then you just need a good clean of the socket wafers by squirting in a dedicated contact cleaning spray and sharply inserting a plug in and out a few times.
Check 2 - Is it the Preamp or the Power Amp: Next, if you put a jack cable into the Fx Return socket you effectively operate those switches and separate the preamp and power amp completely. Try doing that and feed the Fx Send or Preamp Out from a different amp which you have your guitar plugged into in through this cable. You are running your Power Amp with a known good preamp. If you don't have access to another amp then plug the other end into your guitar, or better, feed it from your guitar playing through a pedal which you can turn up to increase the output signal. The signal level may well be low but you will be able to hear whether the amp is working as it should now. If you can still hear the problem then the fault is in the Power Amp section. If the problem goes away then it is in the preamp section. As a second check on this, if you have another amp available put the Fx Send from your TM36 into the Fx Return of the other amp and test. If it works fine then the preamp of your TM36 is ok. If you can hear the fault then the preamp needs looked at.
Your 12AX7 valves may be selected for the specific role they play in the circuit. It is common to have a "selected for low noise" in the first No1 position, a standard workhorse in No2, and a "selected for good balance" one in No3. Keep them doing the job they are suited for as long as they are not faulty. Contrary to internet "common knowledge", I will not swap out preamp valves until they show definite signs of going faulty. It happens so rarely that is doesn't make sense to continuously be throwing perfectly serviceable valves away every couple of years. I always keep a set of spares in my amp bag to change out if I get a problem but the idea that you need to swap them out every 18 months is ludicrous. Valves do not "wear out" through use, they gradually fade in performance with various types of misuse, (old style Standbys are a killer), and overdriving them is not misusing and damaging them at all. I have never experienced a disastrous showstopping fault in use ever. Strangely, almost everyone else I talk to seems to definitely know of them. That's the internet for you.
And on the subject of internet "common knowledge", here is another bit of myth and legend. I recommend you to do the following. You have 3x ECC83/12AX7 and 4x EL34 in the TM36. Of the 12AX7s, two are for the preamp itself and one is the Phase Inverter (PI) for the power amp. The preamp valves are the one under the can No1, and the one in the middle next to it No2. The third without a can on the other end is the PI No3. Get a felt marker pen, take the can off with a push down and twist, and number them 1-3 on the top, a small number or even a number of dots will be fine as long as you can recognise what you mean, then label the power amp valves 4-7 in the same way.
IT IS A COMPLETE INTERNET MYTH THAT YOU WILL DAMAGE YOUR VALVES BY DOING THIS, OR EVEN BY SIMPLY HANDLING THEM WITH YOUR BARE HANDS. VALVES ARE NOT THE SAME AS HALOGEN LIGHT BULB CAPSULES. THEY HAVE NO SENSITIVE COATING AND THEY ARE JUST A THICK HARD GLASS CYLINDER. LABELING VALVES IN ONE WAY OR ANOTHER HAS BEEN DONE BY EVERY ENGINEER, EVEN THE REAL OLD VALVE EXPERTS FROM THE DAYS OF OUR WORSHIPPED NOS VALVES, SINCE VALVES WERE INVENTED WITHOUT ANY PROBLEMS!Check 3 - The Preamp Valves: So let's move on to the valves and try to rule them out. We'll start with the Preamp valves. Do these tests with a cold amp so you can be sure the problem should be audible. If you have access to a spare known good 12AX7, you can always borrow it from a working amp just for the test, swap it for each of the preamp valves in turn. If the problem goes away you have the faulty valve on the table. Make sure to put the original valves back in their original positions.
If you don't have a spare 12AX7 then carefully swap No1 with No3. Run the amp up and test it. If the problem changes audibly then one of those valves is faulty. Make sure to put them back in their original positions, (the numbering might make sense now). Do the same with No2 and No3. The result is the same, if there is no difference in the fault there is no problem with these valves. It is not the fault going away you are listening for now, just any change that having the faulty valve performing a different task in the circuit will create. Make sure the valves are back in their original positions and you can leave the preamp alone. Remember to refit the aluminium screening cap on No1.
Check 4 - The Power Amp Valves: Now the Power Amp valves, although we have actually tested the PI valve with the preamp ones. Number them 4-7 first just as you did the preamp valves The Power Soak can set your amp up for 18W working. It does this by taking the inner pair of EL84 valves out of circuit. This gives us a way of checking out our valves fairly easily. Test the amp for your problem from cold with the 18W setting engaged. Valves 5 and 6 are not in circuit so it is testing whether valves 4 and 7 are working as they should. Switch off and swap the outer pair of valves for the inner pair, that is swap 4<>5 and 6<>7. Now retest. You can now say that any faulty valve will only cause your problem when it is in one of the outer positions. If you have only one faulty valve then you should be able to identify it by setting them up in different pairs in the outer slots until the faulty one becomes obvious.
I would say that 9 years is a good time to think about swapping out the power amp valves at least. Preamp valves work fine for much longer than guitarists think. I have a 1968 Marshall SL100 which is still using the preamp valves I had in when I bought it in 1970. (Yes, I'm that old!
) The power amp valves have been changed but the preamp valves are still fine. Internet myths are hard to resist by gullible musicians!!! If you find a faulty power amp valve then you would best replace it as a pair with its opposite number, 5&6 or 4&7. Buy them matched as a pair, or even consider getting a matched quad. A new pair will work fine with the other pair as long as you keep each matched pair in either the outer or the inner slots. That's one matched pair in 4&7 and the other pair in 5&6. The TSC will take a little time to catch up with setting the bias accurately but just get on and use the amp and allow it to adjust over a few playing sessions.
This will give us a starting point as to knowing how simple or serious this fault is. It will rule out that really common and simple fault with the Loop sockets. It should also let you know whether your valves are at the root of it. Get cracking and report back with the results.
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