Occasional crackle TM36

Hello - I'm new to this forum.

I have a TM36 which is 6 years old and I hadn't used it a whole lot until more recently.   It has been through a few moves.   I've only used it on 1W mode and now that I'm using it more frequently I notice an occasional pop/crackle.   I also notice some fuzzy distortion at low volumes even with the gain down, but I've read that that is not uncommon.   I really want to get as clean a sound as possible.  I'm want to use this in stereo with my Fender Twin Reverb.

Anyway, I'm assuming it's a valve issue, most likely with one of the center valves.   When I do the "guitar pick" test, I get identical 10 blinks in the center and 9 on one side and 11 on the other.   The valves all seem to glow at the same intensity. There is rectangular blackish discoloration on the glass at the side ports on the valves which I assume is normal over time, but it is much more pronounced on the second tube over from the left looking from the back.

I'm assuming I need to replace some or all of the valves, correct? These are still the original valves.  

Thanks in advance for any additional guidance.

Hi AamirZ, welcome to the forum. You are right, this could well be a valve problem. It should be easy to tie it down assuming you are familiar with how to get to the valves to replace them. If you are not, please ask as there is a correct procedure to do this which makes it easy for yourself while upsetting the amp the least. The testing doesn't need any special gear but a spare 12AX7 valve, maybe borrowed from another amp such as your Fender. It has a 12AX7 in its preamp. DO NOT BE TEMPTED TO USE ONE OF ITS OTHER VALVES! Contrary to what every guitarist and tech knows there is no family relationship between 12AX7 and 12AT7 valves, or any other of the 12A*7 types. They are all different and designed to do different jobs. They are not all 12AX7s with different "gains". (Valves do not even have a gain in their parameters.) They are NOT equivalents in any way. If you managed to fit a Toyota carburettor to a Ford engined car it may well run up but you will risk severe damage by using it in that state! This is exactly the same. Just because the amp works with a 12AT7 in it does not prevent every aspect of the stage's design from now being skewed, one or two of them towards risking damage. You have been warned.

Firstly, your valves measure up correctly on the TSC "pick test". Having a difference of 2 flashes between a pair of valves is within the correct 4 flashes maximum for best sound. Don't worry about that no matter what your valves may look like. Dark patches on valves are perfectly normal and don't necessarily indicate a fault. And don't fall for any nonsense about not touching your valves with your fingers. It's total rot and one of the widest spread myths in the guitar nonsense field. They are NOT the same as halogen headlamp bulbs, they are just like milk bottles that get hot.

To test for a noisy valve you will need to swap the valves in a particular way. Open the top cavity to get access to the valves. This is a perfectly safe thing to do if you have followed the correct procedure. Run the amp up and try it on one of the lower settings, 1W is fine, until you notice the noise. Turn it off, wait a minute or two, then carefully swap the outer pair of the large EL84 power valves for the inner pair. If we number them 5-8 from one side to the other, (the preamps are usually 1-3), that means swap 5<->6 and 7<->8. The outer pair now become the inner pair and vice versa. Run the amp up again and try it. If the noise has gone away your faulty power valve is now in one of the inner pair slots, that is 6 or 7. If it has not gone away then you need to check out the preamp valves.

The 3 preamp 12AX7 valves, (assuming you don't have any silly "equivalent" types in there), are numbered 1-3 with 1 under the aluminium cap. Take the cap off, just push and twist. If you have a spare 12AX7 then swap it with V1 and try the amp. If there is no difference then put V1 back in and swap with V2 and so on. When the noise goes it means you have removed the noisy valve.

If you don't have a spare 12AX7 to hand then you need to swap first 1<->2 and see if the noise changes. It cannot go away with this procedure as you are still using a noisy valve albeit in a different slot. However, if a faulty valve is put in an earlier stage with more gain following it the noise usually gets louder, if it is put in a later stage it gets quieter. So if this gives you more noise the suspect valve is the one now in V1 which was originally in V2. If there is no difference then put V1 back in its original slot and swap V2 with V3. The same thing applies.

You need to keep the preamp valves in their original places as V1 is often specially chosen for low noise as there is maximum gain following it, and V3 is often specially selected for accurate balance as it is the phase inverter. Give these tests a try and see if you can isolate the noisy one. Let us know what you find with the tests and we can point you in the right direction.