Tubeneister 18 Low Volume Output

Extremely low volume normal and lead channels. Jumped efx loop problem solved. Sprayed contact cleaner on plug and installed 15 times on both send and return, plugged back into input jack front of amp, same problem, low output. Plugged into return jack of EFX loop, has normal volume without preamp controls. Looking for any ideas on this. When jumping EFX loop, output is strong. Do not believe any tubes to be at fault.

Sorry I missed this CA, it's been manic here chez moi.  It's dirt on the switch leaves of the Loop Return jack.  It's a common problem on hundreds of amp models.  You have absolutely classic symptoms.  It's no good at all trying to fix this by spraying the plug and inserting it, that only cleans the signal contact points and your problem is in the switching contacts.

The picture shows where it needs a clean.  The signal contacts are in the middle of the leaves where the plug would naturally lift them up.  The switching contacts which carry the signal when no jack is inserted are at the ends of the leaves.

You can clean it by unplugging the amp then flooding the whole socket with contact cleaner so it reaches those points and inserting the plug a few times to open and close them with solvent around, spray directly into the socket opening for a few seconds then GIVE THE SOLVENTS TIME TO EVAPORATE OFF BEFORE TRYING THE AMP!!!  If you are more up for going inside your amp as some people are AND YOU UNDERSTAND THE RISKS OF DEATH IN HIGH VOLTAGE CIRCUITRY then get to the jacks and insert a plug, slip a piece of paper between the contacts, remove the plug then pull out the slip of paper.  This wipes the contacts clean.  A little contact cleaner on the paper helps too.  Do not use any force which might bend the contacts and don't use anything abrasive like emery paper.

Incidentally the loop jack sockets are fitted on the board upside down so with the amp standing normally on its feet a spray into the sockets should drain down and around the contacts as you want it to.

Same symptom here, but I think there is another problem.
My TM 18 head (with 1x12 cab) was working perfect upon yesterday, using it in my room, with attenuator at 1W, normally is impossible to turn volume more than barely minimum, because is too strong.
Today I turned it on, only using return to play with my modeling (same than yesterday), only can get low volume output, turn to maximum in pedalboard and still low volume.
Then changed and plugged guitar direct to input, nothing connected in loop, low volume again. All controls seems to work, treble/mid/bass gain and volume in both channels, sound seems to be OK but at extreme low volume.
Both led lights on the back are OK (on when standby off when amp is on) .
I guess that maybe is a power tube problem, but all tubes seem to have the filament orange lit.
Do you have any advise?

I just posted advice about using a plain guitar cable in the Loop Return and Send to test the sockets but rereading your post that would seem to be unnecessary as you are using a modelling unit into the Loop Return anyway. It is something you might give a try as it takes only a few seconds and it can't do any damage to rule out those sockets.

It seems unusual to find that your power amp section has gone down with both valves together. The fact that all valves are lighting up is no guarantee that they are working as they should but to find both valves going down together and still showing correct LEDs on the back seems to be very odd.

The problem can't be a dodgy Standby switch as you say the LEDs react as they should when that is used.

My only guess at this stage is that your V2 12AX7 valve has gone west. Your amp has a single triode type of phase inverter whereas most amps have a twin triode type. If your PI goes faulty then it would take out both output valve signals together. I would swap V1 and V2 and see if your modeller in the Loop Return can then get a normal signal out of your power amp. If it does that is your problem. If that makes no difference then I would be sure to swap the valves back to their original positions after the test.

Thanks Bordonbert,  
I thought it would be weird too,  but I did the test anyway. Situation is as follows:

- Volume is degraded at VERY low level, in example: Attenuator at 18W everything at max is comfortable to hear at 50cm from the amp. no matter clean or distort channel with or without booster.

- I checked the speaker cable with a tester, almost 0 ohm in lead or ground, seems to be OK.
- Guitar direct to the input: problem.
- Add patch cable between send and return: same problem.
- Guitar to interface (irig HD2) and from it to the return, same problem. I can turn interface volume to max with low output from the amp. (usually the volume at interface is close to minimum and I scares myself for neighbors)
- put a guitar cable to the return and touch the other extreme with finger, I receive a hum noise not too loud. Do the same in input and same amount of hum (even less maybe with the volume not at maximum).

- Still not checked to swap 12ax7 tubes. I will do it in some minutes. But I thought 12ax7´s  has no effect on the signal inserted in the loop return, I know very few of tube electronics, just a the popular belief.

*******. UPDATE SOLVED******

Swapped 12ax7 tubes and amp comes to life again through loop return. You were right.

To take advantage of disassembly, I did the test of knock the power tubes with the screw driver and one of them is damn microphonic.

A new set of tubes will do the trick!. thanks a lot.

Yay!!! That's really good news Jeando. Good to find it's a cheap fix too.

Keep in mind, all valves are microphonic, it's just a matter of at what level. Their mechanical structure means that at some level they must start to pick up vibrations and transmit them into the signal. You can hear noise when tapped with almost all valves so the fact that there is noise there is not the killing factor. It is how much there is and that only comes with experience. It's another piece of "online truth" that is being wrongly interpreted by some and it is increasing the unnecessary sales of valves to people who think that their valve is faulty because they can hear this. No one considers that the right level of this sort of vibration noise is a part of their amp's character and sound.

There are some choices to be made here. Long plate designs of 12AX7 which are now on sale tend to pick up vibration more easily as they have bigger surfaces on their inner parts. These are a little less suitable for input valves where signal levels are low and noise is more of a nuisance. You only have to replace that one valve of course but I'm sure it won't hurt to swap out all your valves with a complete new set, however, don't put them all in to start with. Firstly, only replace the one you know is bad. That way you will be able to hear how well the others are still performing. If the amp sounds good I would guess these are still ok, even the microphonic one may be fine in use. As long as the amp works well with only the PI swapped out then you know you can keep the old set as spares. Valves have been known to just go pop as you have experienced and having a known good replacement to hand is alwasy a good thing.

Good luck with your "new" amp. Don't spend too much time trying to get to the "ultimate" valve makes for tone, there is no such thing. If you dig around here you should find that I'm not a believer in the sound of valves being wildly different, the engineering evidence proves conclusively that there is no difference in circuit between the manufacturers. People out there who have no technical expertise will tell you differently. Ears are fine as a guide but you should always go to the Physics and Engineering to get to the truth. Nowadays, with test equipment and procedures and the way we understand how our simple circuits work, if you think you can hear it but it can't be shown then you are deluding yourself. If you can't show it then it doesn't exist! If you choose to take on board the old myth that they all have a different tone then fine. Just keep in mind the old saying, "perfect is the enemy of good!"

Thank you again for your extremely accurate diagnosis.

I am not a tone geek regarding tubes, before expending hundred $$$$ in NOS tubes I rather prefer to go for an additional different Head amp, but I already have a Marshall VM 2466 and if I buy another amp my wife is going to kill me .

Following the up mentioned I am going for cheap non Chinese tubes that are available for fast delivery here (coronavirus delays leaves us few options).

Then, bought a couple of matched JJ EL84 for the power section and two Tung-Sol 12AX7 for the pre section.

Maybe is an off topic issue, but can you please explain me further the vibration issue of 12AX7 vs. other (ECC83 maybe?), what are the options?.

I'm no afficionado of valve types Jeando, I never consider I really need to be having seen the engineering truth of the whole pile of fishheads, but I do know there are some genuine alternatives out there in terms of factors other than tonal characteristics.  There are also some you should not touch with a bargepole.

There are different ECC83/12AX7s available with different plate structures.  The ECC803 has a larger plate than the normal ECC83 as do some other special versions of the 12AX7.  They all conform to the normal ECC83/12AX7 parameter definition however so they aren't in any way wildly different sounding in circuit.  The large plate will improve heat dissipation though it makes the valves a little more susceptible to microphonics as I said earlier.  It could also increase internal capacitances very slightly.  Whether or not the valve needs extra heat dissipation once it is in a properly designed stage is open to debate.  For my money it makes precious little difference.  These valves are reputed to not be good for the input slot due to the microphonics factor in the spot where signal is weakest.  They are not reputed to be very long lasting in comparison with the normal ECC83 as well.  Most of that makes sense technically though I have never seen it tested rigorously.

There is a slot in many amps called the "DC Coupled Cathode Follower".  We must be specific about this, it is only the DC Coupled configuration that this applies to.  This task, if it exists in your amp, can place a huge strain on any valve put in there at switch on.  If the amp is switched on and applies anode voltage to the preamp immediately as most do then the poor old DCCF has the full HT placed across its grid and cathode until both it and the valve before it warm up and conduct.  That is WAY above the maximum permitted design voltage and what is worse, it is inevitable with standard designs.  That is why these valves go belly up more than any other valve in the amp.  It needs specific protection circuitry built in to prevent it and I know of only a couple of boutique amp models where this is actually applied even though it is a few pence to fit.  The usual advice is to use a Chinese valve in this slot as they seem to cope with this abuse better.  This stage is not in your TM18 though you may find it in other amps that you own.

Other than that I would always advise anyone to NOT fall for the usual advice that 12AT7/12AU7/12AY7 are all 12AX7 "equivalents" and are swappable.  Equivalents they bloody well aren't, full stop!  They are specifically very different valves designed for specifically different jobs.  If you fit one of those into a stage which is properly designed for a 12AX7, everything that the designer built into the stage is now up the swanny.  You may get what is to you a better sounding amp, (I always reckon most people who hype outrageously over this sort of swap change back within a short period but say nothing about that), but you now have a stage where the conditions, voltages currents and frequencies, will be wildly off and could possibly even cause damage in a few susceptible amps.  The only acceptable alternative is the 5751 which is close enough in its parameters to genuinely be considered a "low gain", (not a good description for valves but that is technical), version of the 12AX7.  You can go ahead and try out these swaps of course but they are not good from a reliability and engineering perspective.  Designers design in the way they do for good reasons, to achieve stated design aims.  It's odd to consider that someone with no experience in electronics is considered the "guru" to listen to in preference to the amp's designer when it comes to issues of this sort.  It's rather like listening to your parcel delivery guy rather than a trained mechanic for advice on cars and engines because he drives one all day and the mechanic just works in a workshop.

Personally I just use good old JJ 12AX7s, EL84s and EL34s in my amps and I use the tone controls to tweak the tone to what I need.  It has always worked for me, and most other people I know.  I've tried alternatives like the ECC803 and the "s" versions but they never make any significant audible difference to me.  It's difficult to get any meaningful comparison when you are replacing a shagged out valve with a brand new one.  Not many people think of that!!!  Despite attempts from the "guitar forum" brigades, cork sniffing isn't an Olympic sport yet as far as I know.