Signs of a microphonic preamp tube can vary. Most of the time, they will go into saturation mode or instant feedback if there are any vibrations. Sometimes that needs a signal to excite the tube into feedback. That squeal you cannot control and will not go away until you place the amp into standby. Tap across the top of the amp on the front side of the handle with your knuckles, does not require a hard hit. If you hear any ringing through the speaker, you have a microphonic tube or one that is very sensitive to vibration. Some long plate tubes are more susceptible to vibrations than medium or short plate tubes. I had a few bad Mullard reissue tubes in the batch I had bought. Just changing channels was enough to excite the tube into self-oscillation.
If the power tubes have not been replaced in some time, What tubes are in it? I bought mine when Mesa did not offer any 6L6GC tube except for the STR440, they basically shipped every amp with that tube in it. Mark V, it was more common to find the red color codes than others. Yellows and greens of that tube would work out but the greens had more early distortion characteristics than the reds or yellows. Reds have the highest headroom; grey is the lowest headroom. Codes run red, yellow, green, grey, blue. Depends on the tube in question. I believe the set of STR441 tubes I am running in the V are green coded tubes. They actually sounded really good in the V which surprised me as they sounded lame in the JP2C (this amp needs the early distortion, green or grey and will clean up too much with the reds or yellows. Keep in mind the JP is a Class AB and not Simul-class like the V). I did not care much for the STR448, STR445 or STR443 in the V. Even the famed STR415 tubes were not ideal which is epic in the JP2C amp. Best sounding tube I ever ran in the Mark V90 were the SED =C= 6L6GC tubes as they had a 3D complex harmonic structure and low-end sound that was just amazing. The Ruby 6L6GC version of the STR440 also had a different sound, Those were good too. Similar to the STR440 but deeper tone.
Flubby tones, that could be weak preamp tubes and power tubes. Typically, with power tubes, there will be a tone change when they are needing replacement, losing the low end is the one indicator or losing the top end. Depends on the tube in question. I have only used the STR441 tubes for a few hours, so I do not know what its characteristic is when they reach EOL. They can also cut out on you from time to time, usually an indication they are going to red plate and blow the fuse. If the power tubes are not at the correct bias, they can sound flubby or too bright. Mesa amps are fixed bias that will mean the tubes need to fall within a range of use. Nothing wrong with using tubes that do not have the Mesa branding (if the amp still has a warranty, best to stick with Mesa power tubes, not sure what the rules are on the preamp tubes). I have run many power tubes from different suppliers, Tung Sol, Mullard, SED, Svetlana, EHx, Gold Lion etc. Just be specific what amp it is to be used with when you buy non-Mesa branded tubes. Yes, Mesa will brand those other tubes if they meet their requirements. Depending on volume of the tubes, they may be specific to Mesa as they are a Special tube request or STR that may be different than the stock tubes you find in distribution. Also, it may be different based on how the tube is selected and sorted.
Preamp tubes, they can do some wonky things. When they wear, the preamp tubes will lose some luster in sound, not as crisp or bold and may not provide the gain you used to get. They tent do sound dull. Almost like comparing a fresh set of bronze strings on an acoustic guitar compared to an old set that has lost its brilliance. Flub can be a result of the Mesa 12AX7, it was more evident in the RA100 on the Hi gain channel. It can also be an issue with the Roadster, and Mark V90 but not as dramatic. Dial back on the bass when raising the level on the gain, may need to adjust the 60Hz slider down a bit too. The flub character can be due to the preamp tube, sometimes a different tube used in specific locations will make a huge difference on how much low end you can dial in before the flatulence becomes a tone burden. With the Roadster, the key tube position is V2 which has the cold clipper circuit (V2B triode). That cold clipper is the sub-harmonic generator.
How I loaded up my Mark V90: Use it as a reference.
- V1 + V3 = Svetlana 12AX7. They are much on par with the Mullard CV4004 but with a slight less gain. These tubes removed the 60Hz hum I was getting on CH1, it was rather annoying with any of the other tubes.
- V2 = Tung Sol 12AX7. You can run any tube here, I just wanted something a bit brighter than the Mesa branded JJ tubes. It will only affect CH2. It also only uses one triode.
- V4, V5, V6 = Unobtanium 6N4-J (Chinese Beijing Square Foil getter 12AX7/7025 that Mesa used back in 1990) I have fresh ones from Ruby Tubes purchased from Doug's tubes about 10 years ago. Also have some used ones I ran in the Mark III with the Mesa logo on them.
- V7 = Mullard long plate reissue. It is the better version of the Sovtek LPS tube that most like to run in the phase inverter position. Any tube will work find here. Mesa 12AX7 or you name it. Some tubes may have more low-end gain than others. Some are more balanced or there will be more midrange or less. Depends on the tube structure. The effect will not be very dramatic, but a little change can go a long way as it does not take much to go from tight to flub.
- Power tubes: STR441 green
Some alternate tubes I have tried (just about every current production tube available but these were a help for a while)
- V1, V3 = Mullard CV4004
- V4, V5, V6 = Mullard reissue 12AX7 (long plate)
- V2 = Mesa stock tube (jj ECC83s)
- V7 = Mesa stock tube (jj ECC83s) or the Sovtek LPS.
- Power tubes: SED =C= 6L6GC (Mesa did sell them as STR454, not easy to find)
When the Saturation mode became a thing, I loaded the amp with all Mesa 12AX7 tubes and swapped V4 with a JAN/Phillips 12AT7. That was ok for a while. Cuts out the treble on CH3 and helped to cut out the ice pick. I was using the STR440 green 6L6GC tubes. I did try the Gold Lion KT77 with the 12AT7 in V4, did not favor that. The Mesa 12AX7 in V4 sounded better.
I usually run a bench test with the stock tubes if at all possible. Mesa 12AX7. Then I start replacing one at a time with something different. It can consume a lot of time, but the end result is enjoyment of the amp if you have issue with it from the start. Most of the time I see no need to change anything. MWDR, JP2C, Mark VII, Badlander sort of fell into that category, no need to change anything. With the other amps in my list, it was just a simple change in preamp tubes and or power tubes if needed that made it just ok to really good. RA100 is one of those that became a favorite amp once I found the tubes that work the best.
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