Pro Jr III Going microphonic with 12AT7 tubes!?

Discussion in 'Amp Central Station' started by syltaldo, Jul 29, 2021.

  1. syltaldo

    syltaldo TDPRI Member

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    Hey there. New member here.I have a Pro Jr III that started making rattling sounds in the speaker on certain frequencies.

    I tried knocking gently on the tubes and decided to change position 1 (First preamp tube, first tube on the right.) I have 3 new unused 12AT7 tubes and wanted to see if one of these would tame the amp a bit so I installed it and powered up. The amp became way more microphonic and started feedbacking on its own.
    I changed to another 12AT7 and the same thing happened.
    Finally I gave up on my plan and just installed a new 12AX7 instead. Now the amp sounds good and the initial noise is gone. But I'm left wondering.. Did I buy 3 damaged 12AT7 tubes? Or was it just not compatible with the Pro Jr III.
    (I've seen people using 12AT7s instead of AX7s to ease off amps before..)
     
  2. dougstrum

    dougstrum Friend of Leo's

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    When some tube starts ringing in my
    Pro Jr; it has always been an El84.
    Not that a pre amp tube can't go microphonic, but el84 seem prone to the problem.
     
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  3. jimgchord

    jimgchord Tele-Afflicted

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    Could be bad tubes. Keep in mind though that while 12a*7 variants have the same 9 pin base, their operating characteristics are different. The plate resistance is way lower in an at7, it also draws more current so that alters the plate voltage. The plate dissapation is also different. When you put a different tube in a circuit that is center biased for a 12ax7, it will bias differently and the results may or may not be good for your application
     
  4. PCollen

    PCollen Friend of Leo's

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    I don't know what the cause of your problem was, but the 12AT7 is NOT a good tube to use in the V1 position of ANY guitar amplifier...it is not a good "tone" tube. It works OK as a phase inverter tube, and for reverb / FX loop drive or recovery but not very well as a pre-amp gain stage tube .
     
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  5. 11 Gauge

    11 Gauge Doctor of Teleocity

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    The 12AT7 has higher plate current than a 12AX7, which is what makes it so good as a driver or phase inverter.

    For lower voltage gain at V1, it would probably work well just by using what's in a 5E3 - try a 12AY7 there.
     
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  6. E5RSY

    E5RSY Doctor of Teleocity

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    As noted above, the 12AT7 should never really have been tried. To quote the Aspen Pittman book: "The 12AT7 is rarely used in a tone-generating stage, but rather in function stages..." And you found out firsthand the reason why.

    Try the 12AY7. I almost guarantee you'll like it.
     
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  7. archetype

    archetype Fiend of Leo's Ad Free Member

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    Welcome to TDPRI!

    The root cause of the problem hasn't been established, but it's likely that the original 12AX7 was going bad. If the problem shows up again, the EL84 tubes are more likely the source of the trouble. Generally, those tubes aren't as robust as other types.
     
  8. Lynxtrap

    Lynxtrap Tele-Afflicted

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    So how would a 12AT7 cause the problems mentioned in the OP?
     
  9. Timbresmith1

    Timbresmith1 Tele-Holic

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    Different design/operating parameters. Rp (plate resistance) of a 12ax7 is 5x that of a 12at7. So component in circuit designed for AX tubes are not designed for AT tubes. We think of these substitutes as identical, but they are not. They may be compatible in similar circuits, and less than ideal in others.
     
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  10. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Yes, although plate resistance is not a constant.
    Doesn't explain microphony and feedback IMO.
     
  11. syltaldo

    syltaldo TDPRI Member

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    Thanks for all your input folks.
    I had a new 12AX7 tube so I installed that and that worked.

    Yesterday I went to the store and bought some more 12AX7 and also a few 12AU7’s. Any thoughts on the AU tubes? They were out of 12AY..
     
  12. 11 Gauge

    11 Gauge Doctor of Teleocity

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    The 12AU7 is super low gain (significantly lower than even a 12AY7).

    I've only ever had one in a bass amp as the phase inverter tube. It was fine for that, but was horribly microphonic when I tried using it in the V1 slot with other amps.

    My guess is that the 12AU7 is probably similar to a 12AT7, in that it's typically chosen to be a driver or phase inverter.
     
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  13. telemnemonics

    telemnemonics Telefied Ad Free Member

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    The several 9 pin miniatures designed to be preamp tubes, include provisions to avoid microphonics.

    The 12at7 and 12au7 are not on that list, are not preamp tubes, and are more likely to be microphonic along with being poorly suited for preamp use.

    Users and bloggers get confused because the little buggers look similar and are all lined up in a row in our amps, where we might assume that the big ones are power tubes so the little ones must be preamp tubes.
    Then if the amp works with a 12at7 or 12au7 in a preamp stage, we go online and repeat that those are preamp tubes because they work.

    For lower gain preamp tubes, choose 12ay7 or 5751.
     
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  14. archetype

    archetype Fiend of Leo's Ad Free Member

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    Internal design. They're not designed to mitigate microphony, as they are not designed for the front-end stages of an amp. They're typically used for driver and reverb stages where microphonics don't matter. If you've got a microphonic 12AX7 you can usually move it to either of those two stages without problems.
     
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  15. schmee

    schmee Doctor of Teleocity Ad Free Member

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    Not unusual for old tubes or cheap new tubes to be microphonic.
    FYI: I have not had a Mullard RI that was microphonic yet out of many.
    I've had many highly coveted vintage tubes that are.

    But pay attention to the comment about EL84's going microphonic... they seem to do that a lot for power tubes.
    I've never payed much attention to the "12AT7 is not a tone tube" thing. My opinion is "nobody told the electrons" :>)
    I tried many in my Ghia clone and I liked the 12AT7 better than a 5751 or 12AX7 many people use. Go figure.
     
  16. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Wouldn't the same 12AT7 be microphonic used in an LTP phase inverter as when used in the preamp?
    The OP's case was pretty severe, it seems strange that a 12AT7 would make an amp oscillate on its own.
     
  17. Dacious

    Dacious Doctor of Teleocity

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    A lot of 12AT7s have a large plate structure. They can sustain higher current and more plate volts. 12AU7s are small plate but draw less current. They can sound 'off' in signal shaping stages designed for 12AX7s as Schmee, Telemenomics and others have noted.

    I bought an Artist Cremetone amp for my buddies rehearsal studio which uses a 12AU7 for the drive circuit. It's a 1.5 channel amp with a piggyback drive channel with gain and level sharing eqs.

    We got it cheap coz someone had stuck a 5751 in that slot. Conventional wisdom would be that 'should' sound better with more gain.

    Guess what - it would run for twenty minutes, then go tzzzzt! and stop working for a while. I suspect it's being used as a cathode follower. Putting an aged Miniwatt 12AU7 in fixed it up. Runs for hours no problem. Traps for young players.

    I remember reading the guy who engineered this amp for Artist who has them made in China specced a 12AU7 and I'd say he's designed the plate load and cathode resistors to suit. It sounds good in a modern chugga-chugga Mesa sort of way.

    12AY7, 5751 yes..12AU7, 12AT7 can be problematic. Fender uses a 12AT7 in the George Benson Hotrod and Twin Reverb V1. He obviously likes that flat sort of tone.

    The Pro junior is a Bruce Zinky design I think, he's done some good work with Prooonics and the like, he loves to push the envelope of amp design I feel. So I'm not surprised if you run gain high it would sing a bit.
     
    Last edited: Aug 6, 2021
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  18. telemnemonics

    telemnemonics Telefied Ad Free Member

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    Well microphonic is a mechanical thing, not an electronics thing, so the resulting signal function issues are not the same when doing one task as doing another.
    The PI is inverting the signal back & forth to push one side and pull the other side of the sine wave, which is just so different from taking a tiny input from the guitar pickup and boosting it 40X.

    I'm not an engineer though so that's just my rudimentary understanding and not suitable for engineers!

    What I do when buying old stock preamp tubes is I first try them in V1 of a known working amp with a known good preamp tube in V1.
    If it's microphonic in V1 I swap it over to V2 (usually tone stack recovery section of the preamp where it's often NOT a microphonics problem. So where a tube is used nets different things in the range of microphonics)

    Many great tubes are microphonic, and microphonics are not the primary reason a 12at7 is not a preamp tube.
    That's just the main question the OP asked.
    Some 12at7 are NOT microphonic, but they are still NOT preamp tubes.

    And just as we can get away with doing all sorts of stuff "the wrong way", there will always be somebody with a story about how they did something the wrong way and it worked.
    I've run a 12at7 in V1 and V2 now and then, more when I didn't understand that the fact that it works doesn't mean it belongs there.

    Hell I'd say anyone who plays Rock or Country has already chosen a WRONG path!
    Whenever I meed someone and they say they're an artist or musician, I say "My condolences".
    It's just a really bad idea if you want a good life, and I would never tell a young person they should be a musician.

    However, if they have no other choice, they will go right ahead and do that wrong thing no matter what cooler heads tell them.
    Same as this discussion.
    Ask a question and the answer is that the 12at7 and 12au7 are not preamp tubes but will work with very large changes in amp function, and not do any "damage" aside from a big loss of gain.

    (Of course if you sneak in to the rehearsal space and replace all the 12ax7's in your buddy's amp with 12at7's, he or she will probably assume it's broken since it no longer "works properly")

    When discussing right and wrong approaches, a discussion is theoretical so we can do anything we want, since no money or product is changed in conversation.

    At that discussion point, one might step back and ask why a subject is choosing to use an amp with too much gain, then reduce the gain with improper tube use?
    Are there examples of great results gotten by choosing the wrong amp and neutering it with function tubes in preamp slots?

    Or is the subject in Siberia where there is no guitar shop or postal service?
    Is there even a bunch of youtube demo's of Blues Jr's neutered with a row of 12at7 function tunbes in the preamp, showing how much it "improved" the amp?

    Can anyone do a comparison and legitimately find a Blues Jr is "improved" by all 12at7?
    The Blues Jr is the most popular amp to mod so every possible thing has been done to them!
    Modding and youtube gear chat is the new live music shows...
     
    Last edited: Aug 6, 2021
  19. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Interesting thoughts. I brought up the LTP, as often when the ”can I use a 12AT7”-topic is discussed, we forget that the tube is performing amplification duty in that PI. About half the gain of a ”normal” preamp triode gain stage. The inverted triode usually has a little more gain, that’s why its plate resistor is often made smaller, like the 82K-100K ratio in BF amps. Dumble for instance used a potentiometer to balance the gain between the triodes.

    The George Benson HRD is an example of an amp model that comes with a 12AT7 as V1.

    As for myself, I have tried 12AT7’s in many preamps. Never caused any problems, but I have never found them to sound particularly good.

    I just find it odd that a batch of new AT7’s should cause this kind of phenomenon in a Pro JR.
     
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