12ax7 Alternative For Less "fuzzy" Jcm900?

Discussion in 'The Workbench' started by guitarwizard93, Sep 22, 2016.

  1. guitarwizard93

    guitarwizard93 New Member

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    Hi all,

    I've been wondering about this for a while, and finally decided to look into altering the sound of my JCM900 4100 head. Please bare with me here, I'm not very well versed with the inner workings of valve amps so I may not be able to do what I want to do.. And all work will be done by a professional tech, don't worry!!

    So ideally, I'd like to change the sound of preamp overdrive on channel 'B', without changing the sound of the clean channel... can I do that?

    The reason being that I've always found the overdrive to be a bit too fuzzy, too gritty, where I'd prefer something a bit smoother, creamier, without loosing the sustain you get from pushing the gain up. I almost exclusively use the preamp for overdrive, not the poweramp, so figured this would be the logical place to start.

    So I guess my questions are...

    1. Is there an alternative to the stock 12AX7 that would be a good match for the sort of sound I'd like to move towards?

    2. Does changing a preamp valve require rebiasing like changing power valves does, or can I just drop a new one in?

    3. Can I even do this?!

    Thanks for your help! :)
     
  2. wakjob

    wakjob Well-Known Member

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    You can try some old black-plate 5751 tubes in one or all three preamp spots.

    I think they worked well in the 900's I put them in.

    A grey-plate RCA worked just as good too.
     
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  3. jimmyjames

    jimmyjames Well-Known Member

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    JJ ECC823 in one or more of the preamp positions may work for you, leaves the clean channel as is, tames the gain especially in V1 and V2. May lose some other dynamics though....
     
  4. BowerR64

    BowerR64 Well-Known Member

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    Changing preamp tubes isnt really hard to do. You can try ones with less gain.

    What are all the things you have tried so far? have you tried the patch cable in the effects loop and turn the little level control with a screw driver till it gets the most amplitude when your playing it?

    Have you tried turning the gain up and backing your guitar volume back about half way? (i never wanted to do this i was always a wide open player but backing the volume back really changes the tone and can help remove the fuzz alot imo)
     
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  5. bigyinuk

    bigyinuk Active Member

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  6. myersbw

    myersbw Well-Known Member

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    I've been inside the combo version of this amp a LOT over the past few years. A local blues player has two and I've come to really not like the 900. Sure, it has it's place for some, but after seeing your post, here's my take with it. Truly, it is not an all-tube path where great cascaded gain sections of days of old are a reality. It's hybrid. Much of your signal path is within a couple chips. The clipping is mostly diode-based unless you dime the amp.

    Yes, changing tubes will vary it somewhat and you might find what you're looking for with tube swaps...but, don't be surprised if you don't.

    Am I a pro tech? No, but I have an electronic engineering background and several years of tube amp repairs, mods and some builds. For a tech to significantly mod this amp for you? You'll have more money in modding than the amp is worth and it might be the last time a good tech works on it! :) Some board changes changes involve removing the whole board (removing the controls assembly, too). It is a pain to work on.

    Personally? With that amp...if it's working good (as it should) now...sell it and buy what you really want. As for some nice tone AND a nice build on a budget? I'd take a 5150 II or III anyday over the 900...it's made a bit more rigorous and VERY serviceable. (Techs will love you for it!) Now...interestingly enough...the best straight-path tube tone I've ever heard (and suited my personal tastes) came from three amps....the first was a 1972 Orange 120w head I owned in 1976 (paid $150 for)...dimed through an Altair power attenuator. The second...a 1971 Marshall Major I did a one-wire mod to (which I could never afford). And, the third was...believe it or not...a small Bugera V22. It holds ok for light gigging. But, it's overdrive staging of gain was really sweet.

    There is a fourth, too...a Marshall 18 watt TMB build I did a year ago. But, really...that V22 had a deeper smoother overdrive. :)

    I have this love/hate with the 900's. They've provided me some nice extra cash with repairs, but I'm glad they belong to someone else! :O :)
     
  7. royslead

    royslead Well-Known Member

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    The cheapest route is to try the 12AT7 or the 5751 pre amp tube swap, and you don't need a pro to do that, take the back cover off and change them. Be mindful of their orientation, and if you can change a light bulb, you can do it. Myersbw is right, though, you may not get what you are seeking. If not, you are out the cost of a few tubes, it may be easier to sell the amp and find something that better suits you. You don't like the "fuzz" of the 900. Have you had the amp long? Did you ever like it? Maybe something else is going on in there that needs service; but if you do not like the gain structure of it, maybe it is best to move on. Determine what type of gain you want and start researching amps that have "it."
     
  8. myersbw

    myersbw Well-Known Member

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    Oh, just one more comment on the 900's...if you ever have a very low level 60hz hum that NOTHING seems to cure? (Grounding adjustments, pots, etc.) Look to see if you have a SovTek in the preamp section. Same player had one that drove me nuts...he was fine with it, but I can't stand anything that shouldn't be there. I swapped out preamp tubes one at a time...no change. Then I grounded the FX loop (shorted it). Gone! The culprit? Any SovTek 12ax7 in any preamp position...didn't matter. And, same level. To the point where you just know the transformers are inducing it mechanically into these tubes. Swapped out all of them and it was fine! Go figure? :) And, btw, the tubes I used in the preamp section were all current production JJ's...with those, the amp sounded as good as it ever will.

    Also, if you're having a tech dive into it...have them bias it cold. 99% of your distortion will come from the diode clipping circuit so cold bias will help keep your output tubes healthy for a much longer period. You won't likely hear the difference.
     
    Last edited: Sep 23, 2016
  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Take out the clipping diodes. That's what causes the fizz...
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The 900 preamp compresses noise...noise will be amplified just like the audio.
    Any noise on the input...will be compressed...and amplified.

    The effects loop: open capacitors in the 15 volt supplies will cause parasitic oscillation in the effects loop.
    This high frequency oscillation (the frequency is so high you can't hear it) will cause the output tubes to turn on.
    When the output tubes turn on: you will hear a constant hum. The hum is the ripple in the B+ supply (normal).

    Normally there is audio and you don't hear the hum from the ripple. The music is much louder than the hum. (it masks the hum)
    BUT when there is no music.....output tubes turn on (from oscillation)....and the hum is audible.
    Go back to your 15 volt supplies, and get the ripple out (change the filter capacitors). Confirm that there is no HF oscillation in the effects loop.

    This is a problem: because the 15 volt supply has 16 volt filter caps. The caps go bad really fast.
    The caps should be 25 or 35 volt...not 16 volt. The caps open up and the oscillation starts at the effects loop IC.
    You will see the high frequency noise in the effects loop with a scope.

    The 900 sounds way better: if you overdrive the output tubes. If you do not overdrive the output : you are completely missing the good tone of this amp.
    You don't "even know" how good it can sound. You never heard it before.

    Biasing cold: creates output tube crossover distortion. The amp will sound fizzy and weak.
     
  11. big dooley

    big dooley Well-Known Member VIP Member

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    i wouldn't use a lower gain tube in V1 in the DR.. the first tube is in fact a last gain stage with a DC coupled cathode follower, meant to introduce a lopsided sinewave which in turn adds even order harmonics... pleasant to the ear

    try getting the highest gain 12AX7 you can get and compare that with say a 5751 or even a 12AY7... do not use a 12AT7... these are designed for higher frequencies which will add even more fizz... they're great for hifi or full range stuff, as i've experienced when using them as phase inverters in my stero full range stack, but a dead giveaway in a high gain guitaramp
    the 12AT7 is also not fond of high Uk/f, which is exactly what they'll get in the cathode follower position, unless heatervoltage is elevated in reference to ground... which won't be the case in a 900
     
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  12. myersbw

    myersbw Well-Known Member

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    Yes and no. The bottom line is...you never totally eliminate that notch completely without risking a red plate condition. Especially in this push-pull class. Some like to bias at 70%, but that's elusive and often you can jump down to 50-60% and be fine. Much depends on the plate voltages used. The crossover distortion notch will also be impacted by load (and reflected load) if you switch from say a 4 to a 16 ohm cab...even using the correct tap. And, relative to how hard you push it.

    But, let's take the clubs of today. It's not usually like days of old outright diming the amp and covering the speaker cab with shipping blankets anymore. Hence, the extreme preamp clipping circuits we have today and less of a need to push the hotter-side range of output tube bias.

    Sure, we don't want either extreme. But, my reference to 'cold bias' is not extreme...but, rather consider dropping down to 60% and, yeah, it will reflect on tube life and I doubt you'll hear the difference...unless you push it to the extreme end. But, that's a whole other world of playing, too.

    It does get interesting playing with the bias a bit. And, yes, I have dropped the bias back 5ma or so and get just a little conservative from the norm. My ears have not heard the difference and I don't see the amps back for quite a while for new tubes and a bias check. (A pair I regularly service are played 2-4 nights weekly for 2-3 hour gigs. So, they're used.)

    Basically, I agree with you from a general consensus...I likely should've clarified that I'm not talking about too extreme a change. As for open caps? Yep, that sure can do it. The amp I referred to above is one I replaced all the electrolytics in. Wasn't a cap issue at all (for my hum issue)...it was some specific Sovtek 12ax7's creating havoc....bye-bye Sovtek & hum vanished. (Lol, especially after I'd spent time creating a better star-ground in the amp. It was dead quiet to the ears in regard to anything, but an audio signal.)
     
    Last edited: Sep 26, 2016
  13. tman

    tman Well-Known Member

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    I would put a 12ax7 in v-1 and a 12ay7 in v-2.It sounds great!
     
  14. Durden

    Durden Well-Known Member

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    I'm running 3 EI Yukoslavia ECC83 12ax7 in my 4100, sounds unreal.

    _20160927_091414.JPG
     
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  15. myersbw

    myersbw Well-Known Member

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    I love oddball tube combinations! :) Only issue is finding replacements when ya need 'em. :(
     
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  16. ampmadscientist

    ampmadscientist Well-Known Member

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    To change the sound of overdrive is largely solid state.
    The preamp is actually op amps, not tubes. The tubes come after the SS preamp.

    To do this, there is a separate section for overdrive channel which can be modified.
    If you need to do anything, you need to take parts off the circuit board.
    This requires special skill and tools, because the board can be damaged from the soldering iron.
    It would be better to have an experienced tech do any mods. (unless you are expert at soldering PC boards)

    After the SS preamp, there is SS clipping diodes, which create a compression of the signal.
    Then there is SS EQ, which is factory preset, cannot be changed without modifying.
    Only after then, does the audio enter an actual tube amp stage.

    No wonder changing 12AX7 tubes (to 12AT7, etc...) does not make a lot of difference....It's almost all in the SS preamp...SS clipping...SS EQ.

    The only thing you "can" do with the tube stages, is add some bass and lower mids.
    You "can" change the level of the effects loop....which is quite helpful.

    So you see:
    It's not like a traditional tube amp at all. Although it is possible to change the sound...it's all in the op amp SS circuits, mostly not the tubes at all.
    4100 is a Solid State amp, tone-wise. It's a tube amp, output-wise.

    However with some modifications, it really can be quite a good sounding amp.

    FYI: there is some secret tweaks that increase the gain to outrageous amounts.
    But most guitar players could not handle it....it's too powerful.
    These tweaks are primarily for Metal Heads.
     
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  17. charveldan

    charveldan Well-Known Member

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  18. myersbw

    myersbw Well-Known Member

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  19. ampmadscientist

    ampmadscientist Well-Known Member

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    The tone of this amp is all in the preamp.
    And the preamp is solid state.
    Changing the tube won't do what you expect.
     
  20. BowerR64

    BowerR64 Well-Known Member

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    I want to change mine, but i dont want to make it a "metal" amp i actually want less gain with a more full sound.

    Right now the way i like it is to put the preamp on 8 or 3:00 and then roll the guitar volume way back. This give it the fullness without all the distortion.

    I know i got the wrong amp, i kinda bought it sight unseen so i didnt really have a change to test it. I want more of a midrange vintage bark.

    It gets even thicker when you also turn the gain up to noon, but then its so dirty i cant stand it.

    I kinda like the sound of the DSL15H after the eggz mod it has a nice midrange bark but im sure my 2500 (not slx) is different then the DSL

    I have a good soldering iron and i can solder pretty good. I use solder wick to remove the solder and it works really really well at cleaning up the traces so removing and replacing parts is a snap. IMO removing the board to get to the solder traces is the bitch. The awkward weight of the transformers and stuff can be tricky also.

    You say the basic gain structure is in opamps are those the little 8 pin chips? Maybe i could remove the ones that control the most of the voicing or distortion and put in a socket so i can swap a few around? I have a little stash of 3-4 different ones i tried on a different amp
     

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