2W Power Amp w/ 12at7

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by 6stringblues53, Jun 4, 2013.

  1. 6stringblues53

    6stringblues53 Member

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    I have a old PA amp that is currently useing 2x el84's but I would like to run 12at7's. I want the lower output. I know that East Amplication runs 2x 12at7's and gets 2W. I would rather do this than get a attenuator. I shouldnt need to change OT just rewire sockets, right?
     
  2. mark norwine

    mark norwine Supporting Member

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    I'm betting that you will almost undoubtedly have to change the OT...

    The plate resistance of an EL84 is a great deal different than that of a 12AT7

    On the other hand, this isn't exactly an orbital reentry vehicle...you might get lucky.
     
  3. WaltC

    WaltC Silver Supporting Member

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    I'm with Mark, as long as you can get/keep the plate voltage to a level that the 12AT7 is happy with and bias them correctly I doubt that the OT will be bothered enough to worry about.
     
  4. 6stringblues53

    6stringblues53 Member

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    Alright well I'm sold. Time to give it a shot. Im gonna buys some 12at7's and some 12au7's for the preamp. I really want to keep the volume down. Ill give an update when all the tubes are installed.
     
  5. jazbo8

    jazbo8 Member

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    Another option might be a smaller pentode like the Russian 6F3P or 6F4P, just need to re-wire the sockets. And the OPT impedance matching will be better. I think they will sound better than the 12AT7, unless you are looking for a triode sound.
     
  6. WaltC

    WaltC Silver Supporting Member

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    Gerald Weber (Kendrick amps) is using 12BH7s in his new low watt amp with good success.
     
  7. Prattacaster

    Prattacaster Member

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    Although I think its interesting to run a quad 12at7 I agree with jazbo to explore different pentode options. Too low of a reflected impedance and higher voltages/current capability might pose problems with baising.

    Two 12bh7's might be too much wattage since its triodes in push pull gets 2-3 watts usually. That'd be a 5w amp which can be loud.

    Obviously you would need to switch the cathode resistor, or you'll blow up most anything your going to use with a 120-150ohm resistor(just guessing)

    About the preamp tubes: I hear you want to keep the volume down, thats largely going to be controlled by the power tubes. 12au7 will work fine if you want a clean 2watts maybe just a little hair when cranked. If you want a good bit of overdrive at low volume you might want a 12ay7 or 12ax7 mixed in there with the 12au7.

    Does this amp have tone controls? Another thing to consider when designing your preamp and its overall voltage gain and tone.
     
    Last edited: Jun 4, 2013
  8. jazbo8

    jazbo8 Member

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  9. WaltC

    WaltC Silver Supporting Member

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  10. 6stringblues53

    6stringblues53 Member

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    The 6f3p looks promising but does anyone have experience with it? Im also not sure what "OT impedance matching" means. My only knowledge with OT's is swapping out a Valve Jr OT. Thanks for all the info guys.
     
  11. jazbo8

    jazbo8 Member

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    The 6F3P (AKA ECL82 or 6BM8) have been used in many small amps with good results and can easily work with the 8K OPT that you have, whereas if you use the small signal triodes like 12AT7, 12BH7, etc., they would usually like to see a higher OPT impedance. By impedance matching, we mean, to get the optimum output power transfer to the speaker. If you use an OPT with the "wrong" impedance, you get less power and/or more distortion. Hope this helps.
     
  12. 6stringblues53

    6stringblues53 Member

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    OK I understand. One more question, if I decide to go with the 12at7 the sound quality wont be as good but will it cause any damage to the OT? If it doesnt cause any damage to the OT then Ill get a pair of both 12at7's and 6F3P's to see which sound better.
     
  13. jazbo8

    jazbo8 Member

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    It won't hurt the OPT, try them and let us know which one you prefer and why...
     
  14. Prattacaster

    Prattacaster Member

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    Short answer: NO

    Having to high of a reflected impedance could result in flyback voltages, which at the current pull of a 6f3p or 12at7 wouldnt be much to worry about. This is not the case for you.

    Your reflected impedance might be too low which would be tougher on the tubes output tubes. The 6f3p would match up fairly well so you would not have to worry about either one if you use that tube.

    I didnt realize the 6F3P was an ECL82. Those are really incredible tubes. Bogen used them in some of their PA back in the day. Compared to 12at7 its going to sound like a "power tube". Try em both tho, depending on your music style you might like the 12at7. BTW, why not try a 12au7, it would give you more wattage and match up better.
     
  15. 6stringblues53

    6stringblues53 Member

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    I recently bought a couple of 12at7 (Im also gonna try the 6f3p or ecl82) but Im a little confused about how to actually wire them up. Since the 12at7 and it looks like the 6f3p and ecl82 are all pentode/triode then how would I wire it? In pentode like other power tubes or in triode?
     
  16. WaltC

    WaltC Silver Supporting Member

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    the 12AT7 is a dual triode and you have to wire each triode separately as it's own tube. You can't make it a pentode, not built for that internally.
     

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