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  1. #16
    flood is offline IGT Addict
    Join Date
    Jun 2009
    Quote Originally Posted by ironman12 View Post
    ^12AX7 is a preamp tube right?

    is it fine to run it in the power amp?
    yes and no....

    the 12AX7 was never designed to be run in power stages. however, the thing about tubes is that they can be run in a number of different ways; e.g., you can use VHF tubes in audio applications (although performance may suffer drastically), or preamp tubes as power amp tubes, like the 12AU7, which i used in my "firefly" and is REALLY. FUHCKING. LOUD.

    the 12AX7 however has too high a plate resistance to let any appreciable current through, so while there is a large output swing, it is less than suitable to drive small loads. i guess it would work as an audio power amplifier with a 45k primary on the output transformer, but you wouldn't hear it at all. i'm guessing the power would be in the mlli or microwatt range.

    however - this is exactly why the vox valvetronix systems use (if one is to believe the claims) the 12AX7 as a power tube. at the very low power levels it is safe to use an "emulated" load (possibly even digital) and then let DSP handle the speaker cabinet etc.

    so for all practical purposes, you cannot use the 12AX7 as a power tube.

    EDIT: what do you mean by "THE power amp"? in a specific power amp? do not use your 12ax7 in place of an EL84, you definitely will kill the tube and possibly the output transformer too!

  2. #17
    ironman12 is offline Newbie
    Join Date
    Mar 2010
    ^ya tht wat i thought...ya n the el84 have 10w per tube power i guess..n obviously i wont use a preamp tube in power...but u should look at the randall xl series if u can...they have 12at7 in power amp section..maybe to add warmth to the otherwise solidstate poweramp..
    don exactly know wats the logic thr..

    and one more thing...i will be getting the engl840/50 poweramp next thts got 6l6 tubes..i was wondering if its possible to mod it to house KT88 tubes...they r supposedly more powerfull n fuller sounding...and do u know how to do it??

  3. #18
    flood is offline IGT Addict
    Join Date
    Jun 2009
    the EL84 does not have 10W power output. it has a maximum anode dissipation of 12W and a max output power of 6W in class A single ended (just one tube in operation). that too at very high grid currents... output power of 4-5W is more common with siingle EL84 amps.

    the 12AT7 used in the randall is used as a phase inverter section. some consider the phase inverter to be the most important tube, tonally, in the amplifier, and that a lot of power section distortion derives from the phase inverter breaking down and overdriving. the 12AT7 is a great match for this section,, because it provides much better current gain. i first saw this arrangement on some "name" bass amp, i think markbass or eden or something nice like that. solid state preamp and power amp, but a tube phase inverter.

    the KT88 swap depends on the amp.... going to have to read up on it first.


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