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Vox AC4 ef86 el84 further thoughts

Discussion in 'Glowing Bottle Tube Amp Forum' started by shinyphil, Jan 24, 2021.

  1. shinyphil

    shinyphil TDPRI Member

    Age:
    70
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    Dec 26, 2020
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    Enniskillen , Northern Ireland , UK
    Merlin Blencowe suggests a 12ax7 before ef86-to bring out the full character of ef86.
    I used the classical 12ax7 circuit anode/cathode loads 100k/1.5k. Ended up with a parallel i/p amp. 2 i/p jacks: jack 1 to one triode, jack 2 to both triodes. Separate cathode biasing at 1.25v,1.75v,2v & 2.25v. With switching to select all 4 possible choices. Add in 1uf or switched 25uf cap for bass cut or FAT. Gone OTT.

    Next stage. Tone control seem to be a good idea. A cathode follower 12au7 driving a James/Baxendall circuit & a gain make-up stage: 12au7. Noticed the loss of the james circuit is 0.071. gain of make-up stage 13.5. .071 x 13.5 = 0.96. one is almost the reciprocal of the other, how convenient. The follower is very unlikely to be overloaded by 12ax7. Put the i/p volume after the make-up stage.

    Next stage. Use Merlins design. Put a master volume after ef86. going to el84.

    Summary
    The i/p stage gives a thick signal. Tone controls (loss about 0.8). EF86 can have as much
    signal on grid 1 as required. If not enough gain , i/p stage gain can be increased. o/p from EF86 controlled by master volume. A 'power soak' to attenuate the o/p from the el84. Not to annoy the folks living above. Power soak, good idea me thinks.


    All theory so far. Next have a look at el84
     
    2L man and dan40 like this.
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