1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Vox AC4 ef86 el84 further thoughts

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

  1. shinyphil

    shinyphil TDPRI Member

    Dec 26, 2020
    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.

    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.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.