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Vox Ac4 ef86 el84

Discussion in 'Glowing Bottle Tube Amp Forum' started by shinyphil, Jan 18, 2021.

  1. shinyphil

    shinyphil TDPRI Member

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    Been pondering for a long time about building AC4. The datasheets for the 2 pentodes
    were copied. Put together & the AC4 was born. My approach would be the same. It worked for Vox it should work for me. No need to re-invent the wheel. I think I've
    found all the drawing errors & mods. EF86 seems to have noise problems. No experience
    with Vox gear. Any thoughts would be appreciated. Many thanks.
     
  2. Squawker

    Squawker Tele-Holic

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    EF86s can be microphonic, especially when used in a high gain configuration as Dick Denney / Vox did. Merlin discusses it here:

    http://www.valvewizard.co.uk/pentode.html

    There are many tales of people (also on the forum here) sorting through EF86s for low noise/low microphony - but personally I must've gotten lucky and have never had any trouble. I built a pretty accurate AC4 from the Vox schematic, but I did build a head, not a combo, which automatically helps with microphony. I also built a TubeTown Molly which is a Vox EF86 channel into a PP ECC99 AC15-alike. It which works great - no problems with noise/microphonics.

    Svetlana Electron Devices (SED/Winged C) made a very nice EF86 until they stopped valve production in 2013 - still available NOS at decent prices under its Russian designation 6J32P / 6Ж32П. Electro Harmonix and JJ (EF806S) both make EF86s, but I haven't tried them extensively.

    Thoughts:
    - on my AC4 build I never used the tremolo, so losing the ECC83 and making the circuit as simple as possible is a "good thing" in my opinion
    - the Selmer TruVoice 6 is another variant on a simple EF86/EL84 amp
    - SS rectification should be a no-brainer on a simple SS amp, but there's something aesthetically pleasing about a glowing EZ80/81.
     
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  3. shinyphil

    shinyphil TDPRI Member

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    Yesterday went to hospital to review my cancer treatment. My PSA 1000+ to 2.75, good news. Does not change my long term diagnosis. Also the side effects of my treatment can be reduced, more good news. I'm feeling lucky. Looked at Selmer. I have a mains transformer+ choke that would be suitable. Ill look at the data sheets for both tubes. I suspect the Selmer is closer to the datasheets than the Vox AC4
    I would like to add a tremolo. Never heard the effect live. Ill look at the alternatives tubes. The idea of ef86's alternatives did not occur.
    You never stop learning. Many thanks.
     
    mojek likes this.
  4. Nickfl

    Nickfl Friend of Leo's

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    The 5879 is another small signal pentode to consider if you are open to alternatives to the EF86. I've used them in a couple of builds including this loosely AC4 inspired build from a couple of years ago.
     
  5. mojek

    mojek Tele-Meister

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    20yrs ago i’ve built AC4 clone with ss rectifier on a pcb with lightweight 10” Celestion(90dB only) in home-made plywood cab from a friend.
    Used PT and OT out of an old radio.
    Great home-made amp project, sounds fantastic, great tremolo (echo-like), goes well with pedals and it’s very simple. I made it with pcb, but next time would use ptp turret board.
    Regarding ef86 i had never an issue, even they tend to be microphonic.
    It’s great sounding recording amp too.
    The only problem i had was high pitch hiss from OT at high volume settings, which is caused by loose wire on OT(it’s junk OT from an old radio so no wonder):)
     
  6. shinyphil

    shinyphil TDPRI Member

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    Thanks for the reply. Purchased Merlin Blencowe's designing tube preamp. Merlin suggest 12AX7 as i/p then ef86. Advice not to be ignored, progress. thanks once more.
     
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  7. mojek

    mojek Tele-Meister

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    I wonder how mich differenece is between ef86 and 12ax7 in sound..? From what i heard that (aside it’s microphonicity) is pentode more linear and triode distorts sooner. Would say triode tube distortion is softer or pentode is more dynamic? Nevertheles, i can say that in ac4 with ef86 you can hear more of output tube distortion. It’s clean till 1-2pm on volume knob and then it starts nice break-up, which sounds very interesting when you change a dynamic of playing. It’s also very goos to hear when using delay with slightly increased level of repeated sounds. With good resonant cab and 10” speaker you have very organic/living amp, almost 3d. Kind of the things you should know yourself, how it behaves under particular setup and since it has just volume and tone, it’s much more fun than classical 6 knob amp.. Real hidden treasure, inspiring and perfect for creating new song ideas;)
     
  8. shinyphil

    shinyphil TDPRI Member

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    Merlins book explains the ef86 pentode. This chapter need to be looked at to looked at several times. Sound depends upon anode load, where its biased, screen grid decoupling & lot more I suspect. the general for guitar applications keep gain to about 100.
     
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  9. mojek

    mojek Tele-Meister

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    Thank you!! Will check it;)
     
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