Decisions on AC15 Build Project

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mattbedrock, Sep 4, 2019.

  1. Mattbedrock

    Mattbedrock Silver Supporting Member

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    I've been doing some researching and considering options on an upcoming AC15 build. I find Vox designs and schematics intimidating - very unlike the Fenders and Marshalls I have built in the past. And AC15's seem to come in many flavors and types. I was hoping to find a number of AC15 build threads here but have found few to steal from.

    As a baseline, I'm considering using the AC15HTVH schematic. This handwired reissue from a few years back has both an EF86 and a Top Boost channel and no reverb or trem. I want to build it as a head to save weight and space. I have a Ceriatone DC30 clone and really like the EF86 channel. Am I missing out by not having the standard non-Top Boost channel (12ax7 based)?

    The HTVH has a bass shift and a two position bright switch on the EF86 channel. The bass shift basically allows you to change from a 100nf coupling cap to a 1nf which I'd guess would pull a lot of the lows out. The bright switch allows the choice of no bright cap or a 200pf or 270pf cap. I'm considering replacing all of this with a 6 position rotary tone control like a Trinity TC15 or Matchless DC30 instead.

    Power supply filtering options seem to be all over the map by different builders. The Vox schematic shows 2x 22uf and 4x 10uf caps with a choke. Others are using various combinations of 40, 33, 22 or 16 uf caps. I prefer more robust front end filtering to make sure the amp has some grunt, so I'm planning on a 33/33 cap can and 4x 10uf caps. I might switch those to 16's. I did buy a choke.

    Another choice I've been pondering is the selection of cathode bias cap and resistor. A 130 ohm resistor (5 or 10 watt) seems to be the consensus but cap sizes are all over the place from 22uf all the way up to 220uf. The Vox schematic calls out a 47uf. What are the implications of different size caps? Sorry - I'm used to fixed bias amps.

    Feedback appreciated - I'm out of my comfort zone in Vox land.
     
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  2. Glitch Magnet

    Glitch Magnet Supporting Member

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    I can't help much. Only that, a Normal channel would be desirable if you want clean headroom. On my AC15HW1X, the Normal with Bright switch on can sound very close to the TB channel, but the TB breaks up easier.
     
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  3. HotBluePlates

    HotBluePlates Member

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    Probably not.

    I have a '65 AC10 Twin and a '63-64 AC30/6 (non-Top Boost). I love the harmonics in the EF86 channel of the AC10, and find the Trem Amplitude control is really important for dialing-in the most gain from the EF86 (which equates to volume & harmonics before it even gets to obvious distortion). The 4-sectioned high-pass filter after the EF86 (to control trem thump) probably contributes to a pleasing bass roll-off in that channel, too. I know I like playing in the Vid/Trem channel of the AC30/6, even with the trem off because lows are more controlled (mine is a Bass version of the circuit).

    I don't think I'm missing out by not having the Top Boost, as it would probably be most useful if I were playing the AC30 a lot louder (to really push the output section).

    Sounds to me like too much in either direction. The typical coupling cap seems to be 0.01µF or 10nF. My Bass-version AC30 has a 1000pF coupling cap for the Brilliant channel, and that's plenty small enough for me (I don't know if I'd like the 500pF).

    Sounds like a plan to me. I don't know that I would need 6 options of bass-cut, but I'll admit not having tried them recently either.

    AC15HTVH schematic
    AC15 schematic 1960

    If I were building an AC15, I'd stick with the 16µF filter caps for the power section and 8µF elsewhere, as with the original amps. Maybe you'll hear a little more noise, but more filtering changes the supply voltage & feel of the amp. You could buy a modern Vox amp for probably less than what a build would cost, so why shoot for the modern Vox sound?

    Stick with the vintage 47-50µF.

    The goal of the cap is to keep the bias voltage stable as you crank the amp. To do that when you play loudly, the cap should be huge (500µF probably wouldn't be out of line for an amp played very loud into Class AB). However, bias shift and the impact of cathode bias on output stage performance is "part of the sound" so the 47µF is probably plenty (and double what the tweed Deluxe has in an output stage pulling the same current through an even larger cathode resistor).
     
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  4. Mattbedrock

    Mattbedrock Silver Supporting Member

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    HBP -
    Thank you for a thorough and well thought out reply. I really appreciate your feedback.

    Matt
     
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  5. xtian

    xtian Member

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  6. Mattbedrock

    Mattbedrock Silver Supporting Member

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    Good call. I am familiar with the Rocket - I built an Express a few years back but also considered the Rocket at the time.

    For a 4x EL84 amp I already have a Ceriatone DC30 clone to scratch that itch. I really want to try another two tube amp - I really like the Marshall 2061 clone I built and want to go that way with a Voxy design.

    But thanks for the input!
     
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  7. Haruki

    Haruki Member

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    That sounds like a cool project. I had an AC15H1TV; I miss it:

    [​IMG]

    Having two (2) classic channels---EF86 and Top Boost---is a great idea.

    I haven't heard it yet, but JJ tubes makes a Telefunken-like EF86, I love their/JJ's ECC803; to me it has the body of an old Mullard and the clarity of a Telefunken.
     
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  8. makerdp

    makerdp Member

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    I can highly recommend the TC15. Fantastic kit and fantastic sounding amp. If you don't want to go the kit route, the schematic is definitely worth considering for your amp, but the magic is in his output transformer that lets go between Matchless and Vox at the flip of a switch.
     
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  9. Haruki

    Haruki Member

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    I was reading about this kit/amp a couple of weeks ago. It seems cool, especially if it is true to the original/vintage VOX circuits.

    [​IMG]
    [​IMG]
     
  10. makerdp

    makerdp Member

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    It's an extremely versatile amp.
     
  11. menlow

    menlow Supporting Member

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    I was cranking mine earlier today. It’s a fun one for sure!

     
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  12. makerdp

    makerdp Member

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    Did you do yours from a kit? The TC15 was my first build. Then I was hooked and here I am with too many homemade amps and not enough time to enjoy them all... lol.
     
  13. menlow

    menlow Supporting Member

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    Yes. I built a Modkitsdiy Mod 102 to get my feet wet, then went with the Trinity. The instructions were spectacular.
     
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