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Is it possible to add cascade mod to vox ac15 or ac30?

Discussion in 'Shock Brother's DIY Amps' started by HaroldBKNY, Sep 2, 2019.

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  1. HaroldBKNY

    HaroldBKNY TDPRI Member

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    I was trying to add a cascade switch to my AC15 (whose circuit is really more like an AC30 with only 2 tubes). I tried taking the signal from the normal channel's input to the long tail phase inverter and running that to the top boost channel input, but it didn't work. The signal would always end up being really weak, buzzy, and attenuated.

    Maybe it's not possible for an amp that only has a single preamp tube gain stage before the long tail phase inverter? I'm thinking that in the couple of configurations I tried, the long-tail phase inverter section either ended up not having a proper input on the normal channel side, or the normal channel ended up phase-cancelling with the cascaded top boost channel.

    Schematic of amp below (not including cascade mod)
    [​IMG]
     
  2. HaroldBKNY

    HaroldBKNY TDPRI Member

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    For reference, these are two of the different switching schemas I tried, which both failed, given buzzy, weak signals on the cascade channel.

    1:
    [​IMG]

    2:
    [​IMG]
     
  3. gusfinley

    gusfinley Tele-Afflicted

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    What did you have plugged into the inputs? If you didn't have anything plugged into the Top Boost Channel, then the switch is shorting it to ground. You might need to plug a bare plug into it to allow the signal to come through if you didn't modify the input switching.
     
  4. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Thanks, but I modified the input so it's not being shorted to ground (you can see in the schematics). If it were being shorted to ground, I'd get no sound, not buzzy weak sound.
     
  5. gusfinley

    gusfinley Tele-Afflicted

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    It looks like you've removed the Ground Reference to one side of the phase inverter when you are in Cascade mode. Usually the unused portion of the phase inverter is referenced to ground through a .1uF capactior.
     
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  6. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Cool, yeah, I could try that. I tried having the unused side of the long-tail phase inverter be connected to ground directly in one of my attempts and through a 680k resistor in another attempt (schematics not shown here), but that ended up significantly lowering the gain of the other side of the phase inverter, the other channel, which wasn't good. But I haven't yet tried connecting it to ground before C14, which I could try.
     
  7. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Cool, I tried this out and it definitely works! Now there's no weird volume drop from either channel when I switch the mod on.

    Now I just have the problem that the cascade mod just doesn't sound too good... It's really really fizzy, and has what I imagine is blocking distortion (I'm not 100% certain because I don't specifically know what blocking distortion sounds like). Unfortunately, it's not a very smooth, cool-sounding distortion. Oh well.
     
  8. gusfinley

    gusfinley Tele-Afflicted

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    Blocking distortion sounds like a farty, buzy, sputtery distortion - definately not good.

    I built three prototypes of low-cost 5F1s ( SS rectified ) when I was interested in teaching a tube amp class. They ALL had this nasty distortion that I could not get rid of. I searched lots of forums and FINALLY found someone who posted the fix for this on their Gibson Goldtone GA-5 Les Paul Junior amp. The fix was to install a 1.5K resistor on the input to the power tube. ALL. THREE. FIXED!!!

    Look into blocking distortion on 12AX7 types and install appropriate values on your inputs and see if that fixes the problem. You already have them on your power tubes.
     
    Last edited: Sep 6, 2019
  9. HaroldBKNY

    HaroldBKNY TDPRI Member

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    Yeah, it's probably blocking distortion. It sounds almost like a zvex fuzz factory when you crank the "gate" knob up. About a sputtery, ripping quality.

    I don't think I'll look into putting grid-stopper resistors on, though, because I really want to keep the amp switchable to completely stock original specs. The amp sounds good when not in cascade mode, and I don't wanna meet with that.
     
  10. dan40

    dan40 Friend of Leo's

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    Adding grid stoppers should not affect the amps core tone. They should be placed tight up against the tube socket for best performance. Simply unsolder the grid wire from the socket, attach the resistor to the socket and then re-attach the wire to the other end.
     
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