Paging Vox AC30 Experts

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Sunnyevan, Sep 10, 2019.

  1. Sunnyevan

    Sunnyevan Member

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    I just came to acquire a 64 AC30! Woohoo. Works for the most part well!

    The amp from what I can tell was an AC30T. The OT was replaced with a "Sampson-Perrota" replacement OT, I guess Mark Sampson may have rewound the original Albion? The choke is Albion and the PT is original.

    From reading: http://www.voxshowroom.com/uk/amp/ac30_6_hood.html, it seems that the AC30T had 220pf bright caps across all three volume pots. When I opened up my amp, I see three bright caps, what the values are, I don't know. They look original. According to this site, the AC30N did not have these bright caps.

    This amp has an add on Top Boost........ and with the bright caps, which I'm assuming are 220pf, it is extremely bright, which is why I started investigating.... which got me thinking.

    When people sent their amps into Vox for Add on Top boost, and they were varying models (Bass, Treble, Normal), did Vox change the caps? The coupling, bypass caps, and treble cap values vary from Bass to Treble To Normal. If you just pop in a TB on an AC30T, it's going to be very bright.

    What would be the appropriate cap values to use with a Top Boost AC30? What is the standard?
     
  2. pdf64

    pdf64 Member

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    The top boost controls should only affect the bright channel?
    Yes a 220pF bright cap plus setting the treble above halfway may be too much for most; the weak ‘tone’ control on later treble versions compounds that (2n2F cut cap). Try 100pF bright cap on the bright channel volume, or just no bright cap at all.
     
    Last edited: Sep 10, 2019
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  3. Red House

    Red House Silver Supporting Member

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  4. Sunnyevan

    Sunnyevan Member

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    Yes you’re correct. There is a 100pf cap. I will give this a try.

    I guess I was more curious what signal caps were used in the TB circuit when it became more standardized.
     
  5. HotBluePlates

    HotBluePlates Member

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    "Standard" is whatever is standard for the base model, I suppose. Your Treble version has 5 caps that make it brighter than the Normal or Bass versions, plus the Volume control bright caps. Aside from its bright cap, the Top Boost circuit is more gain plus tone controls.

    So if you feel your amp is too bright, you might just want to convert it to a Normal or Bass version. I myself have an early 60's AC30/6 Bass version (non-top boost).
     
  6. Sunnyevan

    Sunnyevan Member

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    I read this comment on a YouTube video in regards to differences in the circuit between bass, normal , treble :

    “ The (non-TB) AC-30/6 came in 3 different versions, named "Treble", "Normal" and "Bass". Some cap values were different. And in the Treble-model the input stages of Normal- and Brilliant-Channel had separate cathode-resistors and -capacitors. (1k5 & 0.1µF for Brilliant and 1k5 & 25µF for "Normal"). In the "Normal" and "Bass" versions, both channels shared a 1k5 & 25µF.”

    Is this true? I’m having a hard time finding schematics of the treble version to compare to the normal.

    According to the Vox showroom site, circuit paths should be the same, it’s just a matter of swapping cap values.
     
  7. HotBluePlates

    HotBluePlates Member

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    Double-checked some schematics, and that is true. Separating the cathodes (while also changing the input stage bias for those channels) was probably done to use the 0.1µF cathode bypass to make the Brilliant channel even brighter than all the other changes already did.
     
  8. Sunnyevan

    Sunnyevan Member

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    Awesome. Thanks for the help. Would you be able to PM the AC30T schematics?

    I already swapped the bright cap for a 100 pf; it was previously a 220pf. It is still too bright. I am going to try the cap changes this weekend.
     
  9. HotBluePlates

    HotBluePlates Member

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    North Coast Music has them for sale.

    My copy came in the limited edition of Jim Elyea's Vox Amplifiers book (at significantly more than the original selling price). I don't have a digital copy of it.

    Yeah, 100pF would make the Brilliant channel even brighter than 220pF.

    The Under the Hood page that you previously mentioned already tells you which caps are different, except that it does not note the split cathodes & smaller bypass cap that you asked about. The reference designators are all the same as the standard (non-Top Boost) AC30/6 schematic.
     
  10. Sunnyevan

    Sunnyevan Member

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    I thought that the lower the cap value, the less emphasis on treble frequencies?
    Wouldn’t the 100pf be less bright then the 220pf cap?
     
  11. HotBluePlates

    HotBluePlates Member

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    Capacitive Reactance

    Capacitors pass high frequency very easily. As the frequency goes down, the cap's reactance (frequency-dependent opposition to current) increases. To get a capacitor to more easily pass lower frequency, it needs more capacitance (make it a bigger value).

    1µF passes some low frequency more easily than 0.001µF. Picking 100Hz out of the air, 1µF has ~1,600Ω of reactance while 0.001µF has ~1,600,000Ω of reactance. So 1µF looks like a 1.6kΩ resistor at 100Hz and 0.001µF looks like a 1.6MΩ resistor.

    Easy way to think of it is "smaller values strip out more bass." The absolute effect of any cap depends on resistance in the circuit, though.
     
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  12. pdf64

    pdf64 Member

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    I think that as here, for a given amp and its vol control setting, a higher value bright cap will sound brighter (more trebley) than a lower value bright cap.

    So if a 100pF bright cap is still too bright for your taste, just try it with no bright cap.

    What have you done with the 330k resistor feeding the bright channel vol control? There are arguments for both leaving it in place (a little more bottom end) and removing it / linking it out (a little more gain available).
     
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