Vox AC30 Schematic in The Tube Amp Book

Kevin Killebrew

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Jan 30, 1998, 3:00:00 AM1/30/98

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Looking at the AC30 schematic in Pitman's book... there is a 250k pot
labeled VR3 in series with a .0047 cap labeled c10, and connected
between the outputs of the 12AX7 phase splitter coupling caps and before
the 1.5k resistors leading the the EL84 grids.

Anyone know the purpose of this pot/cap combination?

Also, does anyone know the wattage rating of the 100 ohm screen grid
resistors and the 50 ohm cathode resistor?

Thanks in advance.

Kevin Killebrew

Richard Fowler

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Jan 30, 1998, 3:00:00 AM1/30/98

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The first circuit seems like it might be a "bright" control.

When in doubt, de-rate.....that is I would imagine a minimum of 1 watt
resistors though if I had 2 or 5 watt available I'd probably use those.
Space probably is of little concern in this application. You're always
safer using higher rated parts. I use 600v coupling caps in place of 450v
etc.

Richard

Kevin Killebrew <kKill...@ESC4.NET> wrote in article
<6asrig$3...@bgtnsc03.worldnet.att.net>...

William Boyd

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Jan 30, 1998, 3:00:00 AM1/30/98

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This is the "tone cut" circuit where the 250K pot progressively shorts a
variable amount of the signal above the rolloff of the cap to the other
(opposite phase) output of the phase inverter. Vox had this in the original
plan for the new AC15 reissue (though it really isn't nearly the same as the
original AC15) but took it out at the last minute before production since
the amp already had treble and bass controls & it is really useless - also
probably takes out a little harmonic content. The original amp didn't have
the treble & bass so it needed this tone cut deal.
Bill Boyd

Ralph and Diane Barone

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Jan 30, 1998, 3:00:00 AM1/30/98

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In article <6asrig$3...@bgtnsc03.worldnet.att.net>,
Kevin Killebrew <kKill...@ESC4.NET> wrote:

>Looking at the AC30 schematic in Pitman's book... there is a 250k pot
>labeled VR3 in series with a .0047 cap labeled c10, and connected
>between the outputs of the 12AX7 phase splitter coupling caps and before
>the 1.5k resistors leading the the EL84 grids.
>
>Anyone know the purpose of this pot/cap combination?
>
>Also, does anyone know the wattage rating of the 100 ohm screen grid
>resistors and the 50 ohm cathode resistor?
>
>Thanks in advance.
>
>Kevin Killebrew

Molto bizarro. Does this pot have a knob with a label on it, or is it
buried inside the amp somewhere? Functionally, it can only be some sort of
high-cut circuit.

Ralph and Diane Barone

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Jan 30, 1998, 3:00:00 AM1/30/98

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TONY ROHTLA

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Jan 31, 1998, 3:00:00 AM1/31/98

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The pot/cap combination You're wondering about is the "cut"-control used to
tweak the high-end.I'm not 100% sure,but I think that the 100 ohms resistors
are 2 watts,and the 47 ohms is 10 watts.Hope this helps.
Tony Rohtla

Ralph and Diane Barone

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Feb 1, 1998, 3:00:00 AM2/1/98

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TimVictor

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Feb 2, 1998, 3:00:00 AM2/2/98

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In article <B0F7D99B...@user34.mdi.ca>, bar...@mdi.ca (Ralph and Diane
Barone) writes:

> Molto bizarro. Does this pot have a knob with a label on it, or is it
> buried inside the amp somewhere? Functionally, it can only be some sort of
> high-cut circuit.

Look at p.70 (in my 4th Ed. anyway) to see AC30 control panels with and without
the Top Boost circuit. I believe the control you're asking about appears would
be the "Tone" knob next to the "Mains" switch. That's the only tone control on
a standard AC30. The Top Boost circuit adds bass and treble controls, but they
only operate on the "Brilliant" channel.

Hope this helps,

Tim Victor
TimV...@aol.com