Re: [TCML] Home-made modulation transformer/guitar-driver (VTTC)

[Steve Ward <steve.ward@xxxxxxxxx>]

Hi Christopher,

On Thu, Apr 22, 2010 at 11:01 PM, Christopher Karr
<chriskarr4@xxxxxxxxxxx>wrote:

>
> Hello 'Everyone,'
>
> In a MOT-powered VTTC, would it suffice to make a modulation transformer
> out of, essentially, another MOT (driven by a 100W guitar amplifier)?


Not if you want to keep your amplifier in a working state.  A properly
designed amplifier would be suitable for the task.


> If so, would using the original secondary winding and a new primary winding
> - with fewer turns - be the preferred method? This would, of course, be
> placed with the bias of the guitar amplifier opposing the flow of
> anode-current.
>

Thats an OK idea, youd basically be adding or subtracting from the anode
voltage.


>
> Assuming this might work, in the first place, how might one 'force' the
> VTTC to respond only to their amplifier, in order to prevent the 'hiss' of
> CW 'sparks?'


Good supply filtering for starters.  But the hiss of a CW spark is basically
a thermal instability as far as i could ever tell, and you likely wont find
any way to get rid of that via electronic control.  Some people claim that
running into the few MHZ region gives a more stable arc that doesnt make so
much noise.  If you are a guitar player, i cant imagine a little bit of
spark noise would bother you ;-).



> A biasing DC supply saturating the core on the primary side? Or would this
> even be necessary, since the high-voltage DC energy from the primary circuit
> of the VTTC would saturate the core? Perhaps the varying current-draw would
> act to cause flux-collapsion?
>

None of that made any sense to me, consider clearing up your thoughts here.
But in general, i think your first idea was pretty close to being right.
You'd set the plate supply so that you get some spark, and then your
modulator transformer would either add or subtract from the plate supply
which would cause the spark length to grow/reduce producing sound.  How good
it works is really up to the designer.  And yes, the modulator transformer
secondary will need to pass the "DC" component from the plate supply, but
that is ok because you arent expecting the modulator to present any DC
voltage itself, just an AC voltage that adds/subtracts from the DC level.
Should not present any real issues with saturation.


>
> Or, perhaps using an IGBT rated at 600V on the grid-circuit is the only
> reasonable option?
>

I wouldnt recommend an IGBT for this application (it doesnt make sense
really), but a BJT or MOSFET could work as a higher voltage linear amp to
drive the grid bias point, which could sort of modulate the power out of the
VTTC.  I think this would be very strenuous on the tube, its power
dissipation would go up a lot compared with class C (basically switching)
operation, it would essentially put it in class A (linear) operation, which
always means lots of power burned off to do the modulation.


> Perhaps a pair of IGBTs (rated at 1200V each) with their collectors and
> emitters in series and gates in parallel is a possibility for post-tube
> (high-side?) modulation..
>

Series anything is non-trivial, not to mention your proposed wiring scheme
will blow out the gate on the lower switch once power is applied (you cant
just parallel them when the emitters are at very different voltages).

Steve


>
> Input and knowledge is very much appreciated, everyone!
>
> Thanks,
>
>  - Christopher Karr
>
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