[sdiy] Re: did someone say "tube"?

[metasonix at earthlink.net]

>> If you want to use tubes for frequency control, however, I wouldn't know
>> how to do it for 10+ octaves.
> That indeed gets a bit tricky. The space charge law doesn't permit as 
> drastic changes in dynamic resistance as the exponential behaviour of 
> semiconductor diodes. So VT-Diodes would have a more limited sweeprange.
> The most promising candidate in my opinion is to use the 
> transconductance control that the screengrid does on a pentode.
> Then there is also so called remote cutoff tubes, where the 
> transconductance depends on the negative grid bias. And can be smoothly 
> controlled over a large range.

That's the spirit, I knew you guys could think of something!

And remote-cutoff pentodes are cheap nowadays--almost as cheap
as small signal transistors.
At this very moment, Antique Electronic Supply is having a "blow out"
sale on 4BZ6/4JH6 and 4LU6 remote cutoff pentodes.
50 cents apiece....
Buy thousands of them and make the Craveners happy. (So they don't
have to pay someone to haul them away as rubbish. Hard to recycle.)

 > I don't claim to be a really smart designer, merely a hack. I have been
 > working on gathering information on what/how synths work so I can make
 > something for my son. I have taken all of the published info from your
 > designs as well as several others and it has been extremely helpful. I have
 > preliminary designs using tubes for :
 > Barbour BP VCF
 > Barbour Beam Modulator

I'm flattered.
Please, let me see any tunable LP designs you come up with. If it is
usable I will put it in a TM module, with your name on the panel.

 >     A reasonable 1V/Oct tube keyboard controller is my current endeavor. I
 > have a nice circuit in opamps and transistors compiled from various
 > ARP/Oberheim/Moog designs, but can't seem to get the sample and hold tube
 > circuit to hold well in simulation.

You will find S/H to be VERY difficult. Sample and hold in
the 1950s was always done with a reed relay and sometimes
an electrometer tube, a special device designed to have low
grid current.