Tube Challenge!!

Rene Schmitz uzs159 at uni-bonn.de
Mon Nov 29 11:32:40 CET 1999


At 19:50 28.11.99 -0500, media at mail1.nai.net wrote:
>Most of the ADSR circuits I've seen use op-amps and NAND gates.  I'm sure
>it would take quite a few tubes to replace a 4001.  By "next to impossible"
>I meant that it might be possible, but probably wouldn't be worth it.  If
>you know of an easy way using thyratrons I'd like to hear it.  I'm gussing
>it involves some sort of "thyratron gate."  Eliminating op-amps from the
>design would simplify the power supply (ie. no need for +15/-15).

One problem with tubes is that they can't swing all the way to zero easily.
A negative supply helps here. (E.B. used an extra -150V supply in a few
places.)
So building logic with "saturating" on-off switches is difficult.

>>But I think that I could live with a solid state ADSR running a tube VCA.
And
>>tube VCAs are VERY easy to build.
>
>I think I could live with that too, as I can't think of any audio advantage
>of having an all tube ADSR, besides the "Tube Challenge!!" aspect.

Right. 

>>Sample and hold ought to be fairly easy with high-impedance tubes.
>
>I thought "high-impedance tube" was an oxymoron.

But don't forget that there is a tiny gate current even at Vg=0! (This is
sometimes used for bias generation with a 10-22Meg gate resistor, the grid
then gets a small negative bias. Called "Anlaufeffekt" in the german
literature.) To have zero gate current the gate must be a little negative.
Its from electrons that are emmitted from the cathode with some kinetic
energy. A few of these will fly to the gate and anode causing a "leakage
current". (Or charge the grid negatively if left open.) Although a vacuum
tube has high input resistance, a jFET or MOSFET (or jFET-opamp) will have
a leakage smaller than that. This effect of thermal energetic electrons is
one of the few tube effects that has an exponential law. IIRC the energy
distribution of the emmitted electrons has an exponential shape at the low
energy limit. But I doubt it can be used for an exponentiator, since the
currents involved are tiny. 

Even if the leakage would be low enough for a sample and hold someone would 
have to show how to make a VC-switch with a tube. Certainly not easy.

>>If you could
>>> build tube VCO's, VCA's, VCF's, ADSR's, and LFO's, how
>>> would you mix modulation sources??  For example, how would
>>> you send a mix of LFO and keyboard voltage to control the
>>> VCF??  What's the best way to sum voltages using tubes??
>>
>>What's wrong with doing just like in solid state, with
>>voltage dividers?
>
>I really don't know.  That's why I asked.

A nice idea would be to sum into a low impedance terminal. Like into the
cathode of a grid-grounded amplifier. Sort of a virtual ground as well.

>>Also, keep in mind that tubes with multiple grids can do amazing things
>>with modulation with ease. There are some things that are far easier
>>to do with tubes!
>
>This is true, but otoh there are types of modulation that are easier in
>solid-state.  Look at Rene Schmidt's neon VCO, it's more of a CCO, a
>current-controlled oscillator.  A typical modular is built around the idea
>of patching control voltages where one output can be sent to many inputs.
>While I'm not saying that this isn't possible using tubes, it will
>introduce new problems involving impedance matching.

Well the tube part of the circuit is a CCO, but together with the
exponentiator it becomes a VCO similar to the solid state ones.

>>Actually, the more that I think about it, the more that I
>>feel that we have a very unique opportunity here. We can
>>spend our time refining and "improving" solid state synth
>>designs, or we can instead spend time designing entirely
>>new circuits, using tubes, going where no one has ever been
>>before. Overall, I feel that tubes are easier to work
>>with and design with, and there are a lot of unexplored
>>areas to check out. And some things may not be practical
>>with tubes, and that's fine, because there are going to be
>>some things that we find tubes will do quite easily, that
>>solid state cannot. That's where the biggest reward for all
>>this comes in, when you come up with something completely
>>new, that has NEVER been done before.
>
>I totally agree.  I just took a look at the Hellfire at the Metasonix
>website.  It seems to do things that have never been done with solid-state.
>One thing that confuses me is its PWM control.  I've always thought that
>pulse-width modulation was varying the duty cycle of a square wave, but
>this thing doesn't seem to have any sort of oscillator.

My view to this is: There is a whole world of unexplored circuits employing
tubes. 
But the solid state universe has not been fully explored as well. To me
there is only *one* true universe of synthesizer circuits in which solid
state peacefully coexists with vacuum tubes. Like in my VTVCO. 

Bye
 René




opinionsexpressedher   | uzs159 at uni-bonn.de
einarethatofmyemploy   | http://www.uni-bonn.de/~uzs159
erandnotofmyself....   | http://members.xoom.com/Rene_Schmitz  
                                   
                                   

                       




More information about the Synth-diy mailing list