Tube Challenge!! (3)

media at mail1.nai.net media at mail1.nai.net
Tue Nov 30 00:20:48 CET 1999


At 11:32 AM +0100 11/29/99, Rene Schmitz wrote:
>
>One problem with tubes is that they can't swing all the way to zero easily.

Otoh, they don't swing to the "rails" easily either, which might be one
reason overdriven tube amps are more pleasing to the ear.

>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.

That makes sense.  I wonder how well CMOS can be mixed with tubes.

>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.)

I'm not surprised the Germans have their own word for such a particular
technical phenomenon, but I'm shocked it's only four syllables :)

>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.

Hmmmm . . .I'm not sure if I follow you here.  Exactly what shows an
exponential response??  Is it in response to voltage??  Tiny currents can
be amplified.

>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.

Right.

>My view to this is: There is a whole world of unexplored circuits employing
>tubes.

I agree.

>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.

This is why I would like to see tube modules that use CV standards that can
peacefully co-exist with solid-state modules.  Having 100V control voltages
is dangerous and very impractical.

At 9:51 AM -0700 11/29/99, Doug Tymofichuk wrote:
>
>Eliminating the +/-15V supply is not a big issue for me
>either way.

It would certainly help simplify the power supply.

>I think that you have to approach tube design
>from a slightly different angle, and try and ignore the old
>paradigms to some degree.

That's why I'm trying to dig up old tube circuits.

>HA ha! A geiger counter is a device for measuring
>radiation. *Rough* description: a chamber with one internal
>insulated electrode is sealed and evacuated, and a high
>potential placed on the electrode. When a sub-atomic
>particle (radiation) passes through the chamber, it
>causes one unit of charge to conduct from the electrode to
>the outer shell. (This is the source of the familiar
>clicking sound associated with these devices.) Integrating
>these events results in a radiation/time value. As
>background radiation has a somewhat random distribution
>with respect to time, this produces noise. No plutonium
>required, although the frequency spectrum of the noise
>would increase in the presence of a radioactive substance!
>(Disclaimer; I am not a physicist, the above description
>may not be completely accurate!)

Thanx :)  Would it make enough noise without getting a radioactive
substance??  First off,  radioactive substances are dangerous and hard to
find.  Secondly, they might interfere with the other tubes.  Thirdly, it
sounds like you would need to find a special tube.

>I used this example just to show how thinking about things
>a little differently can result in innovative ideas. There
>are certainly other ways of creating noise with tubes.

I'd think I'd rather go with those!!

>Exactly what I am saying! Anyone interested in tubes should
>really check this unit out:
>http://www.metasonix.com/modules.htm
>BTW, anybody have any thoughts on this module, the unique
>sounds/waveforms?

Not really.  I don't have an MP3 player, and the description seems rather
vague.  However, it seems different from any solid-state module I've seen,
and unlike the Phattytron, might actually be worth the money.

>>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.
>
>I would guess that this is changing the duty cycle of
>whatever waveform that is sent to the input of the unit.

Huh??

>> Can you think of any historical applications where tubes
>> were used for voltage or current controlled resonant
>> filters??
>
>Unfortunately, no. But I won't let that stop me!

Good for you!!

Here's your daily affirmation:

"If a man can build a tube synth, I can build a tube synth!!"






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