Tube Challenge!!

[Doug Tymofichuk]

On Sun, 28 Nov 1999 19:50:14 -0500 (EST) 
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).

Eliminating the +/-15V supply is not a big issue for me 
either way. But I have been thinking about envelope 
generators and if I manage to come up with some working 
tube designs I will let everyone know. However, I will NOT 
be relying much on solid state designs for reference or 
inspiration. I think that you have to approach tube design 
from a slightly different angle, and try and ignore the old 
paradigms to some degree. 
  
> What's a geiger counter??  I thought it was a device used 
> to measure background radiation.  I have no idea how they 
> work.  Getting parts is difficult enough without searching 
> for plutonium on the black market :)

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!)

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 thought "high-impedance tube" was an oxymoron.

The grid on a vacuum tube is a very high impedance 
connection under many conditions.

Tubes work fundamentally differently than transistors, they 
utilise voltage control rather than current control, and 
they operate at much higher voltages than solid state. I 
believe that this is ONE of the reasons for the differences 
in sound. When the first transistor equipment was designed, 
the power supplies were modelled similarly to tube 
supplies, which did not take into account the high 
instantaneous current demands of solid state circuitry. 
This resulted in a sort of current starvation at peak 
demand levels, which sounds horrible. (non-linear 
clipping?) Much lower power supply voltages in solid 
state just makes this situation worse. (less voltage 
through a given impedance = less current.) Installing a 
high current supply in a solid state design makes a great 
difference in sound, not so with tube equipment. 
Unfortunately, unless they're of very high quality, most 
modern solid state audio electronics still have this 
problem IMO.
(Disclaimer; I am not an electronics engineer, the above 
description may not be completely accurate!) 

> 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.

Same old problems, slightly different numbers, no big deal.
 
> My point was that an all-tube synth 
> with all the features of a common monosynth, like an 
> SH-101, would be the size of a small refrigerator and draw 
> just as many amps.

Here is where you need to forget your preconceptions about 
synths, and open your mind to the *different universe* of 
tube synthesis. When Bob Moog began designing synths, he 
did not try and build an electronic pipe organ or piano or 
whatever, he started with new and revolutionary ideas about 
sound creation. I don't want to build a tube based SH-101, 
I want to build something that uses tubes to explore new 
areas of sound generation.
 
>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. 

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?

>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.
 
> 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! 

----------------------
Doug Tymofichuk
dougt at cancerboard.ab.ca