Tube Challenge!!

media at media at
Mon Nov 29 01:50:14 CET 1999

At 1:39 PM -0700 11/24/99, Doug Tymofichuk wrote:
>> Tube VCA's seem easy enough, but I think building a tube
>> ADSR to control that VCA would be next to impossible.
>Why? ADSRs are mostly just switching and charging
>capacitors. I would think that the ADSR would be an easier
>module to design, just using thyratrons.

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).

>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.

>> Sample and hold is another matter
>> entirely! How about a tube noise source??
>That could be fun, you could use a tube geiger counter to
>generate random noise.

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 :)

>Sample and hold ought to be fairly easy with high-impedance tubes.

I thought "high-impedance tube" was an oxymoron.

>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.

>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.

>> So I'm thinking that many of the basic features found in
>> subtractive synthesizers (eg. ADSR, S&H, keyboard tracking,
>> modulation mixers) would be impossible, or impractical far
>> beyond any desire for novelty, to build with tubes.
>I disagree. In fact, the biggest reason that this has not
>been done to date, IMHO, is that very few people have made
>the effort. Thousands and thousands of people have spent
>decades on solid state synthesis. Only a handful have
>spent a few years on tube synthesis. If we used one tenth
>of the time this list has spent on tempcos, for example,
>on tube design, we ought to be able to come up with some
>truly great stuff.

I agree with that.  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.

>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.

Can you think of any historical applications where tubes were used for
voltage or current controlled resonant filters??


P.S.  Here are some tube sites someone sent me: - audio amplifier design  - guitar amplifier design amps and Eric Barbour's
stuff - SPICE models  - tube "op-amps" and headphone amps	- tube basics  - tube data sheet locator

More information about the Synth-diy mailing list