Tubes, Exp Converters, Hellfire, etc.

[media at mail1.nai.net]

First off, I'd like to express my appreciation to everyone who has
contributed to this topic, and thank Rick Jansen for providing this
excellent forum.

>My experiences with Eric began about two months ago, when
>he responded positively to my tube challenge. I found him
>to be extremely helpful, eager to give out information, and
>easy to deal with. He even went so far as to ship me a box
>of tubes, sockets, and other components for me to
>experiment with, for absolutely no charge!

Good for you!!  Otoh, other people have had different experiences.  I've
written Eric a few times in the last several years, and afaik, he has never
answered any of my emails.  The only email I have received from him was a
nasty letter in response to my saying on AH that the Phattyron wasn't worth
the money.

>I think that it is pretty unfair to criticize Eric for
>his efforts with tubes. At least he is exploring new areas
>of sound generation,

I have never criticized such efforts.  I'm all for it!!

>Comparing the Hellfire to a solid state circuit without
>actually hearing it and then dismissing it as nothing
>different based on a few visual sample waveforms is just
>plain stupidity IMHO.

Notice that I defended the Hellfire as being unique and innovative.  I even
defended its price.  However, people should be free to compare it to
whatever other design they like, especially on a list dedicated to synth
design.

>I will go out on a limb here and make the statement that a
>processor like the Hellfire has the potential to become a
>classic piece with the guitar/tube amp musicians. Like it
>or not. And I hope that it does! Anyone who believes that
>they can easily duplicate it's sound with solid state
>circuitry has never really *heard* the sound of tubes.

I believe that some of the things done with tubes can be done with
solid-state and vice versa.  However, lets compare the difference in
approach of the Hellfire to the Serge Waveshaper.  If you tried to put a
Schmidt trigger input before a negative feedback amp and wrapped diodes
around it using tubes, it would be much less elegant than the "sheet beam
modulator" circuit I found on my hard drive.

>I was going to answer this anyway but I now feel compelled to. I won't
>bother you with the details but Eric isn't happy with my claim that the
>"the Serge wave shaper does all of this and more". Under threat of
>litigation for me and DIY (read Rick), I retract that statement and
>apologies publicly to Eric. That was really just my opinion and not truly
>a fact.

That's just plain silly.  He doesn't have a tort.

>The 6BQ7 tube stage that performs the PWM is probably a
>type of monostable multivibrator. When the input waveform
>is applied to it, it causes it to trigger once per cycle,
>in affect it becomes a second oscillator sync'ed to the
>input signal. Adjusting the time constant for this second
>oscillator to a much smaller value than the input waveform
>time constant will result in a lower duty cycle (narrow
>positive pulses rather than equal negative and positive
>pulses).

I think that makes sense.  I'm guessing it's sort of like the frequency
controlled "oscillator" in the EH Microsynth.

>Here's a quote from one of Eric Barbour's mails on Synth-DIY from July
>1997 (took me a while to find it):
>
>>>You have built an expo converter without linearity problems
>>>with tubes ?!?
>
>>No, I didn't. As someone else pointed out, it is not possible
>>with tubes. (Actually it is possible but very difficult--it
>>requires a special tube made by Raytheon in the 1950s, a
>>square-law tube called a QK329. Very rare today.)

Thanx!! That answers one question.  It wouldn't be worth designing an
exponential converter out of parts that can't be found.  Perhaps one could
build a linear VCO out of tubes that would respond to MS style voltages.

>>At no time did I apply all the tired old paradigms of solid-state
>>synth design to my circuit. The VCOs use thyratron tubes, which
>>vary considerably from unit to unit in their zero-point switching
>>thresholds. The only practical way to control them is with a keyboard
>>having adjustable pots in its resistor ladder, allowing an
>>arbitrary voltage for each key (in ascending order).

Imho, that's throwing the baby out with the bathwater.  Having MIDI without
pitchbend seems inadequate.  Nor would it work within a modular system with
the VCO as a destination.  It seems like too much of a loss just to have a
tube sawtooth oscillator.

PEACE OUT :)
MARK


Disclaimer:  I am being controlled by an alien entity :)