Re: Tube E or not Tube E (Long as hell)

From Mark Pulver
Sent Wed, Dec 4th 1996, 15:36

>Ok I know I forgot to mention limited slew rates. I was kind of leaving it
>till I'd chewed it over a little more and forgot to mention it entirely. The
>reason I needed to think about that one a little is thus. A Slew rate
>limiter is a first cousin to a low-pass filter. Only slight differences but
>the same over all effect. Fast transients get slowed down. Or their
>amplitude rolls off. A device for limiting slew rate is usually made with an

But... You're missing the main issue with tubes... There is NOTHING linear
about them, and very few aspects are totally predictable.

You can sit with an analyzer and watch a response on a tubed circuit for a
given input, but what if that input signal is just a hair hotter, or colder
for that matter?

In a tranny circuit, there is a good amount of pure math that can reliably
predict the outcome. In a tubed circuit, math is tossed aside when it comes
to the finer points. There's no common saturation point even across a set
of matched tubes... There's the issue of the plate and grid voltage being
MUCH high than the base/emiiter potential of a tranny. If I have a .1%
fluctuation of base voltage in a transistor circut, while signifigant, it's
nothing compared to a .1% flucuation in the plate voltage of a tube
circuit. How does this voltage change effect the output of the stage? Very
little in a transistor based circuit, quite a bit in a tube circuit.

You also talk about simulating the randomness of a tube circuit digitally...

>From being a low level coder for eons, I can tell you that there is no such
thing as a random digital circuit. I can make something pseudo random, but
not truely random in the sense that we're talking about here. Sure, I can
introduce bit errors into a signal, but keep in mind that we're talking
about (at 16 bits) 65536 steps to describe a 1-2v signal. If you're gonna
only flip the lower two bits, then you're only introducing a .00003v change
in a 2v signal. A  tube circuit is MUCH hotter than this, and wildly
whipping level changes of 100mv or more is much more common.

To take a digital circuit up to that level of change, you need to start
flipping 7-8 bits, and that's gonna seriously cause problems elsewhere down
the line.

A tube will also age, often in wonderful ways, over time. Heat and current
are great aging factors on the tube. Heat and current will kill a TTL circuit.

>It also reminds me of when I use to work in the television industry. I think
>everyone's probably noticed that Cine film looks far superior to video. Why?
>Well cine film has thousands of times more resolution. The equivalent of
>hundreds of thousands of pixels per line.

Wait... You're talking analog and digital here...

In video, these days, you're shooting through a CCD device, pure digital.
The CCD will have a 16/24bit resolution on a "scan line" as it digitizes
the image.

In film, you have (basically) infinite resolution since you don't just have
65536 levels for a scan line, you have all the wonderful stuff in-between.
Plus, there are no "scan lines" at all! The film will react chemically to
the light reflected from the image, a full frame at a time, all in one
split second.

You go on to say this in industry terms, but using film as an analogy shows
the same reasoning on the tube/TLL/digital issue.

Good thread...


 Mark Pulver/Chicagoish, IL             The home of "Son of The MIDI Wall"                 
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