AW: I just spent a few hours........

[Eric at Svetlana Electron Devices]

> BTW. How do one do a exponential converter in tubes? It ought to be
> possible, right?

It is....not easy, though. I know of no such
design using AVAILABLE tubes. In the 1950s Raytheon made
a special square-law tube, called the QK329. It was
widely used to derive exponential functions in early
analog computers. The QK329 has been out of production
for at least 20 years and is now a collector's item.

For a modern design using a tube VCO, I would use
a variable resistor ladder on the controller or
MIDI interface. MUCH easier and more consistent.

> With tubes down the size of an asperine, one should not need to have a
> much larger oscillator than using op-amps... is there any major
> drawbacks about tubes this small?

None at all. Members of the 7077 family are allegedly still being made
by MPD Inc. in the old GE tube factory in Kentucky. They are
quite expensive (about $ 100-200 each) and are used mainly
in very specialized applications--super-low-noise RF preamps,
military equipment that has to withstand a lot of radiation, etc.

 
> I must add that tubes have long been on the wish-list on things I
> would like to construct with, it is access and propper knowledge that
> is lacking. I would like as a thorough description of tubes and their
> properties that I can find about bipolars and FETs.

>What's a "GE 7077 planar triode (the size of an aspirin tablet)"
>and can one use it to make stable VCO's, VCF's and VCA's ?  Is it just a
>really small triode?  Where can I find schematics and/or literature on
>how to build VCO's, VCA's and VCF's using the "GE 7077".  Is their a URL
>with data sheets and such for the device? 

You can't. Sorry gentlemen, nobody has ever developed such audio devices
and published the circuits. Try Duncan's Amp Pages for some
basic data and PSPICE models. The Svetlana website has full data for 
the Svetlana audio tube line.

Now, you COULD make op-amps out of tubes
(more conventional glass dual triodes, probably),
and use them in typical Electronotes-type circuits....

I wrote an article about such tube opamps, it was published in the 3/92
issue
of GLASS AUDIO magazine. The back issue is available from
Audio Amateur Corporation, phone (603) 924-9464, fax (603) 924-9467
in the USA. The 1992 back issue is $23.
(No, i'm not getting a royalty--I was paid when the article
ran six years ago.)

It's the only tube op-amp article I have seen
in recent years where the circuit was direct-coupled from input to 
output, alllowing its use at DC--other circuits to appear since then
have used capacitor coupling. 

If you do get that article, I suggest you use the Figure 1 opamp.
It runs on conservative +-150v supply rails, which are unlikely to
kill even the sloppy DIYer.
(Also note that offset adjustments
are a necessity for these circuits.
See the article for suggested schemes.)

The power supply shown in Figure 5 in the article is
good for running a few such opamps. Be aware that there is an error
in Figure 5--C3 and C4 should be 0.1 uF, 200v film capacitors, not
220 uF. Otherwise, Figures 1 and 5 have been tested and worked
very well, enough to use them in audio circuits.

And don't forget to look at:
http://home.earthlink.net/~mccorquodale/eng_music/articles.html
See my articles toward the bottom of the page.


If you require any more assistance with these designs, I am available
to help.