[sdiy] Using Tubes in SDIY

[larry at unicode.com]

I wanted to take a moment and some space to send a public Thank You to Eric
Barbour, owner of Metasonix.  I had sent him an e-mail back in early October
asking some questions about Vacuum Tube circuits, usable tubes, and other
beginners questions.  He was very kind and responded almost immediately with
an extensive e-mail with data and suggestions.  I'm now working through some
of the material in the hopes of eventually understanding some of the SDIY
synth circuits I've found and adding them to my synth.

For anyone else that may have questions similar to my own, I am going to
share the e-mail that he sent me.  I hope that he does not mind.

Thanks again Eric!

Larry T.

---- From Eric ----
Larry:
Before you go plunging into this, I STRONGLY recommend you
learn a little about tubes. Get an Antique Electronic Supply
catalog and order BEGINNER'S GUIDE TO TUBE AUDIO DESIGN
by Bruce Rozenblit. Do NOT order the guitar-amp books,
most of them are garbage, except for Tom Mitchell's and
Kevin O'Connor's books. Kevin's ULTIMATE TONE books are
fairly good for learning about guitar amps, but you need
to start with the basics.


> In reading the article on the tube VCA
(http://www.cgs.synth.net/tube/index.html), I note that the circuit wants a
0-150v CV and a -150 to +150v modulation input.

The ranges shown are what the circuit will ACCEPT.
The screen grid works quite well if you feed it a CV of 0-5v.
Minimum gain occurs with a screen-grid voltage of
about -1 volt dc, and goes up from there as the
voltage goes positive. Gain of 1 from a small pentode
occurs at about +1v to +3v, depending on pentode type.

The modulation input is less sensitive, unless you use
a "dual-control" pentode such as a 6AS6. I'd recommend
not messing with that. Connect the suppressor grid (pin
8) to the cathode.

Better yet: use 7-pin pentodes.
I recommend 6AU6 or 6AK5 types, or their (many) cognate
versions with different heater voltages. For example,
4AU6s and 4CB6s are excellent for this, and cost very
little since they were used only in cheap TV sets (no
demand today). The CGS schematic shows an EF86
because when I wrote that stuff 7 years ago,
EF86s were the only small pentodes still being made.
Now, even those are out of production.

But cheap TV types still abound in NOS form.
Vast numbers of pentodes will work in that circuit. Even
pentagrid converters will work. And most 7-pin types
have compatible pinouts. If you take pins 2 and 7 as
the cathode, 1 as control grid, 6 as screen grid,
5 as plate and 3 & 4 as heater, you can use tubes with
standard pinouts 7BD, 7BK, 7CH, 7CM, 7EW, 7FQ and 7GA.
(Besides, 7-pin sockets are cheap and easy to get.)


> Also, where can I get more data on tube pin outs and usages.  There an a
tremendous number of 12..., 17..., and 25... tubes out there waiting to be
used.  I'm guessing that the leading number indicates the voltage for the
filaments.

Yes, USUALLY. Need to check a tube manual first. and, you can use
AC or DC on the heater. Also, the voltage applied can be as much
as 30-40% below its "official" rating. You're not trying to
extract maximum performance from the tube, and it so happens that
cathode lifetime increases dramatically as you lower the heater
voltage.

You need a good tube manual. Get the GE Essential Characteristics
reprint manual from Antique Electronic Supply. Part # B-377.
It is $13.95 and is money well spent. It's the most comprehensive
US tube manual ever printed. The RCA manual is good, but
leaves out many oddities/variations that GE, Sylvania, and other firms
made over the years.
Antique is also a good source of cheap tubes and tube sockets.
If you want to save $, use 7-pin miniature types when possible.
They are cheap, and good NOS sockets for them are also cheap.

www.tubesandmore.com


> Aren't there any of these other tubes that would be usable in synthesizer
circuits?

Yes, of course. We use series-string TV tubes frequently.
Cheap and available in the tens of thousands. The TM-2
uses three 4AU6 types.


> And is there any particular advantage/disadvantage to using circuit
voltages in the 220-250v range

No, just safety. I use 140v or less in Metasonix devices becuase of
legal liability issues. It is unlikely to kill an idiot who opens
the box and pokes around. (Idiots do this.)

If you want to learn how to DIY, I recommend you get on the
Synth-DIY mailing list. Several of the regulars have experimented
with tube circuits. You can read how to subscribe at
http://www.euronet.nl/~rja/Emusic/Synth-diy/

--
---
Eric Barbour
owner, METASONIX
senior editor, Vacuum Tube Valley