Tube synth

Doug Tymofichuk dougt at cancerboard.ab.ca
Tue Nov 23 19:37:51 CET 1999


This reply from Eric, while full of information, failed to 
make it to the list for some reason.

Doug:

Yes, a single big supply would work fine. You can
make one by
hooking two large filament transformers
back-to-back, or by using a
120-120v isolation transformer. Inexpensive
1000piv rectifier
bridge and a big 200v capacitor would suffice to
start. The 470uF 200v
twist-lock types are very cheap and commonplace
today, as they
are used in switching computer power supplies.
(Get the polarity right,
or it explodes like an M-80....)

A supply giving 5% regulation from no load to 100
mA load
will run numerous line-level tube
circuits--pentode VCAs, etc.
Two back-to-back 6.3v 3a filament transformers
will do this
with ease. (I have nice, simple plate power supply
circuits
which use no exotic transformers or other weird
stuff. I will
have gifs of them available tomorrow.)

Then, use a separate supply filter for EACH tube
stage in each
module. I recommend a 2k ohm, 5w resistor
(conservative, yes)
in series, going to a 47 uF 250v capacitor. This
works great for most
line-level circuits. For high-gain preamp input
stages, 2 or preferably
3 of these circuits in cascade will give you a
VERY clean 130-140v
DC--noise on this will be in the microvolt range
if it is in a shielded
and properly earthed box.

The VCOs need some kind of regulation. Either
0B2-type gas tubes
or zeners work well enough. You could get fancy
with complex electronic
regulation, but I have not seen the need for this
in my own circuits.
The 2D21 VCOs in my designs use 0B2s and draw only
about
4-5 mA apiece.

Heaters need not be run from DC, except in
high-gain preamps, where
DC is recommended. Most 6.3v heaters work fine on
5.0v computer
power supplies--most small-signal tubes are not as
sensitive to
heater under voltage as you might hear from
"experts". In fact, it is
often claimed that common small tubes usually last
LONGER on
5.0v (it seems to depend on
manufacturer/brandname--it is a gray area
right now--I am considering further work on this.)
2D21s and EF86s work
and last a long time on 5v. The tube heaters in
the PT-1
run off regulated +-15v supplies
with series resistors to drop to 6.0v (approx) or
12.0v, as needed.

As for using 6.3vac, we can discuss that tomorrow
if you wish............

----------------------
Doug Tymofichuk
dougt at cancerboard.ab.ca




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