At an early VSAC meeting in Silverdale, I listened to Dave McDonald's ST-70 which was highly modded. Here is his description:
"I tried other output tubes, including 7027As, 6550s, KT88s and 8417s,
all in 'triode' mode. Since I had 3 db of feedback (local, not global)
from 16 ohm tap to output tube cathode, and all these except the 8417
biased at about 45 to 52 volts (at 50 Milliamperes), that amounts to a
little more than 70 volts peak from the drivers to get to zero volts
grid-to-cathode on the output tube. I searched for a little more
linearity at large swings on the driver. I wanted to retain a single
stage driver, but needed some gain too - so I "compromised" and picked a
6DJ8. I modified resistors and socket wiring to accommodate a 6DJ8.
Later I took out the driver load resistor, coupling cap, and EL34 Grid
return resistor. These three were replaced by the Lundahl interstage
transformer. The interstage gives more linearity than a simple plate
resistor. I also bypassed the bias pot wipers with large caps to ground
(the bias is applied at the bottom end of the secondary), so I can draw
a little grid current on peaks (but by the time you get there, it does
not sound pretty, it just recovers well). Then I removed the 3 dB
feedback - the EL34 is returned through a 1 Ohm current sense resistor
to ground, instead of to the 16 Ohm tap."
The picture shows where he got to by VSAC/Vancouver in 2007(?) or so with 300B's.
His address then was: Dave McDonald <
[email protected]> in case you want to compare notes with him.
Cheers and good luck! Hank in Eugene
