tube regulated screen grid supply

[xk49w]

Interesting. What exactly is the purpose of that Leslie amp ? Must be a clean machine w/ the regulation and local feedback on the power tubes, and what's with the CT on the OT secondary ?

It doesn't really have a regulated screen supply. As you mentioned the OC3 is really like a zener, gives about 115V drop from the plate supply. This was done in other examples of pentode amps too, using other means. I've seen a voltage doubler for the plate and a the center of the doubler used for the screen for example to get the drop. I'd like to know what the difference in the tube performance is with this lowered screen value wrt distortion, power, etc..

As regards the OPT center tap, check out the Leslie 145 schematic. It uses the center-tapped secondary for series voltage feedback to the output tubes' cathodes.

[Andy Le Blanc]

so.... that OC3 has an average voltage drop of 108 -116 v 115-133v break down
and starts around around 100ma, thats whats different with the types start ma.
That series resistor in the leslie scheme is very important to limit current thru the device.
The application notes in the RCA RTM says you can shunt them with a cap but not to use a
value over .1uF or it will start to oscillate, And the regulation with these is
an average from 5 to 30 ma. Its a cold-cathode glow-discharge tube, they
used to make a triode cold cathode for relay on-off switching too.
they all take a 7-pin socket, I found a OA2 and a 0B2 in my stash to try.

[Ears]

I'd like to know what the difference in the tube performance is with this lowered screen value wrt distortion, power, etc..

[edit: to answer some of the question, screen voltage has big effect on tube's transconductance and head room. If you lower Vsc the plate curves will compress (bunch up) and headroom decreases. Richard Kuehnel in his books (e.g. pg111 2nd Ed The Fender Bassman 5F6-A) provides a method of replotting anode curves for various screen voltages
Changes to Vsc should be made with an eye on the chosen load. (BTW The Grommes 6550 example above hasn't a "lowered" Vsc as 320V is about the manufacturer's recommended value).]

I wouldn't want a stiff zener regulated screen supply. I read that one of the reasons guitar amps (as opposed to hi fi) often use, or at least allow, slightly higher than optimal Rplate-plate is because an unregulated Vsc tracks V+ when V+ sags. The result is that in normal operation the load line will intersect Vg=0 slightly below the knee but when the amp is overdriven the plate curves "compress" and the load line intersects the Vg=0 curve at the knee.
If the screen remained stiff an overdriven amp's load line would hit Vg=0 well below the knee with, I imagine, more sudden onset and more severe distortion.
I'm interested in any comments on this brief analysis.