[R-390] SS VR replacement for 3TF7

[2002tii]

Tom wrote:

> Who else has tried the simple DC regulator method and written down
> what the results were versus that of a current regulator?

I have kludged both SS voltage and current regulators into my 390A to
experiment.  A good current regulator works better than the ballast tube,
but the voltage regulator wins hands down.  Still, I run it with a ballast
tube because that's good enough.

> I'd hate
> to think a team of experienced engineers screwed up something as
> simple as a ballast arrangement.

I don't think it's so much a matter of screwing up as not having an easy
way to stiffly regulate low DC (or worse, AC) voltages at significant
current back in "the day."  Remember, they didn't have gain-of-a-million op
amps then, or even zener diodes -- who'd want a seven-tube voltage
regulator there, and what would they have used for a voltage reference?  A
voltage divider from one of the glow tubes?  But those aren't really very
good voltage regulators.  Voltage regulated heaters were not an easy design
problem in the vacuum era.

Best regards,

Don