[R-390] Dead Horse Beat mode - ON

[Paul Galpin]

I was lucky enough to get two R390s, each with its 3TF7 apparently working.

These were tested by monitoring the output current from a variable Bench supply from about 2 volts to the maximum rating. As
expected, the current rose with the voltage to about 9V, and then stayed constant up to about 20V then started going up again.
Stopped immediately this happened.

For one of them, the constant current section was 0.32A, for the other 0.37A. The high current one got retired to the emergencies
only box, and was replaced by a 47R resistor.

So, for any voltage between 9 and 20 across the barretter, the current is constant. In the R390A, the voltage is a nominal 12.6.
Since the same current flows in the 2x 6BA6 oscillators, their heater current is constant.

What this proves, though, is that your 3TF7 may be working, stabilising heater current, but it might not be the correct value of
current!  FWIW, I've seen the same stabilising effect in filament globes, but never a convenient value.

Please, don't anyone say "but it's different with AC"!

Paul Galpin
ZS2PG