[R-390] RT301 current regulator tube

[Jacques Fortin]

Hi Jim,

You are right.
Only a small diode bridge, a LM317 and a resistor are needed.
And no need to use a GND connection...
However, the resultant current waveform into the filaments will be a kind of trapezoidal shape.
The exact value of the LM317 current-setting resistor have to be found by measuring the current value with a true rms meter.

But, I still prefer the 12BY7 / 12BH7 solution, partly because the non-availability of a 12BY6 tube (it doesn't exist, in fact).
I use a 6BY6 in the BFO circuit to replace the 6BA6 / 5749 and use it as a product demodulator as it provides more output than a 6BE6.
The circuit is based on the W3JHR / W7DI ideas.

73, Jacques, VE2JFE in Montreal

No need to rectify and filter.  If you want to go the LM317 rout, all you will need is to add a bridge rectifier to the current regulator circuit.  The AC leads go to the appropriate pins of the RT-301 socket and the +/- leads go to the LM317 appropriate connections.  No matter what part of the AC cycle, the LM317 will always see the correct polarity and limit the current to the set value.  I just checked the circuit at  https://www.bristolwatch.com/ccs/LM317.htm   and see no need for a ground connection, but since I haven't tried that particular circuit, a ground connection for the bypass cap might be prudent for stability.
But if you go from a 6 volt heater to 12 volt heater oscillator tubes, you can save a few watts of actual heat in either flavor of R-390 by eliminating the current regulator all together.

HNY,Jim


Hello Tisha,
When you mention using a LM317, is it used in Voltage regulator, or in Current regulator configuration ?
In both cases, you will need to rectify and filter the 25.2 Volts AC supply, then use the LM317 thereafter to supply constant 12.6V DC to the filaments or 300mA DC, whatever.
And full-wave rectification cannot be used there, unless you modify the BFO
+ PTO filament wiring...
As the maximum voltage across the filtering capacitor will reach about 35V, it also means that a quite substantial heatsink for the LM317 will be needed.
Worst case, 6.72W have to be dissipated from it (in either configuration).

I prefer to use the 12BY7/12BH7 trick...

73, Jacques, VE2JFE in Montreal