[R-390] R390 knobs and more!

[Drew P.]

Paul Galpin wrote:
<snipped>
 
> I have seen, somewhere, a current limiting circuit to
> replace the 3FT7 using just an LM317, a small bridge 
> rectifier, and a specially chosen resistor to give an 
> true RMS value of 300mA. It's one you have to build
> "blind" unless you have a true RMS meter to check it 
> with. (I don't). This circuit is a true replacement, 
> as it only has the two connections, no ground needed.
> 
> does anybody know where I could find it again?

...to which Ren?e Deeter responded:

> Hi Paul-
> yes it still needs a ground, it was in Electric Radio, I
> think VK3??? 
> had it on a web site.
> If you cannot find the info I'll dig it up, scan it and E
> mail it to 
> you. it is nothing special, typical current reg circuit + 1
> diode and 
> 1000uf/50V on the input side.
> it is the circuit I use, I like it better than any of the
> others, i 
> added a gnd wire to one of the unused  pins on the
> socket, it is a half 
> wave  rectifier, no need for the RMS metre- just
> measure the current on 
> the bench then plug and go, and it all fits in on tube base
> including 
> the heat sink.

The circuit described by Ren?e feeds the PTO and BFO tube heaters regulated DC, but Paul inquired about a different circuit, Dr. Gerald Johnson's AC current regulator.  

Dr. Johnson's circuit uses components as Paul described, no big electrolytic filter capacitor.  It requires only 2 connections and no ground connection.  Goto  http://r-390a.net/Pearls/ballast_tube.pdf

You will find Dr. Jerry's writeup and the schematic on pages 9 and 10.
Wei-Li has done us all a large favor by compiling these "Pearls of Wisdom". He has packed in so much collective intellect distilled from postings that have coursed through this forum over the years that it'll make your head spin.

Drew