[R-390] RT301 current regulator tube

Fri Dec 31 13:37:48 EST 2021

Yeah, the ballast tube thread is second in thread count to the infamous Kielballast thread or additionally the best beating a dead horse thread...

Happy New Year!

Les Locklear

"You can have a college degree and legitimate parents and still be a dumb bastard".
An Old Retired Mechanical Engineer

----- Original Message -----
From: Jim Whartenby via R-390 <r-390 at mailman.qth.net>
Reply-To: Jim Whartenby <old_radio at aol.com>
To: <r-390 at mailman.qth.net>
Sent: 12/31/2021 12:27:51 PM
Subject: Re: [R-390] RT301 current regulator tube
________________________________________________________________________________

We are taking a simple current regulating system and making it very complicated because the 3TF7 seems to have been horded and now is hard to find at a reasonable price.  The R-390 engineering team reported that it was not really needed but included it, I assume, to satisfy Signal Corps contract requirements.  
Could those of us who use 12 volt tubes in the oscillators and have bypassed the 3TF7 current regulator please report on oscillator frequency drift?   

Likewise, those of us who use either a power resistor or a 12 volt, 300 mA vacuum tube as a substitute for the 3TF7 also please report on any noticeable oscillator frequency drift?

I am willing to bet that modern line voltage is steady enough that there is no noticeable frequency drift during many hours of operation.

If others also report that the oscillator heater voltage is close to 7 vac then it seems that there maybe a soft failure mode for the 3FT7 which may be stressing the tubes and will be the next subject of discussion,  <grin>

Happy New Year to one and all!
Jim

Too much agreement kills a chat.  E. Cleaver

-----Original Message-----
From: David Wise <David_Wise at Phoenix.com>
To: Jacques Fortin <jacques.f at videotron.ca>; 'Jim Whartenby' <old_radio at aol.com>
Cc: 'R390A' <r-390 at mailman.qth.net>
Sent: Fri, Dec 31, 2021 11:27 am
Subject: RE: [R-390] RT301 current regulator tube

#yiv0906133686 -- filtered {}#yiv0906133686 filtered {}#yiv0906133686 p.yiv0906133686MsoNormal, #yiv0906133686 li.yiv0906133686MsoNormal, #yiv0906133686 div.yiv0906133686MsoNormal {margin:0in;font-size:11.0pt;font-family:sans-serif;}#yiv0906133686 a:link, #yiv0906133686 span.yiv0906133686MsoHyperlink {color:blue;text-decoration:underline;}#yiv0906133686 span.yiv0906133686EmailStyle19 {font-family:sans-serif;color:windowtext;}#yiv0906133686 .yiv0906133686MsoChpDefault {font-size:10.0pt;}#yiv0906133686 filtered {}#yiv0906133686 div.yiv0906133686WordSection1 {}#yiv0906133686 LM317 regulation of unfiltered rectified AC is poor.  Really you need a large reservoir capacitor (and a ground), but then the regulator needs to throw off even more heat.    If you want good regulation without the heat, you have to go switching.  Kurt Schmid (sigmapert) sells a voltage regulator which requires a ground, or you can build my 3DW7 current regulator which does not.  The 3DW7 uses fifteen to twenty cheap/abundant parts and can fit in the 3TF7 form factor. It regulates RMS AC, by dropping cycles.  Schematic and firmware image free on request.    Dave Wise SWL in Hillsboro Oregon (inactive)    From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net>On Behalf Of Jacques Fortin
Sent: Thursday, December 30, 2021 8:16 PM
To: 'Jim Whartenby' <old_radio at aol.com>
Cc: 'R390A' <r-390 at mailman.qth.net>
Subject: Re: [R-390] RT301 current regulator tube    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 athttps://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

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