[ARC5] 24V B+ supply for "command" receivers.

[Leslie Smith]

The Topic (from John Rose)  BC-454 - screen voltage

> > JR has a receiver that is (or was) ..  "designed to run on 24 volt B+ and is loaded with tubes 
> >designed to run at this level (26A6, 26C6, 26D6 etc).... [He asks]  Will the tube really run at that B+?"

I say:  "Yes".  Note the following:

>Mr. D. Stinson says: "I routinely test Command Set receivers in completely "stock"  condition 
and run them for hours on 24 VDC B+.   Sensitivity is a little down and
audio is headphone level
> ( I use an amplified external computer speaker -  cheap as dirt at yard sales )."

>" I'm not trying to work DX with a Command Set receiver, so a minor reduction in sensitivity is no issue.  In fact,
> they run cool, stabilize faster and the noise floor is better.  I've run some for years this way and never encountered
> a "downside" I considered significant."  (D. Stinson)

To the above I add:  I have run all the "command" sets from a 60V B+
supply.
A report on this practice is available on ARC5_Radio (Yahoo group) in
"files".
I report the same general experience as DS (although at 60V B+, not 24)
but I got lesser quality audio and lower sensitivity at 60V with the
UNMODIFIED receiver.

Looking at the circuit diagram, notice a higher value of dropping
resistor in the plate circuit of the BFO and high frequency (12K8)
oscillator.
Measuring the drop across these resistors suggested the oscillator was
"starving" for voltage.  I reduced these and the audio quality (and
general sensitivity) improved.
I choose resistors that gave the same plate current as the original
design, and took whatever voltage that rule gave me.

The dropping resistor for the 12K8 is "buried" inside the oscillator
"can" in the coil set.  
Not easily found. 
I run the sets using a 24V AC supply (on the filaments).  I double this
(using a half wave doubler cct - commonly seen) to give 68V
(unregulated).
I regulated this to 60V DC.  See the yahoo web-site for the circuit
diagram.
At 60V the set runs wonderfully cool.

Les
-- 
  Leslie Smith
  vk2bcu at operamail.com

-- 
http://www.fastmail.fm - Send your email first class