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

[Richard Knoppow]

     FWIW, the ability of a tube to run at low plate
voltages is related to a property called perveance. The
perveance depends on the area of the emitting surface of the
cathode and the spacing of the first grid; the larger the
cathode and closer the grid the higher the perveance.  It is
the increase in perveance that allows sweep tubes to deliver
high power output at relatively low plate voltages.  Many
"modern" receiving tubes have high enough perveance to
operate with reasonable efficiency at surprisingly low
voltages.  Of course, they are not optimum from the
standpoint of maximum gain and minimum noise but will still
allow satisfactory operation from much lower voltages than
popularly supposed.
See:

 http://en.wikipedia.org/wiki/Perveance

http://www.john-a-harper.com/tubes201/


http://mysite.du.edu/~etuttle/electron/elect27.htm
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com


----- Original Message ----- 
From: "Leslie Smith" <vk2bcu at operamail.com>
To: "David Stinson" <arc5 at ix.netcom.com>
Cc: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Saturday, March 09, 2013 3:35 PM
Subject: [ARC5] 24V B+ supply for "command" receivers.


> 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
>
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