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

[john rose]

I have read the responses to my questions with interest. None of them have addressed my questions. I shall restate them.

The R392 uses a series of tubes designed to run at 24v on the filaments and B+. However the tube spec sheets I have read all go out to at least 200v. Question; will a tube designed for 24v B+ SAFELY run with B+ at 200v or more? Question; if it does, then is this series of tubes really 6/12v tubes "in drag" so to speak? Comment; obviously these tubes run on 24v B+ and run very, very well. In my (not even close to humble) opinion, the R392 as an SWL rig is very nearly the equal of the R390. Collins designed both and if you lift the lids on both, they very nearly look the same except for size. Oh, BTW if you compare the selectivity graphs of the 85kc IF of the '453/R23 with that of the 392 at 2kc, they are virtually identical. 6 gain stages at 455kc will do that.

This series of tubes is not "Space Charge" tubes. Space Charge refers to the electron cloud that forms around a hot filament or cathode. The 'Space Charge Circuit' put + voltage on the grid closest to the cathode (usually the control grid) of an 'ordinary' tube to attract and accelerate the electrons in the general direction of the plate. Circuit control is passed to the next grid in line. This series of 24v tubes operates as you would expect from a normal tube. I.E. the first grid is used as the control grid as any normal tube. What became commonly known as "Space Charge Tubes" were developed by Tung-Sol in the 1950's and had an extra grid (cathode grid) placed in front of the control grid. They have a limit, as I recall, of 30v or so on the plate or you will need to open your wallet.

----------------------------------------
> From: vk2bcu at operamail.com
> To: arc5 at ix.netcom.com
> Date: Sun, 10 Mar 2013 09:35:43 +1100
> CC: arc5 at mailman.qth.net
> 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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