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

Sun Mar 10 13:18:06 EDT 2013

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

>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

** The R-392 used all regular tubes, (with the exception of the 26A7) at 
reduced voltage, the 26A6 is a 6BA6, the 26C6 is a 6AT6, and the 26D6 is a 
6BE6. The 26A7 is built especially for 28V use and has a 50V maximum 
rating.. The 6AJ5 is an optimized 6AK5 for 28V B+ but rated to 180V, and Im 
sure we all know what a 12AU7 is. All tubes were available in 1944-46 so 
were ready for the later 40's R-392 design.

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.

** It took far more IF stages than normal just to provide the necessary gain 
and had nothing to do with selectivity. That was provided by Collins 
mechanical filters as in the R-390A. The HRO-60 can do it in 3 stages at 455 
without mechanical filters altho those were available as a plug in option to 
steepen the skirts particularly for RTTY.

>
> 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.

** Far from an absolute; many were run as a standard tube, others as you 
mentioned, and those with an A suffix had and extra grid added.

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.

** The 12V tubes could be quite troublesome in auto radios and often 
developed microphonics or intermittent shorts. I get a fairly steady traffic 
of these in as Ive been servicing auto radios since the 60's.

 Carl

>
> ----------------------------------------
>> 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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