[ARC5] Space Charge Tubes vs. Low Plate Voltage Tubes In Military Sets

[john rose]

I'm not familiar with the 28D7 so I binged it and got this result:
Size 28D7 Bras - Fine Lingerie, Underwear and Bras

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> Date: Sun, 10 Mar 2013 17:51:30 -0400
> From: kk5f at earthlink.net
> To: arc5 at mailman.qth.net
> Subject: [ARC5] Space Charge Tubes vs. Low Plate Voltage Tubes In Military Sets
>
> The very capable R-101*/ARN-6 ADF (16 tubes) uses 28 vdc as the B+. These are
> standard tubes like the 12SK7, 12SY7, 12SX7, etc., except for the 26A7GT AF
> output tubes that are rated for plate voltage maximum of 50 vdc.
>
> No military set with which I am familiar uses space charge tubes. I believe
> that "space charge" devices did not enter into any design unless operation with
> B+ in the **12 vdc** region was required. I'm not familiar with any space
> charge tube applications anywhere when B+ is as high as 28 vdc. There was
> a 14 vdc version of the BC-1206-* beacon band receiver in WWII known as the
> R-76/ARR-13, but its use in a 14 vdc aircraft required an external 250 vdc
> supply that the 28 vdc BC-1206-* eliminated. If space charge tubes of the
> mid-1950s era been available, the B+ supply could have been eliminated.
>
> The POWER tubes designed for low B+ all have a characteristically very high
> filament power consumption. The improved cathode emission no doubts aids
> operation at low B+. Typical filament power consumption in POWER tubes:
>
> 25L6 25.0 vdc, 300 mA, 7.5 watts (Plate voltage max = 200)
> 26A7GT 26.5 vdc, 600 mA, 15.9 watts (Plate voltage max = 50)
> 28D7 28.0 vdc, 400 mA, 11.2 watts (Plate voltage max = 100)
>
> Note the reverse relationship of filament power consumption to
> max rated plate voltage above.
>
> SMALL SIGNAL tubes with, typically, 26.5 vdc filament ratings have published
> plate voltage ratings from 26.5 vdc up to a maximum voltage that is typical
> of *any* similar tube, regardless of filament voltage ratings. As expected,
> these small signal tubes consume filament power that is typical of any similar
> tube, regardless of filament voltage rating. Typical SMALL SIGNAL tube data:
>
> 26A6 26.5 vdc, 70 mA, 1.9 watts (Plate voltage max = 250)
> 26C6 26.5 vdc, 70 mA, 1.9 watts (Plate voltage max = 250)
> 26D6 26.5 vdc, 70 mA, 1.9 watts (Plate voltage max = 300)
>
> For comparison, a standard tube like a 12SK7 consumes the same filament power
> (12.6 vdc, 150 mA, 1.9 watts).
>
> The most common rationale for *real* space charge tubes was for use in car AM-BCB
> radios of the late 1950s and early 1960s. These typically use space charge tubes
> in every stage except for the transistorized AF output stage. I bought several of
> those BCB radios at an auto salvage yard in 1965 ($1 each). They made some of
> the best performing AM-BCB radios that one could find...ugly, but with excellent
> sensitivity and selectivity. I still have a couple.
>
> The military sets like the R-392/URR, RT-380/AR (18S-4A), R-101*/ARN-6, BC-1206-*,
> etc. do not use space charge effect tubes. They use some POWER tubes rated for
> low plate voltages. One would NEVER want to apply that suggested 200 vdc to a
> 26A7GT or 28D7, even if the associated circuitry could survive it.
>
> Mike / KK5F
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