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

[Mike Morrow]

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