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

[Geoff]

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


Used in mostly civilian equipment. I built a nice QRP transmitter using 3 
that were in an apparently unused elevator control module I picked up some 
years ago. One as oscillator and buffer/multiplier and the others with all 
sections in parallel. With 90 and 45V on plate and screen they easily run 8W 
output on CW while not exceeding dissipation specs. After about 5 years of 
intermittent use the output hasnt changed.

Carl




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