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Questions about RCA 5751tubes

I am needing some help..

I ordered a pair of Sylvania 3 mica Black Plate 5751s for $150
and was sent RCA JRC-5751s instead. Please bear with all my questions.

1. I was told these were better tubes and matched within 2.5%
One tube has a sticker with 1.5/1.5 and the other says 1.6/1.7
Can I tell if these are matched from these values?

2. What do these values mean? Are they an indication of life left
on the tube? If so what values are considered NOS and what
values are considered 50% and 75% life left?

3. Are RCA JRC-5751 Black Plate, 3 mica, (smooth edge top mica)
as good as Sylvania 3 mica Black Plate 5751s?

I was surprised to get a different tube than I ordered but wanted
to ask everyone if this really is a better tube? Anything you can
tell me about this tube would be very appreciated..

More info on these tubes:

The top of the tube (nipple of the tube)is painted half green/half red.
I assume these are military markings..??? What do they mean?

The silk screening is all white but 90% worn off so you can't
read anything but part of the RCA logo, 5751, and JRC

2 black plates

Each tube seems to have 2 paper clip looking supports (one at each
end) to tighten up and better secure all 3 mica disks.

square halos.
tom92602

Showing 1 response by almarg

almarg's avatar
almarg
9,670 posts
 
"1.5," "1.6," "1.7," and "1.9" do not appear to me to be representative of filament draw. The 5751 is listed in my older GE tube manual as drawing 0.175 amps at 12.6 volts. If the 1.5 were amps, it would mean nearly 19 watts filament dissipation, which is obviously wrong for a low power dual triode. I don't know what those numbers would represent.

That tube manual lists gm (transconductance, also referred to as mutual conductance) for the 5751 as 1200, without specifying whether that is average, minimum, etc. My Hickok 800A tube tester lists 1250, specifying that as being for an "average tube," NOT an "average new tube," which it indicates will "be higher." So 1300/1350 would seem to be reasonably good numbers, while 1640/1740 sounds excellent, although the difference between the two sets of numbers leads me to question how well matched they are. The numbers before and after the slash, btw, I would assume are for each of the two triode sections within the tube.

HTH,
-- Al
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