[Mr. Atoz]

While I love a good tube investigation for its own sake, my interest on this
one is not just academic; I have 4 pristine NOS tubes that may be RCA
7581a/KT66's, or they may be RCA 5881's. I don't want to give them 7581a
voltage if they're 5881's. If you consider yourself an expert in the fine
details of old tube identification, here's your chance to step up. I will
describe as well as I can, but I can send photos if anyone wants to see
anything for himself. I just didn't want to do a binary post the first time
I uncloak.

So here's the story: They are labeled Amperex, made in USA, and screened
7581a/KT66 in completely intact red paint. The box is the usual yellow and
green, says 7581a/KT66 in the spot where the type is stamped, and "Amperex a
North American Philips Company" on the box. Both tubes and boxes are
wonderfully mint.

An objective description of the tubes:

Black base, with a slight raised 'ring' in the plastic a few millimeters
from the pin end. Other than that, the base is unremarkable -- it's nice,
shiny black bakelite, with no scratches. The guide pin has a hole in it,
about 2 mm diameter.
Plates are grey, darker than most grey plates I've seen, but not black. They
are matte, not glossy.There is a single ring-shaped side getter, with the
usual side getter flashing. There is a U-shaped metal clip on top, which has
the oddity of looking like shiny stainless steel or chrome, rather than
black metal like others I've seen. The upper micas have, as you look in
through the top, 4 cuts through them -- at noon and 6, they are convex
"parentheses", and at 3 and 9 are concave parentheses. This feature applies
to the lower micas as well.
The envelope is not especially large: It is shaped like the old Tung Sol
5881's. It has the slightly flattened top that 5881's usually had, as
opposed to being rounded like GE 6L6GC's, say.

Why I think they're RCAs: The micas, mostly, because no GE's or Sylvanias
I've seen have the parentheses/reverse-parentheses cuts, but RCAs do. Also,
there is a thin vertical crimp rib on one side of the plate, that I have
never seen on anything else that wasn't an RCA. Also, that ring on the base
has, in my experience, only occurred on RCAs. If other makes did that, I
haven't seen it.

Potential evidence that they're NOT 7581a's, as I see it: The shape. They
are small, and 5881-shaped. In actual fact, they're no smaller than a lot of
GE short-bottle 6L6GC's I've seen; but they do have that characteristic
flattened top, which I've never seen on a known for sure 6L6GC or 7581a, but
which most 5881's seem to have.

As I said, my interest is more than academic -- they're nice immaculate old
tubes, and ought to be used correctly, whatever they actually are. I don't
want to kill a beautiful 5881 because it can't take 500 volts. OTOH, if they
CAN take 500 volts, well... Has anyone ever seen a known 7581a this small?
By the same token, has anyone ever heard of a US make so egregiously
mislabeling a tube as to call a 5881 a 7581a when it isn't?

Thanks, fellas. Looking forward to the analysis.

Mr. Atoz