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.
3. There is a reason the Sylvania's are higher I would imagine. They sound beter but that can be subjective? I had some RCA black plates and liked them ok but preferred GE black plates. I am talking about triple mica.
1. JRC is just the military designation for RCA. Tyey are IMHO just as good if not better than the white letter RCA branded tubes. These are not like getting a JAN Phillips tube instead of an RCA.
2, They are close enough values I find to call it a match. The difference that you would note if any is a difference in volume from one channel. The number refers to filament draw and is a better estimate of the life left in a tube than a striaght gain or Mu reading. I don't know it as well as I should. My testers are Hickoks and use GM. The Filament is what causes the tube to boil off electron from the anode to the cathode. If it is shot, the tube is virtually useless.Typical NOS is given as 1.9 but is dependent on whose testing and which tester thyey didn't callibrate.
Sylvania which may come as JHS if triple Mica Blackplates are considered by most a better tube than the RCA or GE. The Sylvania is hard to find and is a full tone tube with good extension and a sweet mid. I also like them the most. I have a pair in my rig now.
The mystique surrounding the GE grey plates that have been hawked here is myterious indeed they have the least defined and least refined sound of any even 2 mica RCA blacks like the Commands.
The green red tips are a dealer's grading scheme. It is not mlitary standard and can mean anything. Red is the Top grade in some dealer schemes.
The poor print is par for a 5751 which are uncommon as NOS.
The tube you ordered is exactly what you got.
If anything the military name like VT-231 for a 6SN7 is preferred. White print was used on the 50s tubes so you have a preferred tube early vs later when red was employed.
The Sound you have to judge for yourself. I think it is just less of the same overall presentation the Sylvania provides. Typically a littlle leaner but very clean.
If you can scare up a few bucks try the JAN 70s and 80s , get some grey plates beg for Sylvania triple mica blackplates and for Raytheons windmill getter a very popular tube that is also very very clean. Both will likely cost you about $150/pair in good shape today.
The GE triple mica (JG if military) blackplate the premium consumer brand was called 5 star at one point. Sylvanias Gold Brand frequently a 2 Mica grey plate with Blatty bass didn't always come with gold pins the 3 mica gold brand blackplate was always steel pin. All are tubes to try as well.
I can tell you a sampler LIke that will cost you a ton of money and took me a couple of years to assemble.
It takes time to find decent used or NOS- ish tubes. Even dealers pass on some pretty lousy stuff.
Also don't forget the current production. It has a very distinctly 5751 upper end. I think only Sovtek is making it now. Don't recall Shuguang ever doing it.
Finally there are a few Amperex and supposedly Telefunken 5751s . I warn you to avoid European as the 5751 is a truly USA tube and while there is sufficient belief that Amperex did maake them, I own a pair, they look like GE to me. Telefunken????
The only thing I misseed was the silver clip. It means something to one dealer in particular but is common to all 3 major players GE RCA SYl. Finally really there is one other true American brand that is not terribly distinguished for the 5751 that is Tung Sol. Somewhat harder to find but usually not expensive because they are so so. Some dealers will ask money for rarity but demand was low when I bought. figure a $50 tube in ANOS condition.
I am tired if you want more write to me via agon. And remember I am only one jackass amongst the many, we are mostly opinions.
I would also have recommended the GE black plates as the best 5751. However, the RCA JRC is a very sought after NOS 5751 made in the 50's - 60's and was also made as a grey plate version. Vacuum Tube Valley sells the RCA grey plates for $100 per tube, so price wise you did okay. The numbers are demonstrating that the tubes are closely matched, meaning they should have equal strength and current draw, which is what you want. I believe that matching is probably more essential for Power tubes. There isn't any true test that can determine how long any tube will last. These are military grade and more rugged to last longer. I have had 5751's last as long as 7,000 hours and had others die much sooner. My advice is to decide whether you feel the RCA's are equal to or better than the Sylvania's you ordered. If not, you should be able to return them without questions asked.
"Better" is purely subjective, and system dependent. The RCA w/clips may very well sound strident/overly bright in a system that already has that tendency. It does provide the best sound staging of any 5751 type though. In general- many that like 5751s, prefer the RCAs over the Sylvanias. Usually the triple micas w/o the clips however, as they are somewhat warmer(I'm a TLK ECC803S lover myself). To tell anything with regard to tube life from test results, you have to know what that particular tester's tube chart lists as "nominal new", "minimum good", etc for comparison. I seems presumptuous of a seller to send someone tubes other than what was ordered, but- that's just my opinion.
Thanks so far. There were 2 more labels that fell off
the tubes which were in the tube boxes:

Label 1: 1300/1350
Label 2: 1640/1740

Does this affect matching or it this the life left?

So is "1.5/1.5" an average tube value and not all that
Me, if I ordered and paid for a Sylvania 5751 triple Mica tubes, I should get a Sylvania 5751 triple Mica tubes, period!

I do not need someone telling me that the RCA is better sounding than the Sylvania etc. etc. The bottom line is, you should get what you paid for. What if the Sylvania sounds better to you than the GE or the RCA? Only you can tell in your system.

Oh, the label(1 and 2) is the Gm measurements of the tube where (first one indicates triode 1 and the second triode 2 for the 5751 which is dual triodes) which, in most instances, many confuse as an indication of the tubes being the numbers to match so that the tube pair can be called "match pairs".
"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.

-- Al

I love the humility.. And I think we are all jackass's
for being so nuts about musical reproduction. So much
more in life to do and we get all crazy about audio..
You should get what you order no matter what ..No excuses,,,Unless the person selling has a problem and contacts you with an explantion,it ain't the right way to sell......If you order a new Ford and the Salesman drops off a new Chevrolet at your doorstep..You may or may not be too happy.....
I really cannot speak for the group and cast braod aspertions about the general nature of humility. Tom92602 is clearly not a silly audiophile but a very correct and presumambly serious fellow. He is not a Humble Jackass I aplogize.
I missed one Huge point I really made an idiot of myself.
I thought our OP had asked for RCA and got JRC. I see now he asked for Sylvania and got RCA. this is truly no sarcasm intended at all criminal. That kind of behavior earns dealers a questionable charater tating in this case there is no question he is totally wrong by subbing which ever tube he deems fit too. I mean this perhaps that should be punishable with sanctions and a ban from Agon. Our poster should recieve a pair of excellent triple mica Blackplate 1950 vintage Sylvania 5751s. If he is not allowed to exchange them I will offer a pair of my own to barrow long enough to get an idea of what they are like.
OP please contact me through the AGON mail system.
I am not a dealer but have many tubes and may sell some in the future.
That is the only disclosure of potential conflict of interest I can think of. I do not have Sylvania 3 mica BP 5751s for sale only for him alone to borrow. I have JHs, Gold Brand, (? 3 mica) and Sylvania consumer regular. Many have rubbed or warn print but there is a way to identify them that readily distinguishes them from the JRC RCA which have almost Identical plates and getter rings.
A note to all, I sent the tubes back the very next day
and got my full refund in the mail yesterday. It all
worked out in the end..