What do you know and how do you like the RCA 5963 Black Plates?

I think these tubes sound fantastic, especially for the price.  Good ol' made in the USA 50s product.  They have just a nice touch of air on the top end with great imaging and palpable soundstage.  I'm a bit surprised as all of the European made NOS tubes sort of have this "old world, old vines" kind of mysticism behind them--so much so that an ordinary old American RCA was not as valuable.  

I'm curious to see how I like these after 25 hours or so more. 

I also have a stash of these, via Brent Jesse - his description:
“This is a nice industrial type which is nearly identical to the standard 12AU7. RCA made a nice blackplate version of this tube, Sylvania has a gold pin version, and GE has it in their 5-star broadcast line. The plate voltage rating is a little lower than for a 12AU7, but for most applications, it will work fine. It has a rugged cathode and should be long lived, similar to the 5814.”

That specially-designed cathode is said to withstand long periods of time in cut-off without damaging the tube (cathode poisoning). This made them a great choice for NORAD computers, which led to a good supply of them here on the West Coast.

  A great article on using computer tubes like the 5963 for audio use:
http://www.effectrode.com/tubes-computer-applications/

I use them in my TADAC (Tube Audio Designs DAC) and have, like yourself, marveled at the quality and value. They are dead-silent and seem to have an unusually long lifespan. Very satisfied with the sound they produce.
 

Don't know about the  5963 but I had all the different brands of the massive 805 set tubes, and the RCA Blacks were the best sounding.

Cheers George