I received the 6Н21Б from Sotchi and I see the difference with the "classic", AF tubes : they are smaller :-)

I decided not to play with nuvistors and the miniature tubes are right what I had in mind, the object in itself is pretty good looking.

From the seller:

6N21B = 6112 = 6N17B-V ~ 6SL7 Gold Grid Hi-mu Double Triode NOS
High durable low frequency high-mu dual triode
Improved version of 6N17B-V / 6N17B-VR , 1W anode power dissipation ( 6N17B-V / 6N17B-VR only 0.9W )
Very low microphonic noise , 4 times better than 6N17B-V / 6N17B-VR
Different pinout than 6112 / 6N17B-V / 6N17B-VR / 6SL7, see datasheet !
Each tube contains 5.05 mg gold and 0.05 mg silver !

This one didn't come with an etiketka/passport, so I have to rely on other resources. They are easy to find online (which helped me decide on the appropriate reference) and here I copy-paste the data from http://tubes-store.com/product_info.php?products_id=732 (I further the long tradition of copy-pasting it from everywhere).

Name: 6N21B
Type: Double triode, hi-durable
Application: LF voltage amplification
Cathode type: oxide, indirect heating
Envelope: glass, miniature
Mass: 4,5g
Filament voltage: 6,3V
Filament current: 0,36-0,43A
Anode voltage: 200V
Anode current: 0,0035A
Anode power: 1W
Steepness: 2,6-4,0mA/V
Reverse grid current: 0,2µA
Microphnic noise: 15mV
Gain: 65-100;
Socket type: flexible;
Lifetime: more 2000 hours.

I have chosen these because they have a better amplification coefficient than the (slightly cheaper) 6N17B-V. I'm not doing audio and I don't care about linearity ;-) This also means I can undervolt the filament to increase their lifetime and save a bit of energy. Though who in their right mind would use these for 2000h ?

Oh and 200V on the anode ?

I'm not sure if the below curves (found at various websites) tells us the voltage can be lowered:

I might have to learn some Russian, one day.

And it seems I'll have to plot my own curves and find the right operating conditions.

It seems that the stories I heard about early "tube computers" that reduced the heater voltage (to increase reliability) may have overlooked something. From Rod:

"The heater voltage is far more critical than some people imagine, and if too low, the result can be cathode poisoning - a condition where the cathode materials are contaminated by trace amounts of gas."

So in the case of this tube, I'll try to run at the lower end of the range, around 0.35A or so. That's 1/3 of an ampere, or 2W, damnit, and heating 50 of these tubes will draw 100W ! It's ironic since I chose the miniature ones to save power... Yet the insides heat in the 700-800°C ballpark.

I found approximate equivalences. According to http://www.effectrode.com/signal-tubes/subminiature-tubes/, http://www.seymourduncan.com/forum/showthread.php?73998-Tubes-6021-vs-6021W,

"The 6N16B can be substituted for U.S.A manufactured 6021 and 6N17B for 6112 subminiature tubes."

"6021's european designation is Ecc70, and also NJ7P, it only has a gain factor of about 30"

So once again the russians just copied the design though they kept using it much longer than other countries...

Knowing the equivalences lets me extend the research of other compatible schematics and circuits...