RCA Tube Identification

[Johnny10]

I should probably be better at determining tube type by construction but wondering if anyone can tell me what RCA tube this is.
All lettering has been cleaned off.

It has an 8 pin octal tube base.
RCA 66-48 on Base.

[Paul Price]

This looks a full wave rectifier, something like a type 5U4, 5Y3.
I can clearly see there is no grid, two plates and a filament, rather than a cathode.

To verify, find the two heater(filament) pins, they will show a low-resistance and connect to two pins only and if they match the pinout of the 5U4 or 5Y3, my guess is correct.

Most other non-rectifier tubes instead usually use pins 2 and 7 for heater connections.

[Johnny10]

6AS7  ?

[TimFox]

The tube number should be on the glass envelope, either on the side or the top.  The lettering tends to fade with time and can be hard to read.  An old trick, which needs to be applied carefully, is to apply skin oil from your finger to the letters and look carefully, rotating with respect to the light.
Note that other rectifier tubes had different filament connections.  The 5Y3 and 5U4 types have the filament (directly heated cathode) between pins 2 and 8 (tube pins are numbered clockwise from the bottom), but the 5X4 uses 7 and 8.  Other rectifiers, such as the 5AR4, have the heater between 2 and 8, with an indirectly-heated cathode at pin 8.
When you identify the heater/filament connections as suggested above, note that the "cold" resistance measured by your ohmmeter is much lower than the "hot" resistance corresponding to 5V/2A (5Y3) or 5V/3A (5U4) in operation.
The best reference for tube data online is
http://frank.pocnet.net/

[schmitt trigger]

I agree that it looks like a rectifier diode.

Once that you find the heater connections, apply 5 volts DC and measure the current.
Now go to tube specs of a few of the directly heated diodes, and compare your current readings with those of the spec sheet.

With that, at least you verify the tube type.

[Johnny10]

    
6sa7

Amplification factor µ = 2.
Dual Triode
Used as a regulator tube in dc power-supply units, as a booster tube in the scanning circuit of television receivers,
and as a push-pull class A output tube in high-fidelity audio amplifiers.

[Johnny10]

Thanks everyone!

I hadn't had my second cup of coffee before I posted.

Afterwards I thought why not do an image search?

I have 3 of these tubes that were given to me in a plastic container filled with old Tektronix tubes.
Two were RCA and one Sylvania all with similar construction.