Plate Load Resistance For Obscure Power tubes

[pleasantbullet]

Just curious as to how to find the Load Resistance, or Plate to plate resistance for using odd ball tubes for Low watt Power applications.

For instance, i've seen 12au7, 12ax7 used as power tubes. But the plate to plate resistance for choosing an output transformer, is not always available.

I would like to use a 6av6 as a power tube in an amp, now I found a small radio schematic, with one in SE formation, and the OT was labled as 5k:8ohm

How do you go about figuring that information out, when its not available in data sheets.

I'm not looking a rule of thumb answer, more so an answer that would help in using obscure tubes in the future.

[jazbo8]

6AV6 is a small voltage amplifier tube, are you sure that it was used to drive the output via the 5k OPT, that seems awefully low...

[tubeswell]

For centre-biased Class A operation:

Zout = Va/(Pa/Va)

You get Pa from the datasheet.

Va = idle plate voltage.

Say your tube is a 14W tube and the plates sit at 350VDC

Zout = 350V/(14W/350V) = 8,750R (or 8k8. In practice you might use 8k)

Now you can look at a 6av6 datasheet and try.

[pleasantbullet]

TUBESWELL, thanks a million!

Just the info i was looking for.

[Phil_S]

Just wanted to add, 6AV6 is one triode of a 12AX7. You can treat it accordingly as there are a great many examples of 12AX7 in use.

[pdf64]

Have a play with the interactive data sheet http://bmamps.com/Tech_tds.html