[Engineer]

Hi, Vacuumlanders, a 6F6 story...

I am intrigued by the relationship, if any, between mutual conductance
(as tested) and emission percentage (as tested by an emission, only,
tester like the Heathkit TC-2.)

I bought five 6F6's recently on eBay and the seller stated the gm of
each tube (IMHO, honestly), also an average for the type (2000, he
said.) When I received them I ran them through my TC-2 to check
emission. Here's the result.

Stated gm, TC-2 emission%
2300, 65%
2260, 72%
2240, 85%
2000 (no example of a known average gm tube to hand)
1740, 58% (very slow warm up)
1580, 51% (ditto)

Clearly, the last two are poor to bad tubes, right in the "?" range
of the TC-2

Then I tried pairs of the 6F6's in my "Scratch 10" (homebrew)
amplifier (nominally 10 watts using 2 x 6V6's), with the following
results:

Using any two of the good 6F6's from the above, max. power at 100 Hz
was about 8 watts (8 VAC across an 8 ohm resistive load, measured by
DVM), a bit less that the 6V6's but I did not adjust any bias, rather
just plugged them in, neither did I check that B+ remained the same
(it's about 312 VDC with the 6V6's, cathode bias, nominally Class
AB1)
Plugging in the two bad ones: max. power at 100 Hz was only 2.8 watts.

I had another two 6F6's of unknown (to me) gm but they test on the
TC-2 at 60% and 70%, respectively. In my "Scratch 10" they deliver
about 8 watts, the same as the above "good" ones.

While this is rather incomplete evidence, what might one conclude? Of
the good ones, gm and emission seem uncorrelated (actually, inversely
correlated, but that makes no sense), but over a wider range (say, 2250
to 1600, or so, it is correlated.) I also conclude that the two
poor/bad tubes are quite unusable.

Any other conclusions?

Cheers,
Roger