[Engineer]

Patrick Turner wrote:
Engineer wrote:

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.)

(snip)

Patrick wrote

The gm is merely the current swing available for 1V of g1 signal voltage
change.
Usually its measured at the SE class A data figures given in the tube data
sheet.
But sagging emissions give a limited maximum cathode current, and the tube
simply
fails to produce the maximum anode load currents needed for the rated
output power.

if gm is at say 50% of normal is also means Ra has changed upwards since µ
will have remained about constant since this is a function of the relative
electrode distances.
With Ra a lot higher, the gain of the 6F6 output tubes becomes a lot lower
so perhaps the drive amp
cannot provide enough drive.
Basically if the emission is less than 70%, perhaps the tubes are stuffed,
and need replacing.
Some say the running of the cathodes at 12.6V for 20 seconds after they
have been initially warmed for
with 6.3V will "rejuvinate" the cathodes and increase emission and hence gm
for an unknown time.

Patrick Turner.

Patrick, I think you have nailed the problem in spades! I just made
some measurements I should have made before my first post (with the
global NFB off.)
1. B+ is 318 volts and virtually constant with all tube pairs - due, I
think, to the choke input self regulation, it's not sagging.
2. One limit is the phase splitter driver (a 6SN7 LTP with a HV power
transistor constant current cathode source.) On the 'scope, with the
original 6V6's, also the good 6F6's, it start to limit (on both sides)
on the positive swing just below 10 watts output - I need a bit more
headroom on that driver stage (working on it!)
3. With the bad 6F6's the 6SN7 driver starts to limit before the o/p
tubes can deliver more than 4.7 volts across 8 ohms (the 2.8 watts I
alluded to, above.) So, the O/P tube gains are, indeed, far too low.
4. BTW, on the tube emission checker I had to tweak the filament
voltage up to 7.5 volts for a few seconds to "wake them up", then back
to 6.3 VAC to get the readings. I plan to use these two for air pistol
targets!
Thanks and Cheers,
Roger