[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

[Patrick Turner]

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.)

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

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.

[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

[[email protected]]

Hi Alan,

Hi! I still have the tube boxes you sent me back when. They really
improve the sound of my system

Happy Ears!
Al


Alan Douglas wrote:
Hi,
Both testers caught the weak tubes, and that's about the best
correlation you can expect between Gm and emission. One additional
check is to measure Gm while reducing the heater voltage by about 10%.
Strong tubes won't change at all, but I suspect that your weak ones
would have dropped Gm dramatically.

Not to slight Patrick, who always gives good advice, but I haven't
seen a post from Al Marcy for a long time and am glad to renew an old
acquaintance.

Alan