I have a bunch of tube manuals in three or four arms reach -

I suspect the glowing plates on the 6L6 is a Beam tube.
I suspect the Screen voltage is a bit high or the Grid voltage
is a bit in the positive direction (should be negative DC and modulate that.)
If the Grid is near zero and you drive over into the + side - the Grid
pre-accelerates boiling electrons gathering on the hot cathode - IIRC thorium enhanced
book still in shelf.

When the grid pre-accelerates - the screen post accelerates as it should, but at a higher
current density now - due to the assist. The plate physically gets smacked with electrons.
Normal amount and the plates can keep their cool. To much and they get hot or melt a hole.

I don't recommend anyone touch any hot tube if it is half the length of a cig lighter.
More than that, and the wattage can far exceed the capacity of your skin to self cool.
a.k.a. burn.

I myself use 5881's two matched pair to drive my Mono output transformer. :-)

Martin

Gary Coffman wrote:
On Tue, 6 Jan 2004 15:01:54 -0600, "Tim Williams" wrote:

"Gary Coffman" wrote in message
. ..

dissipationless dynamic resistance of a vacuum tube,

So why do my plates glow when they are overbiased? Do you recommend
I grab onto a 6L6 for more than 30 seconds?

Tim


I can't lay my hands on my receiving tube manual at the moment,
but I do have an RCA transmitting tube manual at hand so I'll use
the 813 as my example.

It has a filament voltage of 10 volts at 5 amps. So this converts
electrical energy to heat at the rate of 50 watts. That makes the
tube envelope hot, but is not part of the tube's dynamic resistance.
It is a parasitic loss, necessary for a vacuum tube's operation, but
not part of the energy flow it is controlling. Note that a transistor
wouldn't have this loss, nor does an air regulator.

Static plate voltage is 2250 volts, and static plate current is 50 mA.
So the static plate resistance is 2250/0.05 = 45,000 ohms, and
energy is dissipated at the plate at a rate of 112.5 watts. This is
due to the kinetic impact of electrons on the plate. Again, this is
necessary to the tube's operation, but isn't part of the energy it is
controlling. A transistor would have a similar static collector current
and collector dissipation, but an air regulator would not.

The dynamic resistance is 8,800 ohms, and the energy controlled,
but not dissipated, by that dynamic resistance is at a rate of 573.75
watts. None of that is consumed by the tube. The dynamic resistance
of a tube is the slope of the load line drawn on the characteristic
curves of the tube. It exists because of the throttling effect of grid
voltage on plate current. An air regulator has a similar load line,
or lossless dynamic resistance, controlled by the feedback to the
regulator diaphragm and pintle valve.

Gary


--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder