"Tim Wescott" wrote in message
...
wrote:



I have very little direct tube experience, but from what I've read it
sounds like the grid is emitting. Apparently this happens when some of
the oxide coating from the cathode gets onto the grid, which gets hot and
emits electrons, which tends to pull the grid more positive. The current
flow on the grid heats it up more, which makes things worse, and it runs
away.

In all the old tube books this is a result of mistreating the tube --
running so much cathode current that the oxide vaporizes a bit and
condenses on the grid (although it may also happen from positive ion
bombardment -- hmm). But who knows what 50 years of sitting in a drawer
(or getting carted around) will do?

'Regular' grid current happens when the grid goes positive and collects
electrons, tending to pull the grid more negative -- this is why you can
self-bias a class C amplifier or oscillator with a resistor to ground.

I have no idea of how to actually test this theory, or what other
mechanisms would cause this problem. The best I can do is mention that
while 807's are normally $18 at Antique Radio Supply they're on sale right
now at $10, and 1625's are still a better buy at $5.50.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com


Tim,

I think you are on to something here. I ran across this effect with an
old WWII pulse tetrode, the 4D32. The ones showing this effect also had
little white spec's on the inside of the glass that could be "disturbed" by
tapping on the glass envelope. This was in a ham transmitter, and was worse
with rising frequency. At first the runaway occurred on 10 meters (29 Mhz),
and over time it would occur on 80 meters (3.9 Mhz). One thing that always
helped was to reduce the filament voltage from say 6.8 volts down to about
5.5 volts. Many old amplifiers / transmitters run the filaments pretty hot
with today's line voltages.

Might consider late manufactured 6BG6's for $6.00 each:
http://www.vacuumtubes.com/6BG6.html

These are very similar to the 807, and if the claims are true, then these
are better.

Jim