RadioBanter - View Single Post

June 23rd 06, 11:43 PM posted to rec.audio.tubes,rec.radio.amateur.homebrew

In article ,
says...

Gudmundur wrote:

Hey my man, what you may have is a classic case of the grid coupling
capacitor breaking down and killing your bias on the grid. It is
not uncommon for the grid to go positive and cause the tube to melt!!

You haven't read the thread fully have you ?

Graham

Well let's see, the original poster said the voltage across the grid
resistor seemed wrong, (leaky cap is possible there) and he said the
current kept going up and up (leaky cap getting worse with ambient
heat from the tube) and a blue glow which I find to be typical with
many tubes of the 807 style when there is excess plate current such
as may be caused by A LEAKY CAP between the control grid of the 807
and the previous stage plate voltage. Gassey tube causing runaway,
yes in circuits with high ohmic value grid resistors which is why the
d.c. impedance of power output stage grid circuits is usually low
and and includes an rf choke to keep the rf impedance high. Then, even
with a gassey final power output tube the output stage will be less
likely to run away even with a crappy tube. The original poster
added that after his experience with the runaway tube it seemed to
glow even bluer, no doubt!!!! Guess he liberated even more gass from
the internal overheated elements such as may be caused by 'a leaky
grid coupling cap' in a high resistance grid circuit connected
to a power output tube that is a bit gassey and may be experiencing
such positive grid voltage excursions during peaks causing the
grid to heat and become an even better emitter of electrons causing
a chicken and egg roundabout until the death of a slightly gassey
tube which will find it's final resting place in a landfill beside
some misfit unwanted LEAKY CAPS that I have personally cut out of
some older Seeburg jukeboxes with a problem exactly like the one
the original poster was describing.