In article ,
says...

On Fri, 23 Jun 2006 17:43:40 -0500, (Gudmundur)
wrote:

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.


I suspect that when he says "read the thread fully" he may be
referring to where the OP said "The coupling capacitors are indeed
good and moving the tube around it
follows the tube."


Yes, that post was missing on my newsreader, as were several of
the replies. WinVN newsreader gets kind of weird like that sometimes.
When I posted the first response I had only seen about half of the
total replies that were actually posted. The op's statement that
the problem follows the tube shows two things, the tube is gassey
and the grid circuit resistance is probably to high. Some of my
old books show 47k as the maximum recommended control grid
resistance. Low d.c. grid circuit resistance helps stabilize
the runaway tendancy in power output tube circuits.

Gudmundur wrote:
The op's statement that
the problem follows the tube shows two things, the tube is gassey
and the grid circuit resistance is probably to high. Some of my
old books show 47k as the maximum recommended control grid
resistance.

Just as a compare/contrast:

With 100K grid resistors to the bias supply, the current through the
resistor to a "GOOD" tube is zero. Less than a millivolt drop across
the grid resistor.

And with the "BAD" tube, the current through a 47K resistor to -25V
bias is so high that the drop across the resistor goes from 0 at
poweron to 5V after 10 seconds, and steadily increases over the next 30
or 40 seconds to the point of 300-400mA plate current (at about 0V on
the grid, and 25V drop across the resistor) when the B+ fuse blows.

So, as I see it: with a good tube it doesn't matter much what the exact
value of the grid resistor is. With a bad tube it doesn't matter much
what the exact value of the grid resistor is.

Tim.