Tube failure mode: gassy?

[s***@trailing-edge.com]

OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.

Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.

Being run with 400V on the plate and 255V on the screens.

Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.

But tonight under some listening stress the B+ fuse blew.

Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).

The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.

Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).

Will a certain chunk of NOS 807's simply be this way from age?

Tim.

[Pooh Bear]

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Graham

[s***@trailing-edge.com]

Post by Pooh Bear

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.

Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.

I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).

One thing I want to eliminate as a possibility is that something in my
homebrew amp did this tube in after just a few dozen hours of service:
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

Tim.

[Pooh Bear]

Post by s***@trailing-edge.com

Post by Pooh Bear

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.
Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.
I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).
One thing I want to eliminate as a possibility is that something in my
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

If it's 'friend' hasn't expired I can't really see how your amp can be at fault.

Graham

[Ken Scharf]

Post by s***@trailing-edge.com

Post by Pooh Bear

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.
Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.
I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).
One thing I want to eliminate as a possibility is that something in my
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.
Tim.

swap the tubes around and see if the problem moves with the tube or
stays in the socket. But I bet it's the tube. The getter only
absorbs oxygen. However other gasses can be outgassed from the
metals in the tube and cause problems perhaps.

[flipper]

Post by s***@trailing-edge.com

Post by Pooh Bear

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.
Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.
I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).
One thing I want to eliminate as a possibility is that something in my
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

I've got a 6EM7 that does the same thing and it's visual appearance is
perfect as well so I doubt it's your circuit.

Ironically, it turned out to be fortuitous in my case because I was
testing a circuit design intended to force PP current balance and that
tube was a great test subject. Dern thing works even though it would
run away on it's own.

Post by s***@trailing-edge.com
Tim.

[Pooh Bear]

Post by flipper

Post by s***@trailing-edge.com
One thing I want to eliminate as a possibility is that something in my
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

I've got a 6EM7 that does the same thing and it's visual appearance is
perfect as well so I doubt it's your circuit.
Ironically, it turned out to be fortuitous in my case because I was
testing a circuit design intended to force PP current balance and that
tube was a great test subject. Dern thing works even though it would
run away on it's own.

A servo presumably ?

Graham

[flipper]

On Sat, 17 Jun 2006 16:30:35 +0100, Pooh Bear

Post by Pooh Bear

Post by flipper

Post by s***@trailing-edge.com
One thing I want to eliminate as a possibility is that something in my
390V on the plate, 255V regulated on the screen, about -23V on the grid
to give 38mA idle current, all following the classical AB1 parameters
in the 40's vintage tube manuals. This is MUCH relaxed compared to
typical ham transmitter usage I've done with 807/6L6GC/6146's in the
past (which is 700 to 750V B+, 80 or 90mA plate, class C so substantial
grid current, etc.) but that was ICAS.

I've got a 6EM7 that does the same thing and it's visual appearance is
perfect as well so I doubt it's your circuit.
Ironically, it turned out to be fortuitous in my case because I was
testing a circuit design intended to force PP current balance and that
tube was a great test subject. Dern thing works even though it would
run away on it's own.

A servo presumably ?

Sort of. The version I was testing uses a current mirror. One tube
operates into an Rk, then the mirror, as normal with the second tube
operating into the second side of the mirror. In effect, the second
side of the mirror acts as a variable Rk seeking the value that will
cause the specified (by the first tube) current to flow.

Both bypassed, of course.

Fixed bias would use a conventional servo.

Post by Pooh Bear
Graham

[s***@trailing-edge.com]

Post by flipper
I've got a 6EM7 that does the same thing and it's visual appearance is
perfect as well so I doubt it's your circuit.

Come to think of it I had some nearly-new (back when I was a kid!)
6146B's go into grid-and-plate-current-runaway-at-idle like this 807
too.

Tim.

[Bret Ludwig]

Post by s***@trailing-edge.com

Post by flipper
I've got a 6EM7 that does the same thing and it's visual appearance is
perfect as well so I doubt it's your circuit.

Come to think of it I had some nearly-new (back when I was a kid!)
6146B's go into grid-and-plate-current-runaway-at-idle like this 807

I have seen it with many NOS power tubes in this power class. Doesn't
seem to be a problem with bright filament types but maybe it can be.
Fix is to throw out the offending tube.

If you can't get NOS tubes at Surplus hut prices, my advice is to buy
NEW tubes. From someone you can get a refund from.

[Doug Smith W9WI]

Post by s***@trailing-edge.com

Post by Pooh Bear

Post by s***@trailing-edge.com
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

What condition was the getter in ?

Dark metallic. Not whitish, not even on the edges.
Just warmed it up again, and looked for interelement shorts. No shorts,
just lots of grid and plate current when negative grid bias and B+ is
applied, it seems to "run away" in fact (after 5-10 seconds of bias/B+
the currents start taking off steeply). No oscillations/parasitics on
the scope.
I actually have another GE 807 that's been in my junk box for many
years (presumed "junk"), and it has a similar getter and similar excess
currents (actually worse).

semi-dumb question: where are you measuring the bias voltage? (directly
at the tube, or on the other side of the grid resistor?)

I'm wondering if a bad coupling cap is causing the bias at the tube to
drift towards 0v or even positive.

[s***@trailing-edge.com]

Post by Doug Smith W9WI
semi-dumb question: where are you measuring the bias voltage? (directly
at the tube, or on the other side of the grid resistor?)
I'm wondering if a bad coupling cap is causing the bias at the tube to
drift towards 0v or even positive.

Well, I WANT the bias to be circa -20V or -25V. There's a negative
supply and some pots which form a stiff voltage divider and supplies
grid bias through a 100K resistor.

On the "bad" tube grid current is so high that it does indeed drift up
to 0V in a minute or so. Yeah, I know, there's not supposed to be grid
current until the grid goes positive. I suspect this is the root of the
problem.

The coupling capacitors are indeed good and moving the tube around it
follows the tube.

Tim.

[Mike Silva]

You should also check to see if the coupling cap from the driving stage
is leaky.

[Tim Wescott]

Post by s***@trailing-edge.com
OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.
Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.
Being run with 400V on the plate and 255V on the screens.
Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.
But tonight under some listening stress the B+ fuse blew.
Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).
The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.
I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).
Will a certain chunk of NOS 807's simply be this way from age?
Tim.

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.

[JC]

Post by Tim Wescott
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

[s***@trailing-edge.com]

Post by Tim Wescott
I have very little direct tube experience, but from what I've read it
sounds like the grid is emitting.

This is certainly consistent with what I'm thinking.

Post by Tim Wescott
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?

At the same time Allison says that this is what gassy tubes do, too
:-).

The explanation from Allison that there was a small crack at a pin seal
is a bit more easy to use in terms of timing of my particular problem.

Post by Tim Wescott
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

That's where mine came from, and what inspired my question "are a
certain chunk of 45-year old 807's this way?".

Post by Tim Wescott
and 1625's are still a better buy at $5.50.

In fact I have plenty of 1625's too :-).

Tim.

[Patrick Turner]

Post by s***@trailing-edge.com
OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.
Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.
Being run with 400V on the plate and 255V on the screens.
Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.
But tonight under some listening stress the B+ fuse blew.
Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).
The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.

The violet - orange colouring only ever occurs as a tube dies from gas
intake. Normally
the tubes should have a slight "royal blue" low level flurescence usually
visible in darkness.
the bright blue flickering is the end for the tube; its only good for the
bin unless you
salvage the tube base plug for a multi wire plug.

After turn on with the output tubes in you should measure the voltage
across each bias resistor
from supply to grid pin. Usually the gassy tube draws grid current at idle
with the grid becoming
positive with respect to the bias supply.

Just what has caused the gasiness in anyone's guess but to test the
coupling caps remove the
output tubes and measure dc voltage across the biasing resistors to the
output tubes.
There should be less than a mV after leaving the amp on for an hour.
Using 807 from WW2 NOS could be dodgy since gas leaks could occur; not
enough to turn the getters white
or even slightly brown edged but enough to cause rapid failure well before
the usual 5,000 hrs you should get from a hi-fi output tube.
But then I have just repaired an ARC VT100 and out of 8 brand new Ei 6DJ8
there were two
which were quite faulty because one of the twin triodes in each tube became
gradually saturated
and fully turned on for no reason other than they had been poorly
manufactured in Yugoslavia.
Nice gold lettering on the outside, crummy work inside.

Post by s***@trailing-edge.com
I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).
Will a certain chunk of NOS 807's simply be this way from age?

Youse takes your chances with ancient 807.
They usually were renowned for their ruggedness but heck, anything ancient
must surely be less reliable than had it been made yesterday to the same
spec.

Would you fly in a NOS Mustang P51 that was NOS?

Not me, that's fo sure.

Maybe I got 20 x 807, I dunno their history, and I ain't got around to
using any; maybe I never will....

Patrick Turner.

Post by s***@trailing-edge.com
Tim.

[Gudmundur]

Post by s***@trailing-edge.com
OK, I'm trying to understand the failure of a 807 beam tetrode in a
homebrew push-pull audio amp.
Tubes are GE, about 45 years old, bought as new old stock several weeks
ago.
Being run with 400V on the plate and 255V on the screens.
Both behaved very nicely for a few weeks in my amp, and plate current
at idle followed the curves in the books just fine. They were
well-balanced.
But tonight under some listening stress the B+ fuse blew.
Turning the bias (supplied from a bias supply through a 100K resistor)
down to -40V, the good tube is drawing a small amount of plate current
while the other draws 40mA after warmup and current goes up and up and
up until it hits 100mA or so and I turn the amp off before blowing
another fuse. Check the grid current and as the bad tube warms up grid
current is increasing and increasing as well, to the point where the
-40V is being entirely dropped in the 100K resistor (I guess that means
about half a mA of grid current).
The tube with high current always had some violet-blue spots that
danced on the top but they were on the glass and mica so I figured that
was normal but now I don't know. There was also this strange orange
spot that was on a mica insulator there too.
Is this classic "gassy" tube failure? Towards the end it seemed to be
slightly more blueish on the top.
I've burnt up/blown up a lot of 6146 and other variant beam tetrodes
over the years but now I'm trying to get some understanding of what's
happening :-).
Will a certain chunk of NOS 807's simply be this way from age?
Tim.

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!!

[Arthur Dent]

Post by Gudmundur
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

[Gudmundur]

Post by Arthur Dent

Post by Gudmundur
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.

[Eeyore]

Post by Gudmundur

Post by Arthur Dent

Post by Gudmundur
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.

Fine but he already said he'd checked it and it's not that.

No matter.

Graham

[flipper]

Post by Gudmundur

Post by Arthur Dent

Post by Gudmundur
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."

[Gudmundur]

In article <***@4ax.com>, ***@fish.net
says...

Post by flipper

Post by Gudmundur

Post by Arthur Dent

Post by Gudmundur
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.

[s***@trailing-edge.com]

Post by Gudmundur
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.