Pooh Bear wrote:
wrote:

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.

wrote:

Pooh Bear wrote:
wrote:

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.

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

Graham

wrote:
Pooh Bear wrote:

wrote:


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.

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.

wrote:
Pooh Bear wrote:

wrote:


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.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

Doug Smith W9WI wrote:
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.