Grid Leak

[trem]

10-4
Thank You

[Steve Conner]

One thing that wasn't covered:

Grid current can actually flow in either direction. RG explained negative grid current that tends to turn the tube off. However, there is also positive grid current. This is caused by positive ions hitting the grid, by the grid getting contaminated with emissive material (so it emits electrons) or by leakage across the insulators from the plate or screen grid.

Positive grid current tends to turn the tube further on. It is the reason why old power tubes die by red plating. As the tube ages it gets gassier, the insulators get leakier, and the grid gets more contaminated and emissive, it starts to turn itself on, biasing itself hotter. The extra heat releases more gas and causes more grid emission, thus more positive grid current, which causes the tube to turn itself on more and get hotter still, a vicious circle called "thermal runaway" that ends with a fuse blowing, the tube glass melting, or the OT primary burning out.

It is this risk of runaway through positive grid current that determines the maximum grid leak resistor for power tubes. Most classic amp designs violate it by a factor of 2 at least, so tubes probably don't give as long a service life as they ought to in these designs. The Music Man cathode-driven output stage can soak up huge amounts of grid current, and I've heard reports of extremely long tube life from it.

But preamp tubes can have grid current in either direction. It seems to vary at random between tubes, and also over the life of a single tube. The cathode follower in a Marshall pulls a fair amount of negative grid current because of its low plate-to-cathode voltage.

[trem]

Christ...
I should have seen this coming.
"Emisssive Material".....
Gonna have to look that up first.
You guys are really testing my intellectual horsepower with this "stuff".
Will just have to keep up the best I can.

[Steve Conner]

By emissive material I meant the white stuff they put on the tube cathode at the factory, to make it give off electrons easier. It is a vital part of the tube, but if any gets on the control grid, you're going to have a really bad day.

[trem]

OK Steve -
I figured out about E.M.
Just did not know where it was coming from. Now I do.
I certainly have heard the term "Thermal Runaway"
but this really puts it into perspective.

I am always amazed by the complexity of "simple" 1940's Audio Technology.
You guys continue to be a double edged sword of information.
This Forum is kind of like the classic money pit.
Every time I get One Question answered..... THAT answer raises five other questions.
Coming here has been a virtual Heroin Addiction of an electronics education.
God Bless

[jamesmafyew]

The new hit single from Emissive Material, "White Stuff All Over My Control Grid"

Seriously though, helpful thread. Just posting to subscribe.

[Tom Phillips]

...Just posting to subscribe.

Matt,
FYI. At the top of any thread you can select Thread Tools / Subscribe to this thread and you will get notices just as if you posted.
Cool feature.
Regards,
Tom

[Enzo]

Tube technology was already very sophisticated and mature in the 1940s. Televison was made in the 1930s after all, and during the war we had radar and other things like that in avionics. Fancy things like spread spectrum were things they used.

[trem]

Tube technology was already very sophisticated and mature in the 1940s. Televison was made in the 1930s after all, and during the war we had radar and other things like that in avionics. Fancy things like spread spectrum were things they used.

As normal, you are right on all accounts.
WWII was a boon to (probably) all technologies.
I see now that not only do I lack knowledge of theory, but also the practical Physics of tube technology. (perhaps they are the same thing)
At any rate.....
I do not pretend to fully understand what is discussed on the forum, but I find it fascinating none the less.
Thanks Enzo