What's a 6AS7GYB?

[tvr2500m]

I've looked at the datasheet for this tube. It states and otherwise looks like it's a direct 6AS7 sub, but what does this mean:

“The 6AS7-GYB is a low-mu twin triode primarily designed for service as a series-regulator tube in d-c power supplies. The tube differs from the 6AS7-GA in having controlled zero-bias plate current and a low-loss phenolic base.”

The part I'm curious about is the "controlled zero-bias plate current". What does that mean? Is it direct 6AS7/6080 sub?

Thanks!

[Paul Birkeland]

Zero bias means that the grid and cathode are at the same DC potential.  If a grid and cathode are close together in DC potential, then you have most of the recipe for the grid to draw current (referred to as grid current).  If you have a tightly wound grid (which comes with a high transconductance tube), then the grid current will be greater than a grid with greater spacing.  The 6080/6AS7 have a very tightly wound grid, so it's certainly not a great candidate to be zero biased.

When you draw grid current, that will change the bias voltage of the tube, with more grid current creating a grid that's more positive, and therefore causing more plate current to be drawn. 

There are metallurgical treatments that can be made to the grid to mitigate this issue, and perhaps that's what's been done to the GYB. 

[tvr2500m]

Thank you for the explanation. The last time I experienced grid current draw was when I watched 6550 plates lighting up like light bulbs with the tubes then taking themselves out (and hopefully nothing else) in thermal runaway 

Is the 6AS7GYB a direct sub for a 6AS7 or 6080? Can it be used in the Crack? Should it be used in the Cracky?! Sounds like a bit of hand grenade.

[Paul Birkeland]

Yes, if you plug it in, it will work as expected.