Was there ever a tube/valve available with an insulated grid?

It would be easy to coat each of the grids in its own glass coating. I just wonder if any one ever tried to make one. It would prevent grid current; make for a very high impedance grid input, even more like the gate in a FET, except a lot more linear. And without that DC bias current running through the grids, they'd have less noise.

I'll call it an IGT or IGV.

Maybe I'm a few decades late with this idea!

Bob

What would it be used in?

$10,000 hi-fi amplifiers, along with their $1,000 power cables, for their $1,000,000 owners.

The grids are isolated.

There is not a glass coating but there is a vacuum, which is a better isolator than glass

Originally Posted by ecerfoglio

The grids are isolated.

There is not a glass coating but there is a vacuum, which is a better isolator than glass

Not in this case. The grid intercepts some of the electrons going through the vacuum from cathode to plate which causes a small grid current. Thus his suggestion to add a insulating coating to stop this current.

Originally Posted by crutschow

Not in this case. The grid intercepts some of the electrons going through the vacuum from cathode to plate which causes a small grid current. Thus his suggestion to add a insulating coating to stop this current.

Those electrons will be still going across the insulating coating.

To stop them you will need to screen the grid with a grounded piece of metal. But then the grid will not control the tube anymore

Originally Posted by ecerfoglio

Those electrons will be still going across the insulating coating.

The may be going across it but they won't be going through it to the grid.

Wouldn't isolating the grid from the interrior of the tube defeat the grid entirely?

Originally Posted by kchriste

Wouldn't isolating the grid from the interrior of the tube defeat the grid entirely?

Insulating, not isolating. The grid still has its field effect. No grid current would flow, so the tube would not self-bias.

Ecerfoglio: Yes, tubes contain a vacuum, but current flows through the vacuum from cathode to plate and this current flow is controlled by the field effect (voltage) of the grid(s). After looking at your responses, I can see that you need education about tube theory so that you may opine without embarrassment. Here, read and learn:

Sylvania tube manuals ONLINE!

Crutschow: You understand. Do you think that any static charge on the glass grid inulator surface would have any effect (like the charge on a glass rod in a high school science lab)?

Bob