Zero bias

[vetmed]

Will an input transformer work in front of a tube with the tube's grid grounded? I am considering:    Input transformer- tube biased at 0 volts- IT- output tube-output transformer. The plate curves of the input tube look very promising at 0 grid volts. I suppose it will depend largely on the nature of the input transformer. Any input greatly appreciated

 

[Paul Joppa]

Grid current will be a problem, it's very nonlinear around zero bias. That's why tubes are biased.

Some data sheets have graphs showing grid current - usually as dotted lines on the regular plate curves.

 

[Paul Birkeland]

Will an input transformer work in front of a tube with the tube's grid grounded?

Typically for this kind of architecture you would use an interstage transformer before the zero biased tube so you can select the tube behind it to provide appropriate drive.

I am considering:    Input transformer- tube biased at 0 volts- IT- output tube-output transformer. The plate curves of the input tube look very promising at 0 grid volts. I suppose it will depend largely on the nature of the input transformer. Any input greatly appreciated

For a two stage single ended amp doing something like this, you'd need something like a 15K:150 transformer to properly attempt to do this, but grid current issues would likely make an input transformer unsuitable for this duty.  The large step-down ratio would probably cause gain issues as well.

 

[vetmed]

Thanks for responding, seems likely that when grid current does flow then core saturation will occur, which probably doesn't sound very good :