On Feb 2, 7:05*pm, Graham wrote:
On Feb 2, 6:29*pm, "Antonio Vernucci" wrote:





I cannot give you actual values for your set up, but basically, if you
have already provided optimum bias with your fixed bias voltage, then
you will not need to develop any more bias across a grid resistor.

peter

My fixed bias is set for an idling plate current of 10 mA. The only criterion
behind that bias setting was to keep the final tube safe. So, it may not be
optimum with regard to the final stage efficiency.

On the contrary, it is surely not optimum because, without extra bias, the tube
operates class B. More bias is needed (produced by the grid resistor) to have
the tube operating class C.

73

Tony I0JX

With a fixed bias supply *, do *you *really *need a *grid *resistor as
such ? how is the *-Ve *bias reaching the *grid ? ....with the
cathode earthed .. the fixed bias is *'required' *.. I would think for
a *given drive *level ... increasing the *-Ve *grid bias would *reduce
the *conduction angle and slide the *stage *from class b to *c *as
the *peak drive level *would need *to *overcome the *bias to allow
the *grid to *conduct .. thus the *conduction angle is reduced ?

G ..- Hide quoted text -

- Show quoted text -

The original data has guide lines on the vlaves use in class C
telephony .....

http://www.r-type.org/pdfs/829b.pdf

G ..

With a fixed bias supply , do you really need a grid resistor as
such ?

Yes, because the fixed bias I have chosen causes the tube to operate in class B,
whilst I wish it to operate in class C.

how is the -Ve bias reaching the grid ? ....with the
cathode earthed .. the fixed bias is 'required' .. I would think for
a given drive level ... increasing the -Ve grid bias would reduce
the conduction angle and slide the stage from class b to c as
the peak drive level would need to overcome the bias to allow
the grid to conduct .. thus the conduction angle is reduced ?

G ..- Hide quoted text -

- Show quoted text -

I agree with your statements but they do not help me much with regard to my
original doubts

The original data has guide lines on the vlaves use in class C
telephony .....

http://www.r-type.org/pdfs/829b.pdf

I have several data sheets for the 828B of various manufacturers but with the
grid resistor value they specify I obtain less output power than with a lower
resistance value

73

Tony I0JX

G ..