Nick Gorham wrote:

Don Pearce wrote:
On Mon, 11 Jun 2007 20:54:09 +0100, Ian Bell
wrote:


In some preamp designs I have seen the screen grid of a pentode decoupled
directly to ground and in others to the cathode (which is itself
decoupled). Is there any particular reason/advantage in doing it one way
or the other?

Cheers

Ian


It sort of makes sense to decouple it to the cathode, since it is
modulation of the screen grid to cathode voltage which will produce
audible effects (usually hum) in the output. Taking the screen grid
decoupler to ground means that it effectively runs in series with
whatever cathode decoupling you have, diluting its effect.

d


Yep, as Don said, all the valve knows about is the voltages and current
on its pins, it doesn't know about some external ground point. The only
slight problem with decoupling to the cathode, is that it does provide a
path for B+ noise and signal induced variation to be AC coupled into the
cathode. Not normally a problem, but it does happen. The cathode sees
the B+ signal via a PD of the screen driving resistor and the cathode
resistor. Also depends on if the cathode resistor is decoupled.



Thanks guys. The only other reason I could come up with might be that at HF
you could wire a decoupling cap from screen to cathode directly across the
valve base thus keeping lead inductance to a minimum.

Cheers

Ian