Re: Cathode Bias PushPull

In article <678jttF2mh8mjU1@[EMAIL PROTECTED]
>,
 "Phil Allison" <philallison@[EMAIL PROTECTED]
> wrote:

> "flipper"
> >
> > True, but I wonder how often that happens because a single Rk was
> > quite commonplace.
> 
> 
> ** Shame how many famous PP tube amps use a common grid bias voltage
feed.
> 
>   Same issue.

Phil, you blew it this time, "a common grid bias voltage feed" is not 
precisely the same thing as cathode bias using a common cathode 
resistor.  The difference, which is what leads to potential problems 
with a common cathode resistor, is that with "a common grid bias voltage 
feed" each tube of a PP pair operates independently as far as bias is 
concerned, with no bias interaction between the two tubes, while with a 
common cathode resistor there is an interaction between the bias current 
in the two tubes.  Consider what happens in a PP pair with a single 
common cathode bias resistor if one of the tubes weakens, for an extreme 
example lets say it dies completely, while the other retains its as new 
performance, the good tube will then be operating with what amounts to a 
cathode resistor of one half the value it should have, driving the 
effective bias voltage down and bias current up, potentially resulting 
in the dreaded "red plate" syndrome for that tube.  That does not happen 
with "a common grid bias voltage feed" in a fixed bias amplifier, unless 
maybe the B+ supply regulation is very very poor and the design is such 
that the tubes are normally operating on the edge.


Regards,

John Byrns

-- 
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