Re: Cathode Bias PushPull

Phil Allison wrote:
> 
> "flipper" ...
> "Phil Allison"
> >>
> >>> 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.
> >
> > Well, ok... so? I don't see how that helps answer "how often it
> > happens."
> 
> ** How often  * unpredictable *****  happens is a complete nonsense
> question.
> 
> >>  Same issue.
> >
> > Depends on which issue you mean but with common fixed bias they don't
> > interact with each other like on a common Rk.
> 
> ** In both cases, the only way to get symmetrical  PP operation is to
use
> tubes that are reasonably closely matched  -  ie same make, same batch
etc.
> 
> They can then be expected to stay reasonably matched until they reach
their
> end of life  -  or if one develops a fault then all bets are off re PP
> operation,  no matter what.
> 
> Trying to fix this fact of life re using power tubes in PP  with
individual
> cathode bias is a   TOTAL  WANKER'S   FANTASY.
> 
> The Turneroid Public Menace is one Colossal Wanker.
> 
> As are you.
> 
> ......   Phil

Phil, you have not changed.

You dished out the same BS last time the same subject was discussed just
as fully.

Matched tubes don't stay matched, and they don't necessarily drift apart
at the end of their life.

The fact is that while they may have become unmatched, ie, require
a 10% or more difference in applied grid bias to produce the same Ia,
they may not be at the end of their useful life, and owners want to 
use them until they have gone considerably further from being matched.

Individual bias is the way to go where class A PO is 50% at least of the
maximum PO.

But where class A% is say well under 50% like so many hi-fi and guitar
amps,
fixed biasing is best.

Where you may have a pair of KT66 in a Quad-II amp, with one 
drawing 40mA and the other drawing 90mA with a common Rk,
then this is a real problem. But changing the biasing so you have 
470 ohms and 470uF to each cathode will result in a MUCH lower
difference between Ia of the two tubes, and the sound is far better.

See my website for more details.

Patrick Turner.