Re: Cathode Bias PushPull

On Apr 23, 2:43=A0pm, "Doug Bannard" <ve3...@[EMAIL PROTECTED]
> wrote:
> "John Byrns" <byr...@[EMAIL PROTECTED]
> wrote in message
>
> news:byrnsj-89B769.08132023042008@[EMAIL PROTECTED]
>
>
>
> > In article <678jttF2mh8m...@[EMAIL PROTECTED]
>,
> > "Phil Allison" <philalli...@[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.
>
> >> =A0 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. =A0The 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. =A0Consider 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. =A0That does not
happ=
en
> > 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
>
> > --
> > Surf my web pages at, =A0http://fmamradios.com/
>
> No John, I'm afraid you were the one that "sewered out" this time.
=A0You
> misunderstood Phil completely. =A0What he was referrring to was a fixed
bi=
as
> setup with a single voltage feed for the grids of both O/P tubes. =A0In
ot=
her
> words, a single bias adjust pot with no provision to DC balance the two
> tubes.
>
> 'Twas obvious to me anyway.
>
> Doug Bannard

What's obvious to me is that PP amp without provision for DC balance
of the tubes is a barbarism. It's what comes of discussing guitar
amps. It is this sort of loose thinking that makes noise and the
necessity for NFB. More rigorous thought is required.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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