Andre Jute wrote:
>
> On Apr 23, 2:43 pm, "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.
> >
> > >> 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
> >
> > > --
> > > Surf my web pages at, http://fmamradios.com/
> >
> > No John, I'm afraid you were the one that "sewered out" this time.
You
> > misunderstood Phil completely. What he was referrring to was a fixed
bias
> > setup with a single voltage feed for the grids of both O/P tubes. In
other
> > 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.
Nearly every guitar amp I have serviced
except Vox AC30 and their clones have cathodes taken to chassis
without even a 10 ohm Rk to each tube to measure cathode current easily.
I always instal them.
The bias is usually fixed with resistors. But on many
there is a "hum nulling" pot which slightly alters grid bias voltage
and hence anode current balance to
each output tube. Where the dc of each tube does not balance,
there is a hum voltage across the OPT, and hence it is heard.
There is usually poor B+ filtering on guitar amps.
But balance of Ia isn't critical and there is always some hum
and noise in guitar amps which enhances the performance
if anything, because there's nothing so boring
as a dull clean sound; there must be 20% of artifacts at least before
things sound
hot to trot.
Hi-fi amps are rather different, but where you have 2 output tubes, a
balance pot
is all you need once the right bias voltage has been established for
fixed bias.
But for high class A% PP amps, cathode bias is the best
and with individual R&C to each cathode.
Patrick Turner.
>
> Andre Jute
> Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
> "wonderfully well written and reasoned information
> for the tube audio constructor"
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> containing vital gems of wisdom"
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