Re: Cathode Bias PushPull

On Wed, 23 Apr 2008 17:28:24 +0100, Eeyore
<rabbitsfriendsandrelations@[EMAIL PROTECTED]
> wrote:

>
>
>John Byrns wrote:
>
>> Eeyore <rabbitsfriendsandrelations@[EMAIL PROTECTED]
> wrote:
>> > John Byrns wrote:
>> > >  "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.
>> >
>> > I didn't see where he said it was.
>>
>> Phil's comment "Same issue" implies that he meant they were the same.
>
>I'd call it a similar issue myself. Semantics really.

Phil is funny. He snips the context out and then chides people for
'not reading' the context.

The issue that Casey raised, and I asked about, was one tube
(excessively) hogging current, due to the interaction of a common Rk,
and going red plate.

That's not the 'same issue', or simply 'semantics', as less than ideal
operational imbalance since, in the later case, the amp is operational
but in the other, presumably, not.

In the case of the common Rk, the common Rk aggravates (or causes) the
problem because bias is derived from the current sum of the two but
there is no similar interaction with a "common grid bias voltage feed"
and, ironically, the separate Rks Phil derided is more 'similar' to
the "common grid bias voltage feed" he commented on than the common Rk
he claimed 'same issue' to. 'Similar' in the sense that, unlike a
common Rk, with separate Rks each tube will idle as it would singly
without being affected by the other.


>Graham