Re: Cathode Bias PushPull

On Thu, 24 Apr 2008 00:47:20 +1000, "Phil Allison"
<philallison@[EMAIL PROTECTED]
> 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.

It isn't 'unpredictable ****'.

If it would happen from the 'normal' manufacturing tolerance spread
then it could be predicted from the manufacturing distribution. If
it's a failure mechanism then it could be predicted from the failure
rate.


>>>  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.

That's fine, other than "symmetrical  PP operation" not being the
issue that was under discussion.

The issue, and what I asked "how often it happens" about, was one tube
hogging current and going red plate.

If it happens due to a tube fault, or end of life, and the amp is
robust enough to gracefully survive the incident then I don't see that
as necessarily a problem since it can't be expected to operate anyway.

If, on the other hand, it happens with 'normal' tolerances then it
would be a problem.

>Trying to fix this fact of life re using power tubes in PP  with
individual 
>cathode bias is a   TOTAL  WANKER'S   FANTASY.

No one proposed 'fixing' "symmetrical  PP operation" with individual
(fixed) Rks.


>The Turneroid Public Menace is one Colossal Wanker.
>
>As are you.
>
>
>
>
>......   Phil
>
>
>
>