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shiva
 
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"John Byrns" wrote in message
...
In article t, robert
casey wrote:

Is this the bias rectifier only? As there is an adjustment pot,
the silicon diode giving you more voltage doesn't much matter
as the pot would be readjusted anyway. Higher negative bias
voltage will reduce quiescent current thru the tubes anyway,
so the danger is minimal if you didn't readjust.


It will increase the heater voltage somewhat, which IIRC Scott set at
about 10.0 volts per 12.6 volt tube heater.


Regards,

John Byrns


Hi John -
I can't remember who did the test, or how good the methodology was, but i
remember seeing lowest noise ratings for small signal toobs happening just
around 80% of recommended heater voltage - makes one wonder if all them
7005 regulators are a *good & useful thing*... (i was real happy when I
found out that 06 existed & were cheaper than dirt - now all those 'puter
PS's have one more part to donate!). It makes some sense, though...
OTOH, i'm amazed that no one said: Hey, them dc bal. pots are neat & all,
but how's about using a *bias pot!!!!*. The reason that bias was left
non -udjustable in most (fairly) high-quality consumer gear is, I think,
the sound notion that "if there's a pot, they'll spin it". That, and
(maybe?) higher consistency of toobs "back in the days", and a justification
for "use only Genuine replacement parts".
But there's no reason I can see to rely on a fixed resistor & keeping your
fingers crossed, instead of stickin' in a bias pot & a couple of 1-ohm
resistors at the cathodes of the OT tubes. Otherwise, the DC bal. pot
becomes ... another hum - buckin' pot?
-dim