Re: Brook sliding bias operation.
- From: Alex <apogosso@xxxxxxxxxxx>
- Date: Tue, 1 Apr 2008 17:08:22 -0700 (PDT)
I first tried zeners with a revised ST70 schematic shown at my website.
The trouble with zeners is that they do not react to current. They
react to voltage.
So if a tube short circuits itself, Ek is held down but Ik goes high.
In the ST70 protection circuit I developed, series R between bottom of
zeners
and 0V sends an error signal off to an SCR, and amp meltdown is avoided.
The transistor dynamic bypass stabilizers use very non critical setting
for the
transistor bypass. It only reacts to ac current generated in the low
value sensing R.
So Ek can vary quite a bit before triggering the protect circuit
But if a hard sudden drum beat occurs, the high AB current is nicely
bypassed
and Ek barely moves at all, but with a zener, a considerable rise in Ek
has to occur
before it conducts at all.
Yes, that makes sense. The transistor circuit reacts to the
instantaneous current. The threshold can be higher than a quescient
current, while with the zeners it shall be right on the mark. With the
zeners you do not need 1000uF||750R resistors at all -- it will work
as fixed bias, which is, if I understand correctly, critical to tubes
replacement and B+ variation.
Regards,
Alex
.
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