Reply-To: "George Gonzalez"
From: "George Gonzalez"
Subject: Need BIG inductances, virtual
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
NNTP-Posting-Date: Wed, 25 Sep 2002 13:56:26 GMT
Organization: AT&T Broadband
Date: Wed, 25 Sep 2002 13:56:26 GMT
I have this little challenge-- I need a pair of, say, 1000 Henry inductors!
Background: I'm fixing up this nice old Bogen power amplifier.
It used the last-gasp very latest vacuum tubes, the 8417's.
These have tremendous transconductance, 23ma/V.
Which is good, and bad. Good, as all it takes is a wimpy 12AX7 to drive
Bad as any bit of gas in the tube can lead it into thermal runaway.
As seems to have happened to this amp, as both tubes are melted.
I'd like to supply the grid bias from a "stiff" source, right now the bias
comes thru a pair of 470K ohm resistors, which may be a bit too
flexible if my new tubes (very expensive too) are or become a bit gassy.
But I can't just lower the DC bias resistance, as the AC signal comes from
the high-impedance plates of a 12AX7. Lowering the bias resistance will
lower the peak-to-peak driving voltage available, which may be marginal
to begin with.
Sooo, one solution would be to feed the DC from a source that's got low DC
resistance, but high AC. An inductor would do the trick, but to get
the required high AC impedance (say .5 meg) at 20Hz would require a HUGE
around 4000 Henrys.
And oh yes, it would be nice if that inductor didnt self-resonate at
anything below 30KC or so.
So a real inductor is kinda ridiculous-- anybody have a simple
That can operate off +300 V?, and swing 30v p-p?
(Ach, note to self: a better idea-- try putting an emitter-follower between
the high-impedance source and the lower-impedance grids).