Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Need BIG inductances, virtual
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Date: Wed, 25 Sep 2002 15:51:23 +0100
NNTP-Posting-Date: Wed, 25 Sep 2002 15:51:30 BST
"George Gonzalez" wrote in message
> I have this little challenge-- I need a pair of, say, 1000 Henry
> 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
> 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
> 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
> But I can't just lower the DC bias resistance, as the AC signal comes
> the high-impedance plates of a 12AX7. Lowering the bias resistance
> lower the peak-to-peak driving voltage available, which may be
> to begin with.
> Sooo, one solution would be to feed the DC from a source that's got
> 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
> 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
> pseudo-inductor circuit?
> That can operate off +300 V?, and swing 30v p-p?
Use an RC filter!
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