From: John Woodgate
Subject: Re: Need BIG inductances, virtual
Date: Wed, 25 Sep 2002 21:04:27 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 26 Sep 2002 03:51:15 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that George Gonzalez
wrote (in ) about 'Need BIG
inductances, virtual', on Wed, 25 Sep 2002:
>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.
The 8417 data sheet almost certainly gives a maximum value for
resistance in the control grid circuit, and I wouldn't be surprised if
it isn't 47 kohms.
>But I can't just lower the DC bias resistance, as the AC signal comes from
>the high-impedance plates of a 12AX7.
Well, not all that high; they are triodes, after all. ra under typical
conditions is around 65 kohms. What is the plate/anode load resistor?
>Lowering the bias resistance will
>lower the peak-to-peak driving voltage available, which may be marginal
>to begin with.
I'd be inclined to put a pair of cathode followers (12AU7?) between the
12AX7 and the output stage, have 47 kohm grid resistors and larger value
coupling capacitors to compensate.
4000 H chokes are possible but you would not like the cost.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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