From: Dave Shrader
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Subject: Re: NEED ADVICE: Vacuum tube grid current?
NNTP-Posting-Date: Tue, 07 Jan 2003 22:38:22 GMT
Organization: AT&T Broadband
Date: Tue, 07 Jan 2003 22:38:24 GMT
It's been a long long time <1957> since my vacuum tube days but let me
meander a bit. Grid Leak established the dc operating point for a Class
A amplifier. The grid leak resistor has a distributed capacitor, leakage
and circuit stray capacitance, to chassis and to the plate circuit. In
the tube there are a series of interelectrode capacitors. The Suppressor
grid is generally grounded to chassis to minimize electron backscatter
and to break up the Cp to Cg. The screen grid is heavily bypassed.
Regeneration is achieved by introducing feedback, variable capacitor or
adjusting tube gain, from the plate to the grid with a phase angle
between 145 and 225 degrees and a feedback factor very close to but less
than unity at the operating frequency.
So, I would begin my model by using a dependent voltage source, % of
tube output, with a variable gain, setting loop gain, [forward gain of
the amplifier times the feedback factor] [ratio of capacitors, plate to
grid leakage], to slightly less than 1, and have a phase shift of 180
degrees nominally with a variance of +/- 45 degrees.
Test the model until the circuit just breaks in oscillation then back
off on the feedback a percent or two.
Let me know how it works out.
Deacon Dave, W1MCE
> Hi there.
> I'm doing this project for school that involves a complete mathematical
> analysis of this receiver that I built. The detector (it's a
> regenerative design) uses a grid-leak method on a 6GH8A pentode.
> Let me hasten to say that I'm not asking you for all that! I've got it
> almost done (I hope) except I need a model for the control grid current.
> It's a function of Vg and Vp, I beleive. And no, I'm not using SPICE- I
> find the tailoring of all those fitting parameters more confusing than
> applying Kirchoff ad finitum to the circuit.
> Thank you very much for your time.
> Physics Undergraduate
> Arizona State Pentitenary (oops, I meant to say University)