SB100,101 6hs6 sub and capacitors to correct "Miller" effect

[Stu Lyon]

Hi Ed,
The Miller Effect is the effective capacitor formed by the feedback of the
signal on the plate through Cgp to the grid. It shows up like a capacitor
from the grid to ground and is expressed as Cgp x voltage gain (which is
another long equation). Since the 6CL6 and 6HS6 are connected in parallel
but operate separately (one or the other), the Miller capacitor and
therefore the tuning is going to differant between rx & tx. Apparently that
differance is 3.3 pf.  [Cgp=grid1-to-plate cap.]

Since to Miller capacitor is a function of gain, it will vary with AGC
voltage thus tuning (detuning?) the circuit as the signal level changes.
Using the strong crystal calibrator as a signal source for tuning may not
give optimum weak signal performance (!!).

In the 70's, QST had a H&K for a circuit (that I have installed in my
SB-101) that uses a variable capacitor instead of the fixed 3.3 pf.
Otherwise it operates the same as the Heathkit circuit. I agree with your
circuit analysis.

73, Stu W6CUX
Winnetka, CA
ARRL Life Member

----- Original Message -----
From: "Ed Richards" <k6uuz at JUNO.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Friday, September 15, 2000 6:59 AM
Subject: Re: SB100,101 6hs6 sub and capacitors to correct "Miller" effect


> I just finished analyzing the circuit in question in a HW-101 (no
> manual). please tell me if my analysis of the circuit is correct, if not
> where did I go wrong?
>
> In receive, I see +150 VDC being applied through a 100k resistor to the
> diode anode, causing it to conduct and place the end of the 3.3 pf cap to
> ground, placing it in parallel with the small section of the driver grid
> tuning capacitor, increasing the total capacity by 3.3 pf.
>
> In transmit, the +150 VDC is removed, allowing the -150 VDC bias to be
> applied to the anode of the diode through a 1 megohm resistor, reverse
> biasing it, effectively removing the 3.3 pf cap from the circuit.
>
> What is the "Miller effect", and why does it occur? Why do you need 3.3
> pf more capacity in receive? My education has obviously been
> short-changed. Any response from you engineers out there will be
> appreciated. Thank you.
>
> Ed Richards

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