[HBR] detector /agc

[donald haworth]

Thanks Walt,
I tryed several variations of the infinate Z detector using different
tubes.  I was looking for low distortion on weak signals, as well as the
ability to handle large signals.  All circuits worked -some better than
others-but I finally found a circuit that out preforms everything.  It is a
'precision rectifier' using a LM318 with a cathode follower audio output.
No tubes, I know, but the audio quality is outstanding, as well as the
dynamic range.  I found the circuit by Googleing 'low distortion am detector
/ agc".  It offers a very high Z to the last IF, no loading!  And the agc
outputs -30 volts with 4 volts input to the detector circuit.
-Should be enough for cutoff in any tube IF amp.  BTW my IF strip is using
6EH7's and a Collins machical filter at the input.
-Thanks again Walt!
73's
Don in Nashville

On Thu, Nov 11, 2010 at 5:27 AM, Walt Hutchens <waltah at earthlink.net> wrote:

> Don posted:
> > -New to the list!  I am looking for any thoughts on the 'best' infinate Z
> > detector.....I am looking at the 12at7 as used in the hbr13c....I plan to
> > use a 6cw4.  Trying to get the cathode bias just right for proper
> > cutt-off.....I am runing about 104v on the plate....any thoughts?
>
> Did this truly get NO answer?  Or am I missing some posts?
>
> The infinite impedance detector isn't a critical circuit. As I recall
> the Handbook circuits show parts values and either give a list of
> various tubes that will work or don't mention the tube type at all.
>
> Basically when using a new tube type in this circuit you can just wire
> it up with whatever cathode resistor value is recommended for the
> other tubes and check to be sure it works okay.  The voltage gain is
> nearly one for all such circuits and about the only other problem you
> could get into is noise and an unstable operating point if you got the
> cathode current way too low.  The optimum is very broad.
>
> That's one of the great advantages of inf. impedance: There's no trick
> to getting it to work. (Standard W6TC design philosophy!) I tend to use
> plate detectors but tinkering is generally needed to get good
> linearity with those.
>
> I don't like to use tubes that will oscillate at 1000 Mcs (like the
> 6CW4) in MF/HF/audio circuits because I always suspect them of
> secretly wanting to oscillate regardless of my intent. (I've had
> enough trouble with 12AT7s!) However, if the plate RF ground is short
> (and to the same point as the cathode bypass), it probably will be
> okay.
>
> Walt
> KJ4KV
>
>
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