[Boatanchors] Homebrew Receiver AGC Circuit Question

[Jacques Fortin]

Hi Scott, 
I do not have the 1969 RAH, but I looked in the 1968 one.
There is a five-tube receiver in the 1968 edition (The Junior "Miser's
Dream") that use a similar audio-derived AGC, with a 12AX7 again.
And the W5OMX receiver in the 5th edition (1970) of the "Single Sideband for
the radio amateur" ARRL manual uses the same AGC system (and 12AX7 again).
Wondering if the 6GG7 can be a transcription error in the '69 RAH ??
Please send me the '69 schematic if you can...

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : whitebear1122 at comcast.net <whitebear1122 at comcast.net> 
Envoyé : 23 février 2023 14:56
À : 'Jacques Fortin' <jacques.f at videotron.ca>; 'Boatanchors List'
<boatanchors at mailman.qth.net>
Objet : RE: [Boatanchors] Homebrew Receiver AGC Circuit Question

Hi Jacques, Oh that is intriguing.  I didn't see that before.  The 1967
version uses the 12AX7 like you pointed out, while the 1969 version uses the
6CG7.  While I knew there was a couple of trivial differences between the
two, I didn't pick up on the 12AX7 vs 6CG7.  So obviously I photocopied the
69 version for my build...  Ok, yes that large mu difference is something to
take into account.  I wonder why W1DX used the 12AX7 in the 67 version but
the 6CG7 in the 69 version.   I haven't started wiring the BFO yet so it
would be easy for me to change the 6CG7 wired socket for the 12AX7.  Thanks
Jacques for pointing that out!
73, Scott WA9WFA


-----Original Message-----
From: Jacques Fortin <jacques.f at videotron.ca>
Sent: Thursday, February 23, 2023 12:13 PM
To: whitebear1122 at comcast.net; 'Boatanchors List'
<boatanchors at mailman.qth.net>
Subject: RE: [Boatanchors] Homebrew Receiver AGC Circuit Question

Hello Scott,

I am just intrigued by what you wrote, mentioning that the AGC tube of your
HB-67 is a 6CG7...
A 6CG7 is basically a low µ (~20) tube, a 9 pin version of the 6SN7.
The tube specified for the HB-67 is (from my 1967 ARRL handbook) a 12AX7
which have a µ of 100...
And the other section of this 12AX7 is used in the BFO oscillator (driving
the product demodulator) as well.
If you used a 6CG7 instead of the 12AX7, I understand why the receiver AGC
system is not working...

Please clarify.

73, Jacques, VE2JFE in Montreal