[Boatanchors] Homebrew Receiver AGC Circuit Question

[whitebear1122 at comcast.net]

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