[Boatanchors] Collins 75A-1 Mod info?

[Glen Zook]

Replacing a 6SK7 with a 6SG7 was very common in the "goode olde dayes".  The 6SG7 is a semiremote-cutoff pentode whereas the 6SK7 is a remote-cutoff pentode.  The transconductance of the 6SG7 is twice that of the 6SK7 being 4000 instead of 2000.  Basically, no modifications were usually made to the circuit when replacing the tube.

The 6AK5 and 6BZ6 have the same base configuration except for the fact that the cathode pin and suppressor-grid/internal shield pin are connected differently.  In the 6AK5 the cathode and suppressor are connected to both pins 2 and 7.  In the 6BZ6 the cathode is connected only to pin 2 and the suppressor  is connected to pin7.  However, in the 75A-1 these pins are connected together at the tube socket so either tube will work fine without any modification.  The 6AK5 has a transconductance of 5000 whereas the 6BZ6 has a transconductance of 8000.  Therefore, the 6BZ6 was probably used for the additional gain.  It has been my experience that the 6BZ6 has a little less noise than the 6AK5 and the 6DC6 has even a lower noise factor.  However, the 6DC6 has the same transconductance as the 6AK5.

The 6AH6 has the same base connections as the 6BZ6 and, like the 6BZ6, has a higher transconductance than the 6AK5.  However, it is even more being 9000 rather than 8000 like the 6BZ6.  Again, probably the 6AH6 was added for additional gain.  However, I don't see any real advantage except for maybe a little more injection on 11 and 10 meters.
  
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Roger Shultz <nj2r at verizon.net>
To: boatanchors at mailman.qth.net 
Sent: Thursday, February 9, 2012 9:16 AM
Subject: [Boatanchors] Collins 75A-1 Mod info?
 
I'm working on the restoration of a reasonably good condx 75A-1 that has a
couple mods which I have yet to find any documentation about.

The front end has been changed to a 6BZ6, the mixer is a 7360 and the HF
oscillator is now a 6AH6. The first RF amp has a 6SG7 in place of the 6SK7
though I don't see any socket changes there.

The second mod is the addition of a fast/slow AVC switch neatly mounted to
the right of the Phone jack.

I'm inclined to leave well enough along though the s-meter does not seem to
work as it should however I have yet to check out anything about why that
may be.

I've seen mods for the 7360 mixer for the 75A-4 and suspect the front end
6BZ6 which is close to the 6DC6 may be a copy of that in the 75A-4. I've
searched Electric Radio, on line, Collins archives etc but have found
nothing close so far. Has anyone seen this before?

The really good thing about this radio is that is has the 353C31 mechanical
filter which is not very common as I recall.

Thanks for any info you may have.

73, Roger, NJ2R

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