{Collins} Re: 75A-4 modification

[k2qde@xxxxxxxxxxxxx]

Yes. Ham Radio Magazine.
It was an 'A-4 mod about replacing the mixer. Find the issue and I'll see what I can do.
Stu, k2qde

----- Original Message -----
From: Richard Ferguson 
Date: Monday, July 16, 2007 10:57 am
Subject: {Collins} 75A-4 modification
To: Collins Reflector 

> I have a 75A-4 (serial # 2344) with a modified first mixer. 
> The original 6BA7 has been replaced with a 12AX7 dual triode. 
> In this triode, the cathodes are tied together and the RF input 
> is fed to pin 2 which is the control grid of the second triode. 
> I think the triodes are supposed to be identical. The common 
> cathode is tied to ground through a 560 ohm resistor which is 
> bypassed with a 120 pf capacitor. The common cathode is also 
> connected to the grid of the first triode (pin #7) through a 
> 100K resistor. The output from the crystal oscillator is fed to 
> this grid. The output from the mixer stage is taken from the 
> plate of the first triode (pin #6). 
> 
> My vacuum tube theory is weak but it looks like they are taking 
> the RF off the top of the 560 ohm cathode resistor and mixing it 
> with the crystal oscillator in triode #1.
> 
> The circuit seems to work but I question the 120 pf cathode 
> bypass capacitor. It looks to me like there would be a 
> considerable roll off in system performance as the RF frwquency 
> increases from the 160 meter band up to the 10 meter band.
> 
> As a matter of fact, performanc is poor above 15 meters. I 
> don't know why it isn't more obvious before that but maybe there 
> is enough ALC action early on and the ALC tops out at 15 meters.
> 
> Has anyone seen this modification before? Any comments are 
> desired. Should I rip it out and put the pentagrid convertor back?
> 
> Rich NN7D 
> .
> SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
> http://www.collinsradio.org/html/membership.html -
> paypal accepted-
> .
> NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 
> Pacific SUNDAYS: 14.263 MHz-2000 UTC + AM Net: 29.050 MHz-
> 1200 Central
> 1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)
> 


.
SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
      http://www.collinsradio.org/html/membership.html    -paypal accepted-
.
 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
  SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
       1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)