[HBR] HR-67 / balanced 7360

[Helmut Usbeck]

On Friday 12 April 2002 22:15, John Finigan wrote:
> > The 7360 module I have is setup as a real balanced
> > mixer and is a bear to keep balanced.  Using this
> > tube single ended which most designers do seems to 
> >be defeating the purpose of using it.
>
>
> I think that Squiers-Sanders used it this way, but
> I've never heard of someone doing it in a homebrew
> design.  I'm very curious as to what sort of coils,
> etc it takes??
>


John--

I'm not sure Squires-Sanders used the 7360 as a balanced mixer in there 
receivers.  I know that one of the two wrote up a paper describing it as a 
balanced front-end though.  The circuit I use I picked up from an old RCA 
application sheet on the tube and it's basically the same layout.  Some of 
the component values are different.  Also the RCA sheet had 2 methods of 
getting balanced input and output going. 

-Basically it fed the input signal to the grid.  
-The oscillator output comes from a center-tapped pot of about 500K feeding   
  the deflection plates. (which is a balance control)
-The output from the plates feeds a center tapped IF transformer or in my 
case I used 2 matched caps across the IF coil to simulate a center tap.  The 
IF end which is normally grounded goes to the second plate.  A split stator 
cap was used by RCA and used as a second balance control.  I used two matched 
caps because small value split-stator caps don't grow on trees.  
-A schematic is less scarier  looking than trying to describe the circuit.
-Your coils are pretty much the same as a single ended mixer, unless you 
decide to go for the center-tapped version.  

Running in balanced mode, was pretty hairy.  First the balance drifts out 
rapidly as the tube warmed up and then slowly drifts in and out.  It was 
quite microphonic, perhaps there was some regeneration in the circuit.  Its 
recommended to use a shield with the tube to prevent magnetic fields from 
effecting balance, but the shield itself would cause problems.  Also the only 
sure fire way of adjusting it was to use an oscilloscope.  I got pretty fed 
up with it.  Never had it going on the air long enough to be able to tell if 
it's really worthwhile using.  It sits in a corner now waiting to played with 
again sometime...and I think that's why one never saw a real balanced mixer 
in a commercial ham receiver either. Until someone had the idea of using 
diodes.

Egads, another dessertation!

Regards,
Helm.  WB2ADT