[HBR] Yet Another HBR Project -- Chapter 3

[hbr@mailman.qth.net]

In a message dated 11/16/03 2:53:52 PM Eastern Standard Time, 
waltah@earthlink.net writes:


> My idea of a mixer with one input push-pull and the other single 
> ended, output single ended, won't work.   A mixer can be thought of 
> as a circuit that multiplies the two inputs so (say) negative x negative 
> = positive.   Thus output of one mixer can be added (hooked in 
> parallel with) that of a second mixer with positive x positive inputs.  
> But negative x negative added to negative x positive -- the outputs 
> cancel.  (Jim, I think you tried to say this ...)  

Yeah, that's basically what I was getting at. But it's worth looking at all 
kinds of configurations - that's how the Tayloe mixer came about.

 Basically two of the 
> 
> three connections to a two-device (singly balanced) mixer have to be 
> push pull and the other must be single end.  I have to either to go 
> push-pull for one input and single ended for the other with output 
> push-pull, 

I think that's what G2DAF did

or use two push-pull inputs and single-ended output.
> 
> 
I'm not sure that will work. Here's why:

"Inputs in pushpull" really means "180 degrees apart". So you have Mixer Side 
1 and Mixer Side 2 operating identically except everything in Mixer Side 2 is 
180 degrees shifted (out-of-phase) from Mixer Side 1 and viceversa.

So their outputs are ideally exactly 180 degrees out of phase 
and...cancellation. 


> I only have one tuning cap section available to tune the output, and 
> the signal mixer I was planning (from the G2DAF Mk II) expects 
> single end input.   That favors a premixer that's push-pull on both 
> inputs, allowing single end output.   But I don't know about a Hahnel 
> circuit with push-pull output ... that would likely require more than 
> one envelope.
> 

See above.


> Alternative:  Go with the push-pull VFO, a single ended Hahnel input, 
> take the premixer output push-pull using a separate winding for 
> tuning.   More band switching that way.   Could either use single 
> ended output or push-pull.
> 

Mo' bettah.

> In between alternative -- if you ground one grid of a (say a dual triode) 
> mixer, float the cathode, and drive the other grid, you get much of the 
> effect of a push-pull input.   That is, all the phase relationships work --
> the only thing you don't get is full cancellation of the input in the 
> output circuit.  (The total cathode current has to vary at signal rate to 
> get opposite grid-cathode voltage changes, so the total plate current 
> must vary similarly.
> 
> Another interesting fact -- the 6BN6 gated beam tube has 
> outstanding linearity on either input.   I'm guessing they can't be 
> used as signal mixers because of high noise levels but they ought to 
> work fine as premixers with 100 mV or larger ignals from both 
> oscillators. 

I've got a big jar of them somewhere and I'd love to know how that works for 
you.

  And unlike the beam deflection tubes, the electrode 
> 
> voltages are reasonable.   So two 6BN6's with G3 driven push-pull 
> (the VFO) and one G1 driven single end (Hahnel) with the other G1 at 
> signal ground could have the plates in parallel.   The VFO gets the 
> full benefit of balanced mixer design, the Hahnel only gets some, but 
> those signals are always 5 Mcs on the high side of the desired 
> output frequency -- fundamental is relatively easy to get rid of and 
> harmonics are a non-issue.  And you have single ended output.
> 

Nice!


> There is outstanding info on 6BN6 performance in a product detector 
> in the article "Some Ideas in a Ham-Band Receiver," Arnold & Allen, 
> QST, May 1960, reprinted in "SSB for the Radio Amateur", 1965 
> copyright.
> 

I tried that detector years ago, and it works. But as the authors warn, it's 
microphonic. I went to a 6GX6 self-excited xtal BFO/detector (a la Drake)
for the Type 7


> Definitely time for the next round of headscratching!
> 
Paper is cheaper than aluminum.

One question on the pushpull 1U4 VFO: Will it have enough output to be 
useful?

73 de Jim, N2EY


--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment.  Attachments are not allowed.  To learn how
to post in Plain-Text go to: http://www.expita.com/nomime.html  ---