[HBR] Mixer questions and other tech

[Peter Bertini]

On Wed, Aug 20, 2008 at 11:24 AM,  <match at ece.utah.edu> wrote:
> On 20 Aug 2008 at 4:01, Walt, KJ4KV wrote:
>
>> he entire tuning range of the receiver.
>
> These discussions are the most fun I've had on the 'Net in years. Keep those
> ideas coming.
>
> While I love the discussion about tube rolling, readers should note that the
> receivers we're talking about only loosely resemble the HBRs. The HBRs
> have 2 "magic" features, as far as I'm concerned. Firstly, they are completely
> repeatable. If you follow the design, it's hard to screw up. Secondly, is the
> rather "elegant" front-end. Both these features disapear in the reciever
> design we're discussing here.
>>

I think we'd all agree that if Ted had the time and inclination to do so, the
HBR series would have continued to evolve into what is being discussed here.
It isn't too far of a stretch.. so far...  :)  Certainly many of the
tubes and parts
we're discussing were available by the early 60s.  I'm still pretty intimidated
by having to wind the coils, and getting the tracking and temp. compensation
right. That could be a challenge for me!

>> That means that the 1st mixer almost has to be singled ended, since a
>> balanced oscillator output over this wide range is a very difficult
>> job.  On 10, for example, stray capacitances will greatly affect the
>> amplitude and phase of the mixer drive.
>
> The LO can still be single-ended, just follow it up with a long-tailed pair buffer
> stage. That gets you the balanced output but allows you to use the
> conventional single-ended oscillators that we all know and love, and you
> stand a chance of developing enough drive for the deflection plates of the
> mixer to boot.
>
The problem is trying to drive a balanced mixer properly with
the second harmonic. That's what kind of shot down my plans to use the 6JH8
in my receiver.

> On the other hand, if you use a high-gain beam mixer, you can eliminate the
> RF amp stage, or swap it for a low-gain G-G RF stage with barely enough
> gain to cover the losses in the preselector.

That might involve so changes in the coupling to keep from affecting the loaded
Q and subsequent image rejection?
>
>> 85 kc R-23 IFTs can be cascaded for better selectivity and shape
>> factor. If you're only going to cascade one pair, yes, put them in the
>> second mixer plate circuit, since that protects the rest of the IF
>> chain from out-of-passband signals that can cause crossmodulation.
>
> This statement is more important than it would at first appear. You want to
> have the available reciever selectivity as close to the antenna as possible,
> which means doubling up IF transformers at the front of the IF strip.

It implies using a  simple half lattice filter at the first IF, the
selectivity belongs
immediately after the first mixer. Again, a Heath HR-10 parts set is
worth pursuing...
I wish there was a source for surplus filters xtal in the this range.
I still see the
first IF  as being the weak link in the HBR design.
>
> Cascading the transformers at the 1st  IF strip is at least as important, and a
> sharp preselector is more important still.
>

The Preselector BW  is limited by arithmetic selectivity.   It may be
extremely good
on 160 meters, but not much good on 28 mc.
>
> Now then, we had been discussing how to couple cascaded IF transformers
> together and someone mentioned that a tube works really well. DUH! But I
> sure don't want 4 6EH7s! They'd be just dying to go into oscillation. Other
> candidates in order of highest gain to lowest: 6BZ6, 6BA6, 6BJ6. If you're
> going to use 4 stages, 6BJ6 will be the most well behaved, still provide more
> than enough gain, and they use half the heater current than the others. 4
> 6BJ6s need the same heater current as 2 6EH7s.
>
> Marvin
> KA7TPH

Heater current is rapidly becoming a consideration, at least for me. Besides,
I have a sleeve or two of the 6BJ6s on hand, and I am inclined to
stick with them
Now, for discussion, why did Ted add a first IF amplifier at 1600kc in later
versions?  I don't see the need for the gain at that point? Unless he was trying
to provide a means to achieve gain distribution?

Pete

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>
>
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>
>