[HBR] Mixer questions and other tech

[match at ece.utah.edu]

On 20 Aug 2008 at 4:01, Walt, KJ4KV wrote:

> The W6TC HBR designs yield a receiver that is simple enough
> mechanically and electronically that nearly anyone with reasonable
> proficiency with hand tools can make one work well. However this
> results in some limitations. This is a double conversion receiver with
> a tunable 1st oscillator, that is, an oscillator that must cover (at
> least on harmonics) the 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.
> 
> 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.

> Trying to make this work with
> conventional plug in coils would be a real challenge.  You COULD try
> doing it by building your own balanced plug-in coils like the ones
> used in transmitter push-pull stages. 

Trying to build a balanced LO sounds really fiddly to me. Why bother? I don't 
think you'd ever get it right.

> Maybe use miniductor and tube
> bases?

Or torroids?
 
> These receivers have an RF amplifier.  They don't need a high gain
> mixer.  If you reduce the oscillator drive to the point where the gain
> with a beam mixer is no more than needed, then you've got pretty much
> what you'd have with the original HBR design.

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.
 
> On the lower bands -- say through 20 meters -- the biggest issue in HF
> receiver front ends isn't gain or noise, anyway, but performance when
> there are multiple very strong signals within the passband of the RF
> tuned circuits.   If any stage is driven into non-linearity even by a
> signal you're not actually hearing (because it is eliminated by the
> IFs) the result will be distortion of the signal you're listening to.
> This is 'crossmodulation.'

See the receiver designed by Doug Blakeslee found in a 1972 QST. He 
chose a 7044 dual triode, G-G, with both sections in parrallel for 9 watts of 
plate dissipation.

---snipped stuff about using a 6EH7 in the RF stage---
 
> This is no harder to get working than the original HBR circuit,
> although the greater care you use in wiring the 6EH7 RF stage, the
> more of the potential gain you'll be able to use on 20 and 10 meters
> where high gain is sometimes useful.

Do you think you could get a 6EH7 to work as an RF amp in the HBR with 
plug-in coils? I don't. You can't make the wires short enough.
 
---more snippage---

> 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. 

Cascading the transformers at the 1st  IF strip is at least as important, and a 
sharp preselector is more important still.

> I wouldn't try putting a T-notch between IFTs since tuning the notch
> will change the tuning of the transformers as well. What about a
> select-o-ject tunable audio frequency circuit? Or go with conventional
> placement of the T-notch.

Agreed.
 
> > I had intended to use a pair or even 3 6EH7's in my IF strip, and running
> > cascaded pairs of these transformers at the first 2 coupling points. That 
> > would be 6 transformers total. But now I'm thinking that a strip with 3 or 4 
> > 6BJ6s would be easier to tame. I'll breadboard both and run some tests, I 
> > suppose.
> 
> The 6EH7 is a superior tube because it was designed for IF service at
> up to 40 Mcs in premium TV's where extremely large signals have to be
> handled without crossmodulation. It's really the only pentode that I'm
> aware of developed for wide-range signal handling ability at HF/low
> VHF. Two is plenty for an HBR IF section. Three 6BJ7's might be a bit
> simpler but short leads, careful orientation of the sockets, and good
> bypassing isn't rocket science, especially at 85 kcs. You DO need
> larger bypass caps than you might expect -- .05 mfd or more -- at
> these low frequencies.

Yeah, I keep going back and forth on the IF strip. Each 6EH7 will get me 
about 40 db of gain, and I only need about 120 total for the entire receiver. 
The thought behind using 3 6EH7s was to have plenty for the AGC to work 
with.

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