[HBR] HBR - BFO progress

[B. Smith]

I would like to post these comments on the BFO page. I guess I use a lot 
of RF chokes, ferrite beads are also some of my favorite decoupling devices.
I've had a lot of fun with the receiver works great. Now working on home 
brew IF coils.
I  recommend the HBR project to anyone. But start off simple.
Bread board that BFO before building it.  :-)
k4che

On 1/5/2014 5:13 PM, Walt Hutchens wrote:
> Ron said:
>
>>   I see a problem with the suggested Colpitts circuit, that you
>> provided a link to. If I understand the labeling on the schematic
>> diagram correctly, there is a .01 mfd capacitor, on each end of the
>> RFC. The cap on the left side of the choke, is electrically in shunt
>> with the 180 pf cap, that comprises part of the oscillator tank
>> circuit. This will profoundly alter the feedback ratio, and the
>> resonant freq of the tank ckt.  In fact, the feedback ratio will be
>> altered to the point that I seriously doubt that the circuit will
>> oscillate at all.
> That's a problematic circuit either way. Without the 0.01 mfd cap it
> will oscillate at roughly the frequency set by the tank circuit but
> the choke is in shunt with the 180 mmf capacitor. You really don't
> want a choke in a VFO circuit: They are terribly unstable.
>
> I've never had any trouble as simple as getting the tap right for a
> Hartley. The turns ratio is the same as the proper one for the caps in
> a Colpitts circuit. If I remember correctly the command sets use a
> separate winding for feedback in that BFO; that's another way to do it
> and the turns ratio is the same as the ratio appropriate for a Hartley
> or Colpitts circuit.
>
> You can get rid of the choke in a Colpitts circuit by using a resistor
> instead and for the very highest Q (hi-Q helps with stability against
> tube gain variations due to structural and voltage fluctuations ...)
> tap the coil APPROXIMATELY at zero RF voltage, letting the capacitors
> set the exact feedback ratio and/or tuning range.
>
> As long as the voltage across the resistor is just a small fraction of
> the total the effect on circuit Q will be unimportant.
>
> Feedback ratios really aren't supercritical. The amplitude of
> oscillation in a class 'C' oscillator is set by the load imposed when
> the grid is driven positive over a fairly wide range of ratios. (The
> grid is then basically a shunt diode that eats the excess energy.)
> Only if there isn't enough feedback or it becomes so great that the
> tube goes into blocking oscillations will there be trouble.
>
> (Blocking: the grid capacitor charges so much during the positive
> swing that when the tank circuit swings the grid negative the tube
> cannot conduct again even on the next positive swing of the tank.
> Instead the capacitor must discharge through the grid leak resistor
> for several or many cycles of the RF before the tube again conducts.
> This is a good thing for a superregenerative detector, not so good for
> a BFO, LO, etc. Cure involves some combination of less feedback, lower
> grid leak resistor, and smaller grid coupling cap.)
>
> Well ... long leads and/or the latest UHF-capable tube types can get
> you VHF parasitics if you don't take precautions and these parasitics
> may occur over just a part of a cycle of the intended oscillation
> frequency. But once you've seen this you'll recognize it. Horrific
> crashing noise when you tune the oscillator is one symptom because
> there's very high VHF current in the capacitor bearings or contacts.
>
> Nearly all construction project articles have clear enough directions
> to get you past such issues. It's best to NOT substitute 'better'
> tubes than those specified. 6SK7 replaced by a 6BZ6? GOOD chance of
> VHF parasitics there, unless you use countermeasures.
>
> This is true throughout the RF-IF stages of a receiver. Also in such
> stages 'hotter' tubes routinely need a cross-socket shield to separate
> input from output.
>
> The latest tube types will require extra work compared to those of the
> 50's and if the project you're doing is from a 50's magazine article
> the how-to for dealing with the later tubes won't be in the article or
> circuit. This can be about the most challenging troubleshooting you'll
> see.
>
> All resistors in an oscillator should be the modern carbon film ones:
> They're more stable in every respect than the vintage ones. AVOID
> bypass-type disk ceramics in an oscillator, even for bypass jobs: They
> are NOT stable, indeed their capacitance may vary with the voltage
> across them as well as temperature.
>
> Mica bypasses work well in oscillators, even the larger 'postage
> stamp' micas.
>
> You're looking for a few parts per million stability, right? Receiver
> oscillators are precision instruments: You can breadboard them with
> 'whatever' parts but use only the best for the actual build.
>
> Walt Hutchens
> KJ4KV
>
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