[HBR] (Fwd) Re: [HBR2K Explanatory Footnote

[hbr@mailman.qth.net]

I'm sorry for all the duplicate and other unintended posts lately.   I 
*think* I know how to avoid them in the future.   Part of the problem 
seems to be that mailman is blocking my new ISP (earthlink) so I'm 
switching back and forth and I only have a one track mind.

Jim, N2EY made many good comments:

> > Circuit:
> >  
> >  RF -- 6ES8 cathode coupled.  Will have AGC on both sections.
> 
> MGC too? 

Yes -- The stolen AGC circuit from the Tempo-One includes manual 
control of the AGC threshold by way of MGC.   Best circuit I've ever 
used.

> >  
> >  1st oscillator -- 12AT7, Pierce circuit using the tiny overtone 
> >  crystals from the FT-101.
> 
> You may need a genuine overtone circuit with LC tuned circuits in the output 
> to get the rocks to wiggle at the overtone. The circuit used in the 
> HW-100/101 and other Heath rigs works fine, and the second 12AT7 section can 
> be used as a cathode follower.

Yep -- This is a Pierce with a tuned circuit in the plate.   Although for 
3rd overtone you should only need an L that has low enough 
reactance at the fundamental to suppress that frequency, the fact 
that the circuit must operate with crystals from 7.52 to 35.52 Mcs 
forced Yaesu (and me) to something fancier.

> >  
> >  1st mixer -- 6ES8 Pullen circuit -- a cathode coupled design 
> >  with excellent crossmod characteristics.
> >  
> Also very low noise. So low, in fact, that you may not need an RF amplifier 
> at all. Just tie the two preselector circuits together in a double-tuned 
> arrangement.

In a simpler design I'd do it that way.   However the RF amp is there 
not just to lower the noise figure but to provide a place to do AGC 
and thus control what hits the mixers.  Both sections are controlled 
and I picture this stage as mostly operating as an attenuator. 

> >  2nd oscillator -- Vackar circuit, 6DZ4 triode -- one of the last 
> >  miniature tubes designed for UHF TV tuner oscillator service.  A 
> >  stable oscillator at 9 Mcs is not as hard as one for *900* Mcs.
> 
> I have found the W2YM circuit to be as stable as any hollow-state LC osc 
> around. 

Do you have a reference to this circuit?   

> You might consider some sort of buffer to prevent osc pulling. 

The Pullen mixer seems to be immune to the effect -- can tune in 
World Harvest radio zero beat quite easily (directly into the second 
mixer using my 160 meter full wave loop), with none of the stuff you 
get with oscillator pulling.   The tube operating point seems not to be 
much affected by signal strength and since it's very much like a 
cathode follower, there's little voltage across the osc injection side 
interelectrode capacitances.

I have had pretty good results with other cathode coupled triode 
mixers too, although as I recall there was some slight pulling.

But we will see -- a buffer might be needed with the higher signal 
levels with two more stages.

> >  1st and 2nd IFs -- 6EH7s  

> Great tube - I use them as RF amp and first IF amp in my homebrew Southgate 
> Type 7 transceiver. But even with their low Cgp they are beasties to tame due 
> to enormous gm of 12,500. You need multiple bypasses right at the socket (use 
> different values to break up the resonances) a flashing-copper shield across 
> the socket, and lotsa decoupling. One stage has almost enough gain all by 
> itself. Run the second IF at lower gain by increasing the size of the cathode 
> resistor and/or lowering screen volts. (You are using a screen voltage 
> divider on the 6EH7, aren't you?) btw, great article on RF and IF tubes in 
> QST about 1965 - title is something like "Improve Your Receiver With a Frame 
> Grid Tube" or some such.

I had essentially no trouble except for the initial sloppy grounding.   
The 1st IF now is basically swamped by the 1200 ohm grid resistor; I 
believe the second is so badly mismatched by working into 1/3 of the 
primary of the 3rd IFT that its gain is substantially reduced.   The 
thing is rock-stable now -- not a trace of anything -- no excess ripple 
on the filter passband, for example.   If as I suspect there's still more 
IF gain than needed, I'll do as you suggest and take it out with an 
increased cathode resistor on the second tube.   1% crossmod at 
0.5 VOLTS of signal for 20 volts bias ... wow ...

Of course with the plate detector, the signal level will never get 
anywhere close to half a volt.  

Yes, the screens of the 6EH7's are on a voltage divider.   And they're 
candidates for regulation, if at some point I wind up with the right 
extra tube sections or space available.   Heck, I could probably do it 
now ... I have a spare 1/2 12AT7 and 1/2 12AU7.   Could regulate the 
oscillators at the same time -- their drain is neglegible.

Would have to redesign the S-meter circuit since it depends in part 
on the rise of screen voltage with AGC.   I would really like to have a 
rock-stable S-Meter.  'Course in this set that would depend on 
building a rock-stable DC amp and people spent decades and 
millions on that project (in the analog computer days), with rather 
limited success.    Feedback loop I guess, meaning another tube.

I think I'll get the set working, before opening any of those cans of 
worms.

> btw, perhaps a junker Tempo One could make a good candidate for conversion to 
> a HBR2K. Nice dial mechanism. Pull out the circuit boards and put in aluminum 
> plates. 

Certainly an interesting idea.   The T1 has a separate power supply 
but there's probably room to add it on the chassis.    However the T1 
is a 'value' set -- everything done the cheapest way.   For example 
instead of slug tuned cup-and-core coils in the preselector you have 
paper coil forms and a broadcast variable.   The case is cheaper, and 
so on.  Would there be a significant difference in the result?   
Probably only if you do the measurements ... but considering the 
labor invested in any homebrew receiver and the fact that the two 
sets sell in the same price range as 'parts only' units, I think the FT-
101 is a better choice.  

Someone with a junk Tempo One lying around might well see it the 
other way.

> If you like nice audio, I've had good results using a 12BH7 as a push-pull 
> class A stage. All triode audio sounds nice. If your audio iron will stand 
> it. use two 12BH7s. 

Right -- I don't have that kind of iron.   But it is a great idea and will 
be filed for future use.

> >  Power supply -- a 35 VA 120-120 isolation transformer 
> >  feeding two half wave rectifiers supplying + and - 140 volts or so.  No 
> >  regulation.   Filaments are a Radio Shack 12V/3A transformer.   
> >  Another Radio Shack transformer is used as a filter choke.
> 
> Mounted in the box or external? 

All in the box.   It's not even excessively tight.
 
> Here's another idea: If you find another FT-101 junker, build a matching 
> transmitter...

I have been thinking about 'what's next' and that's certainly one of the 
candidates.   Other possiblities are a general coverage receiver and a 
transceiver.

Tubes have the greatest advantage over solid state in receiver design. 
(Because signal levels are controlled the only serious shortcoming of 
transistors in transmitter design is that they made cheap PLL sets 
possible, meaning phase noise.)   So for that reason I lean toward 
something at least incorporating a receiver.   And general coverage is 
a whole 'nuther game -- a whole different set of design trade-offs, 
which makes it interesting.

But first, to finish this project.    Thanks for the comments!

Walt Hutchens
KJ4KV