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

[hbr@mailman.qth.net]

In a message dated 1/12/2003 4:03:08 PM Eastern Standard Time, waltah@cfw.com writes:

> Jim, N2EY made many good comments:

tnx
 
> > > 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.

> > >  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?   

Yes! Check out:

QST for October, 1960 "Stability With Simplicity" by W2YM for the article that started it all. This design also appears in the first edition of "Understanding Amateur Radio"

QST for February, 1962 "An Easy-To-Build VFO" by W1ICP uses the W2YM circuit and a buffer-doubler to produce a very nice "crystal replacer" for typical GB MOPA rigs. 

The basic W2YM circuit uses a 12AU7A in a High-C Colpitts circuit. Very stable if good components are used. Original design covers 3.5-4 Mc., scale the L and C for other ranges. Two 6C4s may be substituted for the 12AU7A. Output is only a volt or so but a 6AU6A buffer will provide all the drive a GB rig needs. Uses very few hard-to-get parts, and subs are obvious.

Requires 75 and 180 volts regulated, which I obtain from a series string of VR tubes. 

Built myself a copy of the W1ICP VFO using the variable cap from a Command transmitter. Did such a nice job that I use the circuit in my homebrew CW transceiver, with the range shifted to 4.9-5.125 Mc. Quite stable after 15 minutes warmup.
> 
> > 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.

Very good!
 
> > >  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.   

Yup.

I used Command Set IF trans in my earlier CW transceiver designs (Southgate Type 5, based on the Heath 3395 filters. Found one at the 1978 Rochester Hamfest for $5 and proceeded to build a rig around it). Simple change of cap in the IFts let the 2830 units tune 3395. Left the old cap in the can so they could be converted back "someday". Yes, they're single-tuned, but we are not looking for max gain and the selectivity is determined by the filters anyway.
 

> 1% crossmod at 
> 0.5 VOLTS of signal for 20 volts bias ... wow ...
> 
Not really. Any remote-cutoff tube will handle big sigs with lotsa bias. Where the 6EH7 and other frame-grid tubes shine is how they do with big signals and low bias.

> 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.

I've got a cigar box full of mini VR tubes if ya need 'em. I use octals because I like to see the glow.
> 
> 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.

I never understood the need for S meters and AGC. But then, I'm from the W2LYH school of homebrew, anyway. (See ER for July, 1995)
> 
> > 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.  

Ugh. Keep the PS and it's heat, weight, mag fields and vibration separate. Also permits use of one PS with several projects. 

> 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.  

Agreed! Both are Yaesus anyway - T1 was actually the FT-200 in disguise. No provision for any filters other than the SSB one, either. Nice dial, tho.

Of course if somebody hands ya a junker T1 with "it's you or the dumpster", things change...
> 
> 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.

If you've got iron that can handle pushpull 6AQ5s, it can do a single 12BH7. Maybe even a pair of 12BH7s. 
> 
> > >  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.

May be hot, though.
> 
> > 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. 

I'd say each technology has its advantages and disadvantages. An Elecraft K2 has an excellent solidstate rx and uses less power than a 12BH7's heater doing it! But that's just one measure of performance. 

A big part of homebrewing today is to make something that cannot be bought, rather than imitation of what Ikensu has on the market. 

> And general coverage is 
> a whole 'nuther game -- a whole different set of design trade-offs, 
> which makes it interesting.

Yup. 
 
> But first, to finish this project.

And get mine started.

>    Thanks for the comments!
>
Like Durante, "I got a million of 'em!" 

73 de Jim, N2EY