[HBR] Re: HBR2K Explanatory Footnote

[hbr@mailman.qth.net]

In a message dated 1/11/03 3:07:21 PM Eastern Standard Time, 
waltah@earthlink.net writes:

(snip of absolutely excellent description of design philosophy to save space).

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

MGC too? 
>  
>  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.
>  
>  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.

>  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. 

You might consider some sort of buffer to prevent osc pulling. 
>  
>  2nd mixer -- same as 1st.
>  
>  Cathode follower (driver for crystal filters) -- triode section of 6KE8
>  
>  Crystal calibrator -- pentode section of 6KE8
>  (circuit to be stolen from the FR-100B -- surely not a popular theft 
source, 
> 
>  these days ...)
>  
>  Filters -- the three regular FT-101 filters for SSB, AM, CW.  Will be 
>  diode switched as in FT-101
>  
>  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.
>  
>  Carrier oscillator -- 1/2 12AU7, Pierce circuit, crystals are hot 
>  switched.
>  
>  Detector -- 1/2 12AX7, plate detector; for SSB the carrier is injected 
>  on the cathode.   
>  
>  AGC -- 1/2 12AX7, plate detector.   Cathode returns to -140V, plate 
>  is 0 -> negative to suppy AGC.   
>  
>  The detector, AGC, and RF gain control circuit is stolen nearly intact 
>  from the Tempo One, AKA FT-200.   

Interesting!

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. 
>  
>  1st audio/paraphase amp -- 12AX7
>  
>  Audio output -- 2x 6AQ5 in p-p

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. 
>  
>  Audio is conventional; feedback is used from the speaker winding to 
>  the cathode of the 1st audio stage.

Nice.
>  
>  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? 
>  
>  Construction is basically "Obtain an FT-101 and strip to a bare 
>  chassis."   Then replace the top plate of the chassis with 0.050 AL, 
>  drilled and punched for the hollow state design.   I'm recycling the 
>  front end coils and tuning mechanism, bandswitch, all crystals, the 
>  VFO box, capacitor, and dial assembly, and many, many small parts.

I've done the build-from-scratch thing many times, and the metalwork and 
partsscrounging is the toughest art of the project. Which reminds me, I gotta 
dig up some photos to send to 'BCQ.

Here's another idea: If you find another FT-101 junker, build a matching 
transmitter...

73 es GL de Jim, N2EY