[HBR] HBR -- Part 11

[Kees & Sandy]

Oh.
Kees

-- "Peter Bertini" <radioconnection at gmail.com> wrote:
I was thinking that the guy who put together the website and the nice CD could
add it. .. I agree, great reading.

Pedro



On Sun, Jan 18, 2009 at 10:17 AM, Kees & Sandy <windy10605 at juno.com> wrote:
I would still like to see a complete collection of all "Walt's HBR Data"......very imformative for anyone into the larger tube radios.
73 Kees K5BCQ


-- Walt Hutchens <waltah at earthlink.net> wrote:
What a pleasure to see the list becoming a little more lively and
perhaps there are even going to be some more HBR projects.  And I'm
looking forward to being able to post progress more easily, now that
'The Bat!' is allowed as a mailer.

My 'transformerless' HBR variation went well up to the time I
discovered that it was about 40db short of sensitivity.  In the last
post I was thinking the problem is in the IF stages.

However when I measured the gain, stage-by-stage, I found that
the mixer stage had a conversion LOSS of about 20 db. That is, it
takes about 10 times the voltage at the mixer signal grid compared to
the mixer output to get the same receiver output. OOPS!

I've used this same 'push-push' mixer circuit before; typical gain is
~+6db. That is, 26db better than I'm getting here. I won't go through
all the things I've checked and changed -- tubes, IFTs, voltage
checks. No sign of a problem.

I have been using the triode halves of the two front end 19JN8s as a
mixer. (Pentode sections are RF amp and ECO, respectively.) Finally, I
laid out the triode section tube characteristics charts side by side
with those of some tubes I've used successfully. Ahha ...

Down close to the zero plate current line (where mixers have to go
to mix), the successful mixer triodes just cut off. The gain stays
relatively constant (lines of constant grid voltage remain nearly
equally spaced) until the tube cuts off sharply. But the triode halves
of the 19JN8 are effectively variable Mu: the grid voltage lines curve
and bunch together so that as cutoff approaches it takes a much larger
grid voltage swing to produce a given plate current change.

D'oh! (slaps forehead with base end of 9-pin tube) These tubes were
intended to be used as oscillator/pentode mixers in FM sets or in
applications in which the triode was operated with small signals and
well within its linear range. In a class 'C' oscillator, a variable mu
characteristic helps reduces harmonic generation.

And here I thought all triodes of similar Gm, voltage gain, and supply
voltages were pretty much alike ...

Needless to say there are VERY few choices of triode-pentodes with a
0.15 amp filament, some of them have common cathodes for the two
sections, and I'd bet that all of them were intended for
oscillator/mixer service. I will review this point today, but it looks
like I'm going to be rearranging things a bit. I think there's space
to add a separate mixer stage: I have used both the 12AT7 and 19J6 in
this application before.  The 'J6 data sheet specifically refers to
mixer service with plates in parallel.

This would require changing the two front end tubes to single unit
types (say 12BZ6 RF and 6BH6 oscillator) in order to pick up the
necessary filament volts. These are 7-pin types so socket changes
would be needed. I guess it's worth looking for a 9-pin pentode with
an 0.15 amp filament, but I don't have a good feeling about that --
9-pin pentodes came later and tend to be high gain tubes with higher
wattage filaments, not aimed at six-tube AM/FM radio applications.

It's not likely that I'll have enough gain even WITH the mixer change
but it'll be in the ballpark and there's still room for improvement in
the RF stage and probably in the IF's, too.

In other news, I did get the dial lighting installed: Four Radio Shark
12-volt 25 ma. bulbs glued in holes in aluminum angle stock supported
on the Eddystone 898 dial mounting screws and in series with a 3000
ohm resistor did the trick.

Walt
KJ4KV



************************************
Visit the HBR Receiver Web Site with over 100 pictures of receivers and 
construction notes...... via http://www.qsl.net/k5bcq/ 
there is also a mirror (faster response)at http://k5bcq.edebris.com/


Retrieve reflector archived data via http://mailman.qth.net/mailman/listinfo/hbr

************************************
Visit the HBR Receiver Web Site with over 100 pictures of receivers and
construction notes...... via http://www.qsl.net/k5bcq/
there is also a mirror (faster response)at http://k5bcq.edebris.com/


Retrieve reflector archived data via http://mailman.qth.net/mailman/listinfo/hbr



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/hbr/attachments/20090118/d4033fb6/attachment.html