[HBR] HBR -- Part 11

[Peter Bertini]

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
>
>
>
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