Re: Looking for 12V tube Rx project info

From: Ken Scharf on Sun 26 Jun 2005 22:41

>LenAnderson@xxxxxxxx wrote:
>> From: Ken Scharf on Jun 25, 6:26 pm
>>>LenAnder...@xxxxxxxx wrote:
>>
>>>>>I am looking for a nice schematic, and perhaps additional info, on a 12V
>>>>>(fil. & plate) tube superhet Rx; just something fairly simple for CW/SSB
>>>>>reception without bells and whistles, and preferably for 75M/40M reception.
>>>
>>>> Setchell-Carlson managed to do it with loctal-base tubes running
>>>> off of a 24/28 VDC aircraft bus back in WW2 times. No dynamotor,
>>>> just that cute little box referred to as a "range receiver" or
>>>> BC-1206.

>> Converting a "Q5-er" from tubes to FETs would be rather easy.
>> No real need for the 2N2222s. Would have gobs of space left over
>> inside an already compact box.
>
>This is the idea of a cascade fet-bipolar circuit. The drain of the fet
>drives the emitter of the bipolar and the bipolar's base is biased so
>that the base sits about half way up the power supply (two resistors).
>The fet is self biased with a source resistor for the desired current.
>The gain of this circuit is higher than a fet by itself, and the output
>impedance will be higher.
> ____ __
> ->|_ \_/
> | |
>
>> If the 135 KHz IF has an equivalent total Q of 100, the BW would
>> be about 1.3 KHz, not all that swift for CW and too shart for SSB.
>>
> The AnArc-5 Q5'ers had 6 tuned circuits in their If but the '1206
>only has 3 or 4. So it's BW might be a tad wider. The 1206 also has
>no bfo.

The ARC-5 LF version has IF TRANSFORMERS but the insides are
easy to see for detail. The 6 to 9 MHz version IFs (2830 KHz)
had only one tuned circuit in each IF can.

Can't remember what the BC-1206 used in their IF cans, but that
is easy to change inside the present can (it will look just the
same on the outside).

I can't see any advantage of going cascode (JFET-bipolar) when an
insulated gate FET would closely approximate a vacuum tube for
both input impedance (all capacitive) and drain/output impedance
(many hundreds of KOhms). Depends on what is available for the
vacuum-to-solid conversion.

Note: Both input and output impedances will affect the impedance
and Q of the tuned circuits...and the GAINS...for a conversion
with minimum circuit changes. All that has to be taken into
account.

>> However, the BC-1206 tuning range of 195 to 500 KHz results in
>> an image at the converter input of 390 to 1000 KHz away from the
>> desired band. That's worse than the 910 KHz image of an old
>> 455 KHz IF.
>>
>The 1206's dial only goes up to 400khz, but it tunes past that. Maybe
>it goes to 420 or 450khz, I don't think it goes as high as 500khz.
>(so maybe I don't have to expand the range for my needs).

The variable tuning capacitor max:min ratio is roughly 11:1 (no
external parallel capacity). Since the resonant tuning range is
the square root of that, you could have it tune 190 to 570 KHz
with a 9:1 max:min change in resonance capacity. The only real
problem is getting the variable LO tuning to track the front end
since it would tune 325 to 705 KHz (for the high-side) and that
would be a 2.169:1 frequency ratio or 4.706 capacity change ratio.


>> The linear-in-capacity rotation of the 3-gang variable is good
>> for an RX Noise Bridge having an expanded parallel-C range...
>> which is what I used them for. :-)
>>
>True, but the rig is just to cute to canabilze.

Heh heh heh. I agree with you there. It probably is most dense
of any other 1940s era design using 8-pin "medium" size tubes,
including the very compact ARC-5 receivers. Not quite as dense
as the SCR-300 Walkie-Talkie (BC-1000) which stuffed 18 7-pin
"miniature" tubes, a half-dozen coil/transformer cans, and a
five-gang (!) variable capacitor into an approximate 5" x 10"
chassis. [Galvin/Motorola did real good in packaging there]

KEEPING the tubes as-is, with the possible exception of the AF
out (28D7 ?) would yield a straight-AM receiver. ADDING a BFO
via under-chassis bipolar transistor would result in minimum
cannibalization (one extra switch on front panel)...RF and IF
could remain the same. I've forgotten the audio output circuit
of it but recall that AF output power for old aircraft was
rather high (ambient noise in old warbirds was terrible). In a
quiet land environment, the audio output needed is rarely 200
milliWatts into a speaker, much less for headphones. The SCR-300
walkie-talkie (headphones only) had a maximum of 3 mW audio out!

Hint on supply conversion: A 12.6 V "filament" transformer can
do the job for all-12 V filaments in parallel. A voltage
doubling rectifier (even a tripler if need be) will yield the
quite-low B+ for plate and screen. I once did that (long time
ago) with a single transformer and voltage tripler for B+, all
working into a set of power resistor load boxes, all in a half
hour of tack-soldering. Got 30 VDC for B+ at 115 VAC line,
less than 5% ripple. Didn't have the BC-1206 then. A critic
looking on said "that isn't a good enough test...gotta have the
'real' load on it to 'prove' it." :-) [some folks just aren't
convinced until someone else has made a finished production
product!]

LenAnderson@xxxxxxxx

.

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