[HBR] Re: HBR -- Part 3

Sun Oct 12 13:36:25 EDT 2008

This all reminds me of Walt's receiver and transmitter that he built and 
wrote about in the pages
of the early issues of Electric Radio.  Well thought out - well written - 
and well built.  It is sure good
to see you writing and building again, Walt.
Dave w9ocm

----- Original Message ----- 
From: "Peter Bertini" <radioconnection at gmail.com>
To: "HBR Receiver List" <hbr at mailman.qth.net>
Sent: Sunday, October 12, 2008 1:17 PM
Subject: Re: Re[2]: [HBR] Re: HBR -- Part 3

> Are those green sockets Western Electrics?  Your layout looks very
> tidy. We're all awaiting more updates!
> Peter
>
>
> On Sun, Oct 12, 2008 at 6:09 AM, Walt Hutchens <waltah at earthlink.net> 
> wrote:
>> Heathkit-terminology steps since last time:
>>
>> Second mounting of parts -- output transformer, volume control, filter
>> choke, ON/OFF switch, speaker, and misc.
>> First wiring -- filaments and plate power supply
>> Second wiring -- 1st & 2nd Audio stages
>>
>> Only made one mistake wiring the filaments this time! Pins 4 & 5 on 7
>> tube sockets; how, you wonder, could I get that wrong? Well ... I
>> wonder the same thing.
>>
>> The audio section worked right away, though I had the interstage
>> coupling cap wrong, so had a bit too much hum. With that fixed, it is
>> silent at low to mid volume settings, only some hum at full volume.
>>
>> The full volume hum comes from capacitive coupling between the chassis
>> (which is at power line ground voltage) and the neutral power wire
>> which is B- and the audio signal ground for all circuits.  This
>> voltage is of the order of one volt RMS and usually 'trashy.'  With a
>> 750k volume control, just a few mmf of stray capacitance is enough to
>> produce an audible signal.
>>
>> The audio detector has a much lower (than 750k) plate resistance; when
>> it's wired it will lower the impedance at this point, eliminating the
>> hum.
>>
>> The audio stages have a high level of negative feedback and thus
>> relatively little gain -- I think a voltage gain of 3 from the volume
>> control to the voice coil.  But 100 mW of audio for a 4 ohm speaker is
>> about 0.65 volts RMS so with voltage gain 3, volume control set for
>> 10% of max voltage gain, the detector output will only need to be
>> a bit over 2 volts and the 2nd IF output, in the milivolt range.
>>
>> Sharp cutoff tubes are fine in an IF operating at that low level.  But
>> a sharp-cutoff tube in an RF stage is not ideal for ham band service.
>> Does anyone remember how unhappy Hallicrafters was when some people
>> substituted 6AK5's for the RF tube in one of their receivers?  Maybe
>> the HQ-129X?
>>
>> This is not that bad, because the AGC voltages will be correct for the
>> tubes I'm using, but better front-end performance would result from
>> using 6EH7's in all three sockets. Then you'd have an 11 tube receiver
>> that might possibly be done as a 300 mA series string set ... a
>> different design!
>>
>> 'High fi' audio with frequency response shaped for communications
>> purposes makes signals a lot clearer and takes just a couple more
>> parts. Use of 1/2 of a 12AU7 for the audio output stage requires a
>> high efficiency speaker: the modern midget extended low frequency
>> response speakers used for computer and car audio will not deliver
>> enough volume.  Speakers from taxi and CB sets are fine.
>>
>> Most of the circuitry of this set is copied from the earlier band
>> imaging set with six tubes, push-pull oscillator and mixer, 80/40
>> meters.  So I THINK all the problems with 'transformerless' power
>> supply have been resolved.  Yeah, that's what I think ...
>>
>> I plan to wire the audio and AGC plate detector circuits next. These
>> are respectively the triode halves of the 19JN8 1st audio and 2nd IF
>> stages.
>>
>> Following that, I'll wire the local oscillator, so I can start working
>> on the oscillator coil while wiring the other stages.
>>
>> The Eddystone dial that I have is factory-calibrated for six ham
>> bands; I think it came as part of a kit of parts for the G3LOK
>> receiver design. (Think: HBR with bandswitching, using itty-bitty
>> coils ... NOT a great design!) And I'm using the tuning cap that came
>> with.
>>
>> This dial is a nice (silk screen?) printing job on painted metal and
>> the scales are very close to linear so it makes sense to try to track
>> the oscillator to the dial, rather than recalibrating.  However this
>> sort of thing requires a bit of cut-and-try ... probably a week's
>> work, on and off, to get a decent result.
>>
>> This will require a three-point fit. Typically the three parts to be
>> adjusted would be the coil inductance, the shunt (trimmer) capacitor,
>> and a capacitor in series with the tuning cap, called the padder
>> capacitor. These would adjust the low and high ends and the midband
>> point, respectively. The W6TC designs use a tap on the coil rather
>> than a series padder cap, probably because tapping the coil is cheaper
>> than another high quality capacitor. However, moving a tap to get
>> midpoint dial tracking right is tough.
>>
>> On the other hand, changing a capacitor down inside a coil form with
>> an APC trimmer in the top may not be simpler ... I'll probably try
>> using the tap, first.  I bought 10' of PVC for $2.39 plus tax, and I
>> have a bunch of rusty octal tubes ...
>>
>> Just one more thought: that capacitor tracked that dial with the coils
>> that are in that (busted up) front end assembly. Maybe my first effort
>> should be to duplicate the in-set coils and copy the oscillator
>> circuit parts. Hummm ...
>>
>> The tube lineup has turned out to be surprisingly tidy:
>>
>> 19JN8 -- RF & 1/2 mixer
>> 19JN8 -- ECO LO & 1/2 mixer
>>
>> This looks messy, but the RF drives one half of the mixer and the LO
>> drives the other, thus each pentode is coupled to the triode in the
>> same envelope.  The two triode plates and cathodes are joined; the
>> plates go to the 1st IFT.
>>
>> 19JN8 -- 1st IF & AGC clamp (triode is diode-connected)
>> 19JN8 -- 2nd IF & AGC plate detector
>> 19JN8 -- 1st audio & audio plate detector
>> 12AU7 -- 2nd audio & 3.5 Mcs marker oscillator
>> 12AT7 -- BFO & bias rectifier (2nd triode is diode-connected)
>>
>> 117Z6 -- Plate supply rectifier
>>
>> There's also a 150 mA 9-pin miniature with a triode and two diodes
>> with independent cathodes that could be substituted for the 12AT7 if
>> another diode is needed. The 14GT8 was designed for use in combination
>> ratio-detector/1st audio service in AC/DC AM/FM receivers.
>>
>> Another report in another few days ...
>>
>> 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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