[ARC5] Reducing BC-459A chirp: 12A6 replacement for 1626

[J. Forster]

On the TVI thing, IMO it was more a lack of understanding of the set
design than anything.

Most vehicular sets (plane and ground vehicle) were designed to work with
electrically short (capacitive) antennas, hence the roll-a-ductor or
variometer. When run into a long wire or beam or 50 Ohm system, the output
was not resaonated. Hence the TVI got out.

-John

=============





> Hi Henry,
>
> I cannot believe that filament voltage rises due to RF pickup other than
> the
> meter used getting fooled due to the high RF flying around.  Now having
> said
> that, byapssing of all filament lines with additional caps was commonplace
> to reduce TVI.  Hams did this at the same time they rewired the filament
> circuitry for 12V.  I found that my T-22 (after a minimal reversible mod
> to
> the output tank) did not generate excessive harmonics at the output and
> I discovered that the cabinet offered sufficient shielding also.  This is
> in
> contrast to all the articles written in the past about how to deal with
> TVI
> in ARC-5's by adding all kinds of shielding and tank conversion to PI,
> etc.
> The major contributor to the TVI problem turned out to be leakage of
> harmonic energy via the key and power lines.  So I filtered all those
> lines,
> did a final RF sniffer test and all was OK.
>
> Regarding chirp reduction, I didn't understand exactly what WN7RKY was
> talking about from the brief excerpts you provided.  Which tubes were
> involved?  Sounds like a novice ham did a little experimenting and found a
> way in his radio to reduce the chirp by playing with the screen feed.  My
> own tests and experiments led me to conclude there are basically two
> approaches to addressing the chirp.   Either you regulate all supplies
> (starting with the PA screens) or you delay keying the PA until after the
> OSC settles.  The latter can be accomplished either by keying both and
> then
> using a cap to delay the PA keying relay or you can just run the osc
> continuously during transmit periods.  I actually do all the above :-)
>
> BTW, your tube shipment is all boxed up securely and now I need to get
> some
> custom declaration paperwork from the post office.
>
> Dennis AE6C
> On Thu, Jul 22, 2010 at 5:02 AM, Henry Mei'l's <meils at get2net.dk> wrote:
>
>> Found this in on page 64 of Hints & Kinks, Vol. 5 (1955)(WN7RKY). Screen
>> grid voltage derives (from osc. anode voltage?) via a 20k series
>> resistor,
>> bypassed witrh a 0.001uF cap. This is supposed to reduce chirp.
>>
>> (This mod is almost invisible mod and shouldn't cause too much
>> consternation among the purists among us -- and it's easy to retro ;0)
>>
>> Any one tried the this tube switch ?
>>
>> (BTW: On page 40, there is also mention of RF causing 1625 filament
>> voltages to rise to 18-22 volts, cured by grounding to a copper strip
>> running across the chassis and feeding filament voltage via RF-lossy
>> microphone cable with bypass caps at both ends. This is probably old
>> hat;
>> just thought I'd mention it, anyway.)
>>
>> 73
>>
>> Henry OZ1UF, Cph.
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