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

[Dennis Monticelli]

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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