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

[Mike Hanz]

Antenna matching was certainly one of the major factors for TVI.  Also 
remember that hams never did appreciate a two tube 807-ish transmitter 
that only put out 15 watts RF. :-P   They pushed those poor little sets 
to extremes with far higher voltages and currents than they were ever 
designed to take, either from a power standpoint or a heat dissipation 
perspective.  Then they wondered how come why for they put out so much 
TVI and the tubes never lasted very long.  Of course, at 50 cents a 
tube, who cared at the time?  I have a couple here that were modified 
with screen wire over all the louver slots and other unmentionable 
indignities, all because the modifier couldn't keep the tubes from going 
into all sorts of nonlinear perturbations at the power levels he was 
demanding.  But, so what, they would say...a NIB T-15/ARC-5 is only 
$5.95, and after I hack it up for 160 and use it for a while, I can just 
toss it into the parts bin.  Lots more where that came from... :'(

  - Mike

On 7/22/2010 1:15 PM, J. Forster wrote:
> 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 resonated. 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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