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

Thu Jul 22 13:48:21 EDT 2010

Yep, I got one of those ARC-5 xmtrs with screen door wire over the louvers 
some time ago at a garage sale.  Used the parts to restore another 
restorable one.  Mike W6MAB

----- Original Message ----- 
From: "Mike Hanz" <aaf-radio-1 at aafradio.org>
To: "Discussion of AN/ARC-5 military radio equipment." 
<arc5 at mailman.qth.net>
Sent: Thursday, July 22, 2010 10:39 AM
Subject: Re: [ARC5] Reducing BC-459A chirp: 12A6 replacement for 1626

> 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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>>
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>
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