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

Thu Jul 22 14:45:48 EDT 2010

I didn't mention it, but the novice who wrote the blurb got the idea from a 
W7KOG.  Also, wasn't a novice supposed to be running xtal control then?

Actually, I had situations where my TCS had RF in all kinds of strange 
places, such as the plates of the modulator tubes (in CW mode). I don't 
remember precisely, but I think I was loading into a non-resistive dummy 
load with short leads, getting a good plate dip and appropriate RF current 
in the ant. ammeter - RF was indicated by a neon bulb - this was back around 
1961. - Dont remember if this was before or after my pi-net output mod.

We didn't have TVI problems with my 80 & 40 meter ARC-5's back in the 50's - 
even when I was loading into the supporting metal headset of my 6' x 6' 
metal venetian blinds (!)- It worked for NY area QSO's and more). We were 
using a indoor TV ant. no more than  10-15' from my rig and antenna..

Thanks Bill, I'm looking forward to receiving the tubes --BTW I've got about 
twenty five 12A6's some NOS, so if any one needs some, let me know.

Henry, Cph.

----- Original Message ----- 
From: "Michael Bittner" <mmab at cox.net>
To: "Discussion of AN/ARC-5 military radio equipment." 
<arc5 at mailman.qth.net>
Sent: Thursday, July 22, 2010 7:48 PM
Subject: Re: [ARC5] Reducing BC-459A chirp: 12A6 replacement for 1626

> 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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