[R-390] 12BH7A

[Transmaster]

I did this mod to my R390A years ago, I picked one of the tubes listed as
being a substitute and changed the wiring on the socket to match.  It has
worked.  In retro spec a wire wound ceramic would have worked just as well
but at the time 3TF7's where merely hard to find so the idea was to be able
to use the original ballast tube if you found one.


On Sun, Jan 27, 2013 at 5:27 AM, Bernie Doran <qedconsultants at embarqmail.com
> wrote:

> If one is really fussy about appearing totally stock, carefull bend some
> small wires around the pins on the 3tf7, then rub the 12 of of the 12 volt
> replacements. Isn't that being a little silly though? perhaps  you might
> have a larger problem if you are that concerned.   Bernie W8RPW
> ----- Original Message ----- From: "Todd, KA1KAQ" <ka1kaq at gmail.com>
> To: "R-390-List" <r-390 at mailman.qth.net>
> Sent: Sunday, January 27, 2013 12:14 AM
> Subject: Re: [R-390] 12BH7A
>
>
>
>  >
>>
>>> kinda like getting sumpin' for nuttin'
>>>
>>>
>>
>> But if you have to remove a module to solder jumpers into place, swap PTO
>> tubes or whatever else, it's no longer something for nothing. (o:
>>
>> I understand that Ron's radio has apparently been modified already, but it
>> still baffles me that more folks either don't use or perhaps aren't aware
>> of the simple resistor substitution solution (RSS), particulalry since the
>> ballast tube is referred to as a 'current limiting resistor'. A basic
>> 40-50
>> ohm 5-10 watt  resistor (think I used 43 or 45 ohms) is all that is
>> required. Simply bend the leads so they don't come into contact with
>> anything nearby (or dress them with spaghetti if you're really worried)
>> and
>> insert into pins 2 and 7 of the socket. No soldering required, the leads
>> plug in just as tube pins would. Allen Bradley carbon comps might not be
>> the best choice if you receiver lives in a humid environment.
>>
>> If you've ever overcome with guilt or insanity and want to reinstall a
>> 3TF7
>> in the socket, just yank out the resistor. It'll take more time to align
>> and stuff the tube back in. Beyond completing the circuit, the ballast
>> tube
>> serves no real function in today's world. Elevated line voltage is far
>> more
>> of a concern than poor regulation. It was supposed to be removed from the
>> cost-reduced R-390A design but somehow managed to hang around.
>>
>> You can even hide said resistor inside of a tube shield if the Originality
>> Police have you staked out.
>>
>> de Todd/'Boomer'  KA1KAQ/4
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>
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-- 
Kenneth A. Crips, W7ITC, NRA Life member. Home of the Chew Crew,  Red
Headed Irish Terrier House Ninjas, Cowboy, and Ladybug  10th degree black
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