[R-390] Second R-725 on ebay

[Cecil Acuff]

And just in case it's not been discussed...those heaters should be disabled 
anyway....as a matter of fact the last one I rebuilt I removed the outer 
can, insulation and heater and discarded it....as it was banged up anyway.

Cecil
----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <r-390 at mailman.qth.net>
Cc: "Tom Bridgers" <Tarheel6 at msn.com>; "David Wise" <David_Wise at Phoenix.com>
Sent: Monday, May 11, 2009 2:28 PM
Subject: Re: [R-390] Second R-725 on ebay


Tom,

Dave is NOT talking about the FILAMENT.  He is talking about the HEATING
ELEMENT wrapped around the PTO inside the can.  Thereby taking AC away
from the PTO innards!

Bob - N0DGN

Tom Bridgers wrote:
> David,
> How does one "float" the vfo tube's filament up to a positive dc 
> potential?
> Does that mean we should do the same thing to our R-390A's PTO tube's
> filament?
> Thanks,
> -tom
>
> ----- Original Message ----- 
> From: "David Wise" <David_Wise at Phoenix.com>
> To: <r-390 at mailman.qth.net>
> Sent: Monday, May 11, 2009 1:26 PM
> Subject: Re: [R-390] Second R-725 on ebay
>
>
>
>> I read about this in the RCA Receiving Tube Manual.
>> The theory section at the front was one of my first
>> big learning experiences in electronics.
>>
>> Dave
>>
>> -----Original Message-----
>> From: Tim Shoppa [mailto:tshoppa at wmata.com]
>> Sent: Monday, May 11, 2009 10:22 AM
>> To: r-390 at mailman.qth.net; David Wise
>> Subject: Re: [R-390] Second R-725 on ebay
>>
>> David writes:
>>
>>> The purpost of the transformer is to separate the
>>> PTO and BFO heaters from the main heater supply.
>>> (The main supply is grounded.)  This allows the
>>> heaters to be floated up to a positive DC
>>> potential, which has a known beneficial
>>> effect in reducing hum.  (It prevents
>>> filament emission.)
>>>
>> Back when I was trying to make VFO's by using hot pentodes I discovered
>> the same technique.
>>
>> The problem I was having (and floating the heaters up solved) was a 
>> subtle
>> 120Hz frequency modulation. It was sorta like a hum but more like a
>> warble.
>>
>> Tim.
>>
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>
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-- 
Bob - NØDGN

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