[R-390] Second R-725 on ebay

[Tom Bridgers]

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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