[R-390] Second R-725 on ebay

[David Wise]

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.