[R-390] Second R-725 on ebay

[David Wise]

Er, I _am_ talking about the filament, Bob.
Frequently referred to as "heater" in indirectly-heated
tubes.

If the PTO was having FM hum problems, any AC in
the area would certainly be anathema.  It's the
biggest source of residual FM in the General Radio
1009 Standard Signal Generator.  There, the 
power transformer's magnetic field changes the
tuning slug's permeability.  I don't think the
R-725 uses a DC oven, so in that case I have
to guess that the combination of inner shield
and (incidentally) field-cancelling wrap-around
oven winding design minimizes the effect.

Dave 

-----Original Message-----
From: rbethman [mailto:rbethman at comcast.net] 
Sent: Monday, May 11, 2009 12:29 PM
To: r-390 at mailman.qth.net
Cc: Tom Bridgers; David Wise
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