[AMPS] floating a filament transformer

[R.Measures]

On Sep 16, 2004, at 5:22 AM, lncarman wrote:

>
> I've heard over the years of some amp builders floating the filament 
> transformer. I was
> told that you isolate the filament transformer from ground and add two 
> chokes
> in the primary side with bypassing. Since the primary side is 115V or 
> 220V the
> amperes demand would be much less on the primary side. Looks like any 
> RF would
> be suppressed greatly by the filament transformer and you wouldn't 
> need a lot of
> uH in the primary chokes to finish the job.  Has anyone tried this???
>
---  It is common in commercial g-g applications, Larry.  All filament 
xfmr windings are RF-bypassed to the core and the transformer is 
floated on a insulating sheet of ABS or G-10.  A bifilar choke is 
placed in the primary of the transformer.  After installation, the C 
from the core to gnd is measured and this amount is subtracted from the 
calculation(s) for C2 in the tuned-input(s).  An additional choke is 
connected to the fil. secondary CT to provide the DC path for electrons 
going to the the cathode from the –HV.  As I see it, filament currents 
of more than 30A are a candidate for this solution.  Thus, a filament 
choke for a 3cx10,000A7/8160 (7.5v/99A) would typically be  Much  
smaller than a filament choke for a pair of 3-500Zs (5v/30A).   In this 
  case, with 240v mains, the filament choke for the 8160 would only need 
to be wound with #22-ga, wire, while the choke for the 3-500Zs would 
need to be wound with #8 gauge wire.
- cheerz

> Regards,
> Larry N5BIP
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>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org