[AMPS] Fil. Xformer w/o center tap

William L. Fuqua III wlfuqu00@pop.uky.edu
Tue, 9 Feb 1999 09:20:46 -0500 (EST)


  The following is a series of e-mails pertaining to
the use of Filament transformers w/o center taps with
directly heated cathode tubes. There are several
advantages in doing  this.
  
  1. Being able to use inexpensive surplus or home
     wound transformers. I got the 2.5 KW Microwave
     generator for $1.

  2. The use of simple low capacitance homemade
     filament transformers. These are easily made
     by reusing the core and windings of defunct  
     variacs. Usually the brushes are broken or 
     a few windings are burned which can be removed
     and bypassed. 
        Then you wind your own secondary thru the 
     toroid and use some insulating blocks to put
     distance between the secondary and primary.
     Two RF chokes of only a hundred or so micro henrys
     would have to be placed in series with the 
     smaller transformer's secondary. 

   3. In some cases it may be useful to have
      secondary taps on the transformer. And this 
      method makes that possible with out having a
      transformer with 2 sets of balanced taps.

   I felt that this group may find this discussion intersting.
By the way this technique is working just fine so far.


Date: Tue, 8 Sep 1998 08:37:59 -0400 (EDT)
Message-Id: <199809081237.IAA15387@pop.uky.edu>
Mime-Version: 1.0
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To: Old Tube Radios <boatanchors@theporch.com>
From: "William L. Fuqua III" <wlfuqu00@pop.uky.edu>
Subject: Fil. Xformer w/o center tap

  Just removed from a 2.5 KW microwave generator
a very nice 5KVA HV transformer w/ bridge rectifier (no filter)
and a very nice filament transformer.
  The filament transformer has no center tap and I am building
a 4-1000A amplifier. The first solution that came to mind was
to use a pair of matched resistors but then I realized a center-tapped
choke wold be better. Never seen such a thing, but a smaller 
center tapped filament transformer's  secondary should work. It
would not have to be very large. I am going to use the first thing
I found (a 12v/2A ct transformer) even though it should only require
a amp or less rating. Core saturation is not a problem sense the cathode
current should produce very little flux in the core and the primary
(now secondary) will be open.
  Two matched chokes won't work. They would offer a high impedance
to the varying cathode current. It has to be center-tapped so that
the impedance will be low for the common-mode voltage and high for 
the differential voltage.
  I have not seen this before and welcome any useful advice or suggestions.
 

73
Bill wa4lav

  William L. Fuqua III  P.E.  E-mail WLFUQU00@POP.UKY.EDU  Phone (606) 257-4155
  Department of Physics and Astronomy      CP-177 Chem. Phys. Bldg.  
  University of Kentucky ,                 Lexington, Ky 40506-0055




Al Klase - N3FRQ
skywaves@bw.webex.net
Flemington, NJ 08822
Web Page:  http://www.webex.net/~skywaves/home.htm
Message-Id: <199809081606.JAA20927@mail-gw3.pacbell.net>
From: "Arden Allen" <gumbear@pacbell.net>
To: Old Tube Radios <boatanchors@theporch.com>
Subject: Re: Fil. Xformer w/o center tap
Date: Tue, 8 Sep 1998 09:08:47 -0700
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

Hello Bill;

>   Two matched chokes won't work. ...........

Maybe a little filament transformer derived AC could be bled into the grid
return to offset the effect of the unbalanced filament.  Sorta like putting
the grid-cathode potential diffrence mid-span between the filament
endpoints?  Is this the hair-brained idea of a whacko?

Arden  Allen   KB6NAX   Vallejo, CA   gumbear@pacbell.net

From: "Mike Feher" <n4fs@monmouth.com>
To: Old Tube Radios <boatanchors@theporch.com>
Subject: Re: Fil. Xformer w/o center tap
Date: Tue, 8 Sep 1998 12:47:11 -0400
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
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How about a switch? For one month one side is grounded and then for the next
the other and so on. 73 - Mike

Mike Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
732-901-9193

Message-Id: <199809081934.MAA11901@mail-gw5.pacbell.net>
From: "Arden Allen" <gumbear@pacbell.net>
To: Old Tube Radios <boatanchors@theporch.com>
Subject: Re: Fil. Xformer w/o center tap
Date: Tue, 8 Sep 1998 12:36:55 -0700
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
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Hi Ken;

> One solution to a "no center tap" filament transformer is to put
> r.f. chokes in series with the filament leads, then a pair of r.f.
> coupling capacitors in series across the leads between the
> chokes and the filament. The r.f. return connects between the
> two r.f. caps. 

It's not the cathode circuit RF impedance Bill is concerned with, it's the
effect of 60 cycle modulation of the RF signal envelope due to *imperfect
linearity* (power gain shift) with bias.  With a center tapped filament
transformer the hum modulation is cancelled to a great extent due to part
of the filament going positive while part is going negative.

Arden  Allen   KB6NAX   Vallejo, CA   gumbear@pacbell.net


Message-ID: <35F79819.2328@earthlink.net>
Date: Thu, 10 Sep 1998 01:13:42 -0800
From: "Don L. Davis" <dxguy@earthlink.net>
MIME-Version: 1.0
To: Old Tube Radios <boatanchors@theporch.com>
Subject: Re:  Fil Xfmr
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

I think everyone missed the point of Bill's question.  The real question
is how to return the filament's DC current (ie cathode current) to the
HV return (chassis).  Normally, this is done through the center tap of
the fil xfmr to make sure both sides of the fil carry the same DC
current.  One could operate the tube at half the rating and ground only
one side - might work OK.  Seems like putting smaller xfmr across the
circuit might work (at least I can't find any fault with Bill's logic -
although it seems like there "has" to be a gotcha in there somewhere). 
The fil xfmr must carry the fil current plus the plate current
(including modulated peaks) without much loss to avoid changing the bias
point too much.

Another question:  If you're fooling around with a very expensive tube
like the 4-1000 & socket & roller inductor and vacuum caps & etc, why
scrimp on the filament xfmr?  Seems like there are a lot of these things
available reasonably on ebay & the NGs.

73s  Don Davis   DXGUY@earthlink.net

  William L. Fuqua III  P.E.  E-mail WLFUQU00@POP.UKY.EDU  Phone (606) 257-4155
  Department of Physics and Astronomy      CP-177 Chem. Phys. Bldg.  
  University of Kentucky ,                 Lexington, Ky 40506-0055
 
  


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