[Amps] Fil transformer

Jim Thomson jim.thom at telus.net
Thu Aug 10 13:01:09 EDT 2017

Date: Thu, 10 Aug 2017 11:47:58 -0400
From: Kelly Taylor <ve4xt at me.com>
To: Radio WC6W <wc6w_amps at yahoo.com>
Cc: amps at contesting.com
Subject: Re: [Amps] Fil transformer

<Hi Russ,

<Thanks. Just to be clear, I do not require a transformer for providing filament power: it's to fix a design flaw in the Clipperton L, which lacks a centre tap for the filament windings and instead feeds the filaments with RF on one side only, introducing hum. 

<The fix is to parallel a centre-tapped 6v transformer secondary with the filament windings on the main transformer and move the RF input to the new centre tap. 

<The Hammond 166j6 is ideal, as it can be added with no metalwork, and you simply cut off and tape the primary windings. 

<73, kelly, ve4xt,

###   There is no RF on the CT...... its just the return for the B-....  IE:   DC plate current.   But we know what u mean.   Seems  nuts that dentron would  use a fil xfmr winding that lacks a CT.    That will give u gross amounts of 60 hz hum,  with the B-
being returned to the extreme left..or extreme right side of the sec winding.  Dentron must have been asleep at the switch when they speced the xfmr. 

##  I mentioned this b4,  a local buddy  re-wound a lower V, 28 vac  xfmr.... with 10 gauge wire.. plus added a CT.... for his 2 x 3-500Z HB amp.   5 V at 30A  under load.   Typ they are 5.5 vac with no load.   In his case, the loaded voltage was  too high,
so he removed some turns off of the sec... but only at one end of the winding !!    Loaded voltage was  now correct, but the CT was effectively shifted off to one side...producing 60 hz hum.   To do it right, he should have removed an equal number of windings from both ends..so the CT ends up dead center. 

##  what ever xfmr u end up using for your fix... measure the DC resistance  from CT to each end.  They should be the same of course. 

##  The xfmr used in the dentron Clipperton L is just a single xfmr... that not only has the high voltage AC   for the B+..... it also has a fil winding for the 4 x directly heated cathodes.... and also a 3rd low vac winding for the TR relay and misc stuff.  

##  Dont use a small  CT xfmr thats much less than 6 vac.  If its too low, like 3-5 vac, there will be a lot more than 120 vac  appearing on the un-used  120 vac  primary of the retrofitted small xfmr used for the ... fix.  

##  The hammond xfmr  u picked out appears ideal..as it will  fit inside the Dentron amp,   How the heck dentron managed to screw up something as basic as... CT required on a fil xfmr for a GG amp is an...  epic failure !   The only time a CT is not required
is for the case of a grid driven amp..and dentron never made anything other than GG triode amps.  They obviously never tested em properly.   In all fairness though, the RX at the other end would have to be capable of hearing the 60 hz ac component, which in my local buddys  case, was blatantly obvious for a 800 mile radius...and his CT was over to one side of exact center.. by just a little bit  vs  the oem dentron.     One would think QST  would have picked up on that fubar in the 1st 10 mins. 

Jim  VE7RF   

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