Jim,
With a filament winding the resistance difference from side to side is hard
to see. But the true center tap does not have equal resistance on each side
of center.
The voltage is determined by the number of turns on the transformer. As you
get more layers of turns on the core the winding area grows and each layer
requires more wire length for each turn compared with the length of the
lower layers.
So the outer layer turns will have higher resistance than the lower layer
turns. This means that the center tap will not have the same resistance on
each side.
You can easily see this on a transformer primary where it has two windings
(120/240). One winding is usually layered over the other. Each will have a
different resistance while the voltage is the same because of the same
number of turns.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Jim Thomson
> Sent: Thursday, August 10, 2017 12:01 PM
> To: amps@contesting.com
> Subject: [Amps] Fil transformer
>
> Date: Thu, 10 Aug 2017 11:47:58 -0400
> From: Kelly Taylor <ve4xt@me.com>
> To: Radio WC6W <wc6w_amps@yahoo.com>
> Cc: amps@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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