[Amps] Fil transformer

Carl km1h at jeremy.qozzy.com
Fri Aug 11 19:58:58 EDT 2017

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Friday, August 11, 2017 1:01 AM
Subject: [Amps] Fil transformer

> Date: Thu, 10 Aug 2017 07:08:54 -1000
> From: Merv Schweigert <k9fd at flex.com>
> To: amps at contesting.com
> Subject: Re: [Amps] Fil transformer
> <How about the old time trick of putting two low value resistors to
> <ground, one from each side of the
> <filament,  used to be used for a center tap many times in days of old.
> ##  This isnt 1956 anymore.  The B-  has to float above ground a bit on a 
> gg triode amp, in
> order to measure both dc plate + dc grid current.
> The two resistors would have to be wired in series....across the sec of 
> the oem fil xfmr.
> The junction point of the resistors becomes your new CT.

** What does 1956 have to do do with anything?  The 2 resistors in series 
between the filament  and the mid point becomes a virtual CT. I mentioned 
this yesterday.

> ##  two resistors in series will  work, provided they are each real low 
> value.  If say
> a pair of 10 ohm resistors used in series...and perhaps  600ma of plate 
> current flowing,you will
> end up with 300 ma  flowing through each of the 10 ohm resistors.   10 
> ohms X 300 ma =   3 volts.
> Thats another 3 vdc of unwanted bias on each side of the fil, yo-yoing up 
> and down as you talk on ssb.
> Probably no big deal, moot point.   You could also put a small 
> electrolytic across  each resistor. That
> would provide for a near constant  3 vdc, as both caps try and charge up 
> when the plate current peaks.

** If 3V stains your undies use a lower value. It sure as hell beats buying 
a transformer for no value added if something isnt already on hand. I had 10 
Ohm 5W MOX already on hand and the spectrum analyzer shows no effect.

> ##  Its also  .9 watt  dissipation  in each resistor, so if u go that 
> route, use a 3 watt mof resistor.
> ##  I cant fathom why dentron could miss such a fundamental  concept.  No 
> surprise really.  I had one of
> their dentron  160-10L  4 x 811 amps back in 1970. Its was a dogs 
> breakfast right out of the box, new.
> Jim   VE7RF

** With good filter caps there is no hum on SSB and just a trace on AM which 
is where I use my L and when the 300W dead carrier is ~ 40 over S9.
I also use the transistor plus 1W zener bias method used on late run L's and 
all Clipperton QRO's and run the bias for a much lower idle current to 
improve efficiency and reduce strain on the old iron.

IMO changing anything is a waste of time/money for SSB as the several 
thousand Clipperton L's around do just fine with a good PS.


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