[Amps] Fil transformer
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
> ## 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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