----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@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@flex.com>
To: amps@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.
Carl
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
---
This email has been checked for viruses by AVG.
http://www.avg.com
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|