Date: Tue, 21 Feb 2017 14:24:53 -0700
From: MU 4CX250B <4cx250b@miamioh.edu>
To: Bill Turner <dezrat@outlook.com>
Cc: Amps group <amps@contesting.com>
Subject: Re: [Amps] Checking Filament Voltage
The simplest solution for amps with separate filament xfmrs is
probably to install an inrush current limiter in series with the
xfmr's primary winding. These look like disc capacitors and cost about
a dollar. I use them frequently on any circuit that has a initial
surge current, and a constant load current in equilibrium.
Probably the easiest amplifier to install one in is the drake L4B. I
removed the jumper on a terminal strip and replaced it with the
current limiter. Took five minutes, most of which was spent removing
the screws on the bottom cover.
73,
Jim w8zr
## watch out. They run HOT in normal operation, with a normal constant load.
Which is all fine, provided the inrush current limiter is not in the airflow of
any blower.
If it gets air blown onto it, its resistance will start to climb = bad news.
## In the case of the drake amp, the fil xfmr and bifilar is in a separate
compartment
from the pressurized sockets. So the limiter, in that case will work good.
## I used an external 25 ohm step start on my Drake L4Bs...in series with one
leg of the
incoming 240 vac line. Then it step starts both the B+ and the fil xfmr..at
the same time.
## I used a DPST relay to apply the 240 vac. A SPST relay is used to shunt
out the 25 ohm
resistor. Both relays and resistor reside in their own small box. Small
control cable from that box
runs over to the operating position, into a small mini box, with 2 x miniature
toggle switches on it.
Switch #1 turns on the DPST relay. Switch #2 turns on the SPST shunting
relay.
## In normal operation, 1st toggle is thrown, and B+ comes up to 75-80 % .
Tube fils
come up to 75-80% normal brilliance. 2nd toggle is thrown, and both B+ and
fil come up to
full voltage.
## when toggling between low B+ setting..and high B+ setting, the 2nd toggle
is thrown 1st,
putting the step start 25 ohm resistor back into the circuit. Then hit the
rocker switch, that
increases B+ to the higher setting. Then operate the 2nd toggle, and shunt out
the step start
resistor.
Jim VE7RF
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