[Amps] Checking Filament Voltage

[Jim Thomson]

Date: Tue, 21 Feb 2017 14:24:53 -0700
From: MU 4CX250B <4cx250b at miamioh.edu>
To: Bill Turner <dezrat at outlook.com>
Cc: Amps group <amps at 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