[Milsurplus] Protecting Battery Tubes in the Learavian P-10A

[David Stinson]

Been working on a Learavian P-10A portable
AC/DC receiver, circa 1949:
http://www.radioatticarchives.com/radio.htm?radio=10458
Got a schematic through the kindness of one of 
our good members;  thanks again, Rodger!

Installed a primary fuse first thing, of course.
Replaced the bad caps and resistors
And man, did I get tired of being "bit" by this
AC/DC chassis when I had it out of the wood case
.... grumble.... 
Anyways....

The set uses 1.4 Volt minis like 1R5 and 3V4 in a 
50-milliamp, 7.5 Volt series string as well as a half-wave 
117Z3 AC rectifier.
I don't like the way the filament voltage is derived-
by a simple 2.2 K 10W dropping resistor from the 100 Volt B+ line
feeding directly to the series filament buss.
The primary AC voltage feed to the tumbled-down hovel-barn
where I have my work bench varies a great deal.
With the simple dropping resistor, the filament volts across
the 1R5 converter could drop as low as 0.7 Volts, 
stopping oscillation.  And there was the possibility of it
going over 2 volts, given the "hot" line voltages around.
This might be OK when you could buy 1R5s surplus for
25 cents, but not now.  Worse- should one of the serie fils 
go "open," the B+ buss would soar to over 150 volts,
which woud then appear on the open filament buss.
I decided to regulate and protect the filaments.

I dropped the series resistor to 1800 Ohms at 10 watts
to quit starving the fil. buss and give some "swing room" 
for regulation.  
Installed an 8 Volt, 5 Watt Zener diode on the fil. buss.
Assuming the B+ buss might swing as high as 160
the maximum zener current (if the fil string should open)
should be (160-8)/1800= 85 mils, so the zener disappates
.085*8= .68 Watts, so a 5-Watter should be plenty.
Set plays nice now.

Here's the small changes:

http://home.netcom.com/~arc5/P10AFils.jpg

This should cover all bases but one-  what if 
the zener opens?  The buss is probably going to 
soar too high if plugged into 125 VAC.
What's you solution- a second zener?
An SCR with another zener on the gate
to crowbar the buss to ground should it rail above
9 volts or so?  

73 DE Dave AB5S