Tubes in Series was (Re: [Heathkit] Question on 12AU7) LONG

[Kevin Ward]

I was certain my RCA Receiving Tube Manual would clear up the issue of
filament voltage variation, but I didn't find any general tolerance
specifications.  What I did find is the following, excerpted from the
chapter, "Electron Tube Installation", in the first paragraph under
"Filament and Heater Power Supply."

"The design of electron tubes allows for some variation in the voltage
and current supplied to the filament or heater, but most satisfactory
results are obtained from operation at the rated values.   When the
voltage is low, the temperature of the cathode is below normal, with the
result that electron emission is limited.   The limited emission may
cause unsatisfactory operation and reduced tube life.   On the other
hand, high cathode voltage may cause rapid evaporation of cathode
material and shorten tube life."  

<snip>

There was also this:

"In the case of the series arrangements of filaments and/or heaters, a
voltage-dropping resistance in series with the heaters and the supply
line is usually required.   This resistance should be of such value
that, for normal line voltage, tubes will operate at their rated heater
or filament current."  <snip> 

"When the filaments of battery-type tubes are connected in series, the
total filament current is the sum of the current due to the filament
supply and the plate and grid-No.2 currents (cathode current) returning
to B(-) through the tube filaments.   Consequently, in a series filament
string it is necessary to add shunt resistors across each filament
section to bypass this cathode current in order to maintain the filament
voltage at its rated value."

<snip>

There's much more material, of course, including formulas for
calculating resistor values.  BTW, the ARRL handbook recommends keeping
the filament voltage for indirectly heated cathode-type tubes within +/-
10 percent of ratings and keeping higher-power filament-type tubes to
rated voltage as a minimum and +5 percent as a maximum. 

Oh, and the material the heater or filament is made of makes a big
difference too.  My advice would be to look for  receiving and the
transmitting tube manuals at the next hamfest.  Get more than one brand
and keep them on the shelf nest to the Radio Amateur's Handbook.  You do
have one or more of those, don't you?  OK, get a handbook too; an old one. 

Kevin  N2IE