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

[Ian]

Chris,

Think it as two resistors in series.

Our old friend, Ohm said R=E/I
Filament/heatter of 12AU7 resistance = 12.6/.15 = 84 ohms
Filament/heater of 5814 resistance = 12.6/.175 =72 ohms

Put those two in series and hook them up to 25.2 volts, twice the voltage of
12.6 volts which we often call 24 volts when speaking of series strings of
12 volt nominal filament volt tubes hooked up to a nominally 24 volt supply.

So, by our friend Ohm again...
I=E/R    I = 25.2/(84 + 72) = .162 Amps will flow through the series circuit
of the two tube filaments.

So one tube has a bit too much current ---> .162 A - .15 A = .012 A more
than the 12AU7 specifications.
And the other tube has a bit too little current ---> .162 A - .175 A =  .013
A less than the 5814 specification.  

For the 12AU7 the current would be 8% too high.
For the 5814 the current would be 7.4 % too little

Does this make that much difference?  What do YOU think?

How close is your line voltage to that for which the filament transformer
was designed?  Within 5%? 10%? Or ??????????

Perhaps for this example it's "good enough for government work" as the
saying goes...???

Ian, K6SDE



-----Original Message-----
From: heathkit-bounces at mailman.qth.net
[mailto:heathkit-bounces at mailman.qth.net] On Behalf Of Chris Kepus
Sent: Saturday, February 02, 2008 12:59 PM
To: 'Revcom'; glowbugs at piobaire.mines.uidaho.edu; heathkit at mailman.qth.net
Subject: Tubes in Series was (Re: [Heathkit] Question on 12AU7) LONG

Would someone please enlighten me about the use of a series of tubes in a
common filament string that have differing filament voltages and/or current
requirements?  

In the referenced thread on the 12AU7 (vs. the 5814), Rod said, "Now the
5814 draws a little more on the filament, 0.175A at 12.6V while the 12AU7 is
rated at 0.15A at 12.6V.  That would mean you would not want to mix them in
especially series applications, i.e. 24V mil gear. and if you had a lot of
them it might burden the filament supply too much."

I certainly understand one would not want to design a tube series string
that required 3 amps from a filament tranny that only delivers 2 amps.
However, what I am confused about is the requirement to have all tubes in
the string to have *identical* current draw requirements. I know (or thought
I did) that in a series string, the current is the same throughout the
circuit.  Take the case above: put two tubes in the series circuit, one with
a current rating of 0.175A at 12.6V while the other is rated at 0.15A at
12.6V.  If the tube rated at 0.175A causes the tranny to deliver 0.175 amps
to the circuit, and since the tube rated at 0.15A will be "exposed" to that
current flow, will it "live" at that current level? (filament wattage
dissipation rating??)? Are "balancing resistors" something that could be
used if necessary? 

If the answer is, "That's just the way it is per the laws of electronics -
the current draw requirements MUST be identical for tubes in series"  - then
someone say so and I won't use any more of the list members time or
bandwidth on the subject.  

If the answer is - it depends -then please read the rest and if you can
help, I would appreciate it.

I pulled up the GE 12AU7 data sheet at this tube data site
[http://www.tubedata.org/] (Carl, KM1H provided info about this incredible
site...thanks, Carl!).  The only special reference to filament current I
could find on the sheet was a comment that the 7AU7 (equivalent to the
12AU7) "..is specially suited for use in television receivers that employ
600 millampere series-connected heaters."  Hmmmm... So they all HAVE to be
600 millampere??

If there is an explanation of this on the web or in a text, please direct me
to it so I can read it. 
 
Yeah, I've got a lot of TV toobs that have interesting specs with "goofy"
filament voltage requirements.  Since I don't want to use them as plinkers,
I either have to figure out some other way to use 'em or lose 'em.

Thanks,

Chris
W7JPG