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

[John Franke]

If all the tubes are rated for the same current, then you can expect the 
voltage drop across each tube filament to be correct.  If one tube of higher 
current is part of a string, the total resistance will be less and the 
string current will be between the value for the higher current tube and the 
lower current tubes.  The voltage drop across the higher current tube will 
be lower than normal and hence its filament will be a little cooler. 
However the voltage drop across the other tubes will be higher than normal 
and their filament current will be higher than normal.  The lower filament 
current tubes will fail sooner than normally.  The exact change is hard to 
calculate because the filament resistances are a nonlinear function of 
filament current.  The hot resistance of a filament could be as much as ten 
times the cold resistance.  One could shunt each filament with a resistor so 
that each tube draws the same current, but there would still be an imbalance 
while the tubes warmed up.

John  WA4WDL


----- Original Message ----- 
From: "Chris Kepus" <ckepus at comcast.net>
To: "'Revcom'" <revcom at wbsnet.org>; <glowbugs at piobaire.mines.uidaho.edu>; 
<heathkit at mailman.qth.net>
Sent: Saturday, February 02, 2008 3:58 PM
Subject: GB> 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
>
>
>
>
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