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

[kim.herron at sbcglobal.net]

Morning Chris, Et, al.

    Let's simplify this for you.  Ohms law dictates that in a series circuit 
current flow is the same through all the components and voltage drop varies 
to equal the total voltage available..  In a parallel circuit voltage is the 
same through all the components and current varies.
    How this matters in a tube heater circuit is this.  The tube is designed 
to generate a certain size cloud of electrons at the cathode to operate 
correctly.  If the current and voltage available is off, then the tube won't 
operate correctly in the circuit because that cloud of electrons won't be 
there in the first place.  That's why all the tubes in a series circuit need 
to be the same current draw.  Yes you can make up the difference with a 
dropping or ballast resistor in that circuit, but you figure the resistance 
value using the current of the heater circuit as a fixed value.  If you 
change to tubes that are different current spec than the rest of the tubes 
in the circuit, then the value of the ballast resistor is wrong as well 
because the current drop in the circuit is no longer uniform.
    In a parallel circuit, it doesn't matter if you tubes that are different 
current draw, because they are across the voltage source, usually a 
transformer secondary, or perhaps a DC battery source in a vehicle or ship. 
The only consideration here is that the heater supply is capable of 
delivering the necessary current to supply the entire set of tubes.
    Bottom line:  Tubes, in a series circuit, that require more current than 
is available will have a shorter useful life in that circuit and possibly 
the circuit won't operate correctly if the tube is running at its max 
capability.  Stick with the proper heater current values when subbing tubes.

----- Original Message ----- 
From: "Chris Kepus" <ckepus at comcast.net>
To: "'Ian'" <ianwebb5 at comcast.net>; "'Revcom'" <revcom at wbsnet.org>; 
<glowbugs at piobaire.mines.uidaho.edu>; <heathkit at mailman.qth.net>
Sent: Sunday, February 03, 2008 12:00 AM
Subject: RE: Tubes in Series was (Re: [Heathkit] Question on 12AU7) LONG


> Hi Ian,
>
> Thanks for your response, ....and Bob, too. :-)
>
> You said, "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?"
>
> The basic math in figuring the current and voltage consequences of my
> example is now much better understood thanks to your answers and example 
> and
> the answers and examples provided earlier.
>
> Based on the prior posts that recommended not using the 5814A in place of
> the 12AU7 led me to think that even a small percent delta in current
> requirements was to be avoided.  With the math calcs you and the others 
> have
> done, I still remain unsure of why a +/- tolerance of less than 10% would 
> be
> a problem.  What I found looking for an answer on the web before I posted 
> my
> question was that any *voltage excess* provided by a transformer (or AC 
> line
> voltage) to the series string total voltage requirement was typically 
> dealt
> with by a ballast resistor to get it closer with a ballast resistor
> (probably using resistors with no better than +/- 10% accuracy).  Most of
> the articles I found were silent on the current issue.  I think that was
> because all the tubes in the strings described had the same current draw
> requirement.
>
> I have never seen a specification in the tube data sheets that claimed an
> accuracy of regard to the current draw stipulated. Is the actual current
> draw likely to be within +/- 5% of the rating?
>
> At this point, knowing that I don't know the answer to your question, my
> brain overrides my thinking that +/- 10% is OK....so I would err on the 
> side
> of caution and say that in a series filament string that kind of imbalance
> would not be a good thing and I would avoid constructing the condition.
>
> Is that the correct answer?? :-)
>
> Thanks es 73,
> Chris
>
>
>