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

[Chris Kepus]

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