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

[jeremy-ca]

Chris, you are using the wrong tube for your discussion. Neither the 12AU7, 
12AU7A or 5814 has controlled heater warmup. What made you assume that?

Also consider that those tubes (along with the 12AX7, 12AT7, etc) were used 
in mobile applications where heater voltages were far from steady and could 
often exceed the 10% rule. Yet they seemed to fail at no higher a rate than 
their AC line operated brethren.

Octal based tubes such as the 6K8, 6SG7, 6H6 and similar were used in AC-DC 
radios and they did not have controlled warmup time. Yet they worked just 
fine and lasted a long time.

I believe it was one of the Riders publications that had the info on 
designing AC-DC strings using tubes with different current ratings. I'll try 
and find it as I rummage around the book shelves.

If I remember correctly, the controlled warmup came about for TV sets which 
used tubes with a varying current draw and in often confusing 
series/parallel configurations. Those were the tubes with odd first and 
second digits and the requirement was that they had to withstand 4 times 
rated voltage for  XX seconds. Yet, ask any old TV tech about how often 
those tubes blew out.

Carl
KM1H


----- 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
>
>
>