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

[Mike McCarty]

jeremy-ca wrote:

[...]

> 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.

It's very easy. One needs the total current flowing in the string
to be that which the tube with the highest current demand needs.
One subtracts the current needed by each of the other tubes, and
puts a resistor in parallel which absorbs the excess current, given
the voltage the tube needs. One can put a resistor in parallel with
several tubes' heaters if they all need the same current.

For example, suppose we have a set with a series string of 8 tubes,
each needing 12.6 V on the heaters, and we run it off of 120 V.
Suppose that each of the tubes needs 150 mA, except one needs
200 mA. How do we do the design?

Ok, 8 x 12.6 is 100.8 V, so we need to drop an additional 19.2 V
at 200 mA, so we need a "ballast" resistor of 96 ohms. BTW,
true ballasts are made of material which increases in resistance
like that of the heater material, to match things. They are not
just resistors.

Anyway, now we've got our ballast designed. All the tubes but one
need 150 mA, so we need to pass another 50 mA around each tube.
Since the heaters have 12.6 V across them, we need another
12.6 V / 50 mA = 252 ohms in parallel with each of those tubes.

If they can all be put on one side of the "hungry" tube, then
we can also just use one 1764 ohm resistor in parallel with
all of them.

Controlled warm up time is more important for rectifiers than
the other tubes. It's important not to apply plate voltage from
a fast warm up rectifier to tubes which have cold cathodes.

Mike
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