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

[Mike McCarty]

Chris Kepus wrote:
> 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."

Yes, it might.

> 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

[...]

They don't. However, if one does that then one cannot "just"
connect them in series. It is possible to fix things up so they
work, by putting resistors in parallel with the tubes which "want"
less current, so the higher current tubes get what they need.

Mike
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