[Amps] Regulated filament current

[Manfred Mornhinweg]

Bob,

I'm 100% with Jim here.

If the tube manufacturer tells you "use 12.6V, plus/minus 5%", then do that! It 
means that the manufacturer tested his tubes with higher and lower voltage, 
trimmed the filaments to work best with 12.6V on them, and as far as possible 
all manufacturing tolerance is managed in such a way that ALL those tubes will 
work well with 12.6V +/-5%, even if the current they draw could vary over much 
more than 5%. If you feed such a tube with a constant current, then the voltage 
will probably move out of the manufacturer-recommended range, and you will get a 
reduced tube life, poor emission, or both.

My approach is to use a well regulated VOLTAGE, and limit the inrush current to 
a safe level. In many cases, using a suitably sized (not oversized!) filament 
transformer is all you need, if your line voltage is fairly constant. You might 
want to pick a transformer with a slightly higher output voltage, and then 
adjust the precise voltage with a rheostat in series with the primary. Simple 
and effective. The transformer's resistance, plus the rheostat if used, will 
limit the inrush current to a safe value.

If the line voltage is too unstable, an actively regulated voltage supply is a 
good idea. A cheap open-frame switching power supply is often ideal, if the tube 
works well with DC at the filament. It should have a current limit above the 
tube's maximum rated filament current. About 50% larger is fine. That provides 
reasonably quick warm-up while being gentle to the tube. After warmup, the 
filament should always operate in constant-voltage mode, unless the tube is 
rated for constant-current operation, but all the constant-current tubes I know 
are receiving-type tubes, and were massively used in antique radios, with all 
filaments in series, and connected directly to the AC line. They needed to be 
constant-current rated in order to allow series connection, and they even had 
matched warmup times to avoid having a fast filament light up like a lamp, while 
a slower one took forever to come up.

Manfred

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