[ARC5] Filaments - How Low Can You Go?

[J. Forster]

[snip]
> Further comments:
> - Some poking around among WE data sheets tells us that the filament
> supply for their tubes is often tightly specified, and even made to be
> like a constant current source for some tubes.

This is to give the tubes a "slow start" and minimize thermal shock on
startup.

> - The 3-9 mc "ARC-5" receiver I had as a novice in the late 50's still
> has its original tubes installed.  I would expect them to work fine
> now (even though I have not run it for a long time.)
> - The EIMAC publication "Care and Feeding of Power Grid Tubes" has a
> section on filament management for transmitting tubes.
> - To test radio performance with respect to filament voltage, I
> suggest testing with a variac  on the filament supply to detect a
> noticeable reduction in audio output, AVC off.  The above article
> indicates that when emission limited performance is reached,
> presumably due to long service or low filament voltage, the noise of
> the tube increases.  Maybe a measure of the noise of a receiver would
> be a good test when investigating low filament voltages.

The emission v. filament voltage can be measured on some tube testers.

> - Reference to the RCA Handbook 3 sections on tube life- and
> performance- testing will shed light on filament management for
> receiving tubes.
> - I think that receiving tube failure/end-of-life is more often due to
> reduced cathode emission than to filament failure.

It is clear you can trade off increased filament life for emission, but
actually filling in the nubbers is probably hard.

> - I've noticed that European tubes of the dual triode sort (e.g. 12AU7
> and 12AX7) start up with a brief brightness seen from the exposed
> filament wire whereas American made ones don't.

That may just be more exposed filament wires, not covered by the cathode
sleeve. I would expect the cold resistance of the different tubes to be
similar, hence the inrush current would be similar too.

Best,

-John

============
> - For those who want to try out the WE=416A or B, www.tubeworld.com
> has them for $23 to $25 each.  Not bad compared to the $125 or so they
> want for the 417A.
>
> Roy
>
>
> Roy Morgan
> k1lky at earthlink.net
> K1LKY Since 1958 - Keep 'em Glowing!
>
>
>
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