On Sat, 6 Dec 2003 09:29:13 -0800, skipp isaham <nospam4me@juno.com>
wrote:
>I'd like to read any or your opinions about
>trying to drive 4 directly heated cathodes
>(572b) connected in series?
_________________________________________________________
It might be ok, but you need to do some checking first before you
commit to this design.
The resistance and warm up time of the heaters needs to be close to
identical on all tubes, and on any future replacements, or there will
be an unequal voltage drop across them. Temporarily connect them in
series, apply the voltage and measure the voltage across each one,
both during warm-up and when fully warmed up. If any of them are
outside the manufacturer's specs, don't do it.
This was a problem with receiving tubes back in the days of
series-string heaters in TV sets. Originally, even though the tubes
supposedly had identical current ratings for their heaters, it was
found that there was too much variation in their warm up times and
some were being stressed beyond their limits. Someone (RCA I think)
developed what they called a "controlled warm-up" heater, and such
tubes were identified with an "A" suffix. The 6AU6 became the 6AU6A
and so on. Problem solved.
This could be especially troublesome if you ever mix manufacturers on
your 572B's.
--
Bill, W6WRT
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