On Apr 22, 12:17=A0am, "Iain Churches" <Iai...@[EMAIL PROTECTED]
> wrote:
> =B4> What's the alleged advantage of limiting heater current ?
>
> Longevity
Iain:
Longevity in what way? I have a Dynaco ST70 (obligatory possession
IMO) that has many thousands of hours on it, is used nearly every day,
and has - as far as I can tell - the original Dynaco-branded Mullard
(6CA7 & 5AR4) and Sylvania (7199) tubes. On my Hickok 539B, they still
test at 130% of minimum-acceptable and the amp itself performs as well
as expected.
My Scott LK150 gets about the same use, it has been in my hands for
only about 5 years and about 3 thousand hours. I had to replace both
5AR4s on receipt - Chinese junque was in place - but the OEM Scott-
branded (GE) 6550s are still in place, as well as the Scott-branded
(RCA) 7199s. The 6550s test at about 160% of minimum.
Now, Dynaco is known to run tubes at about their maximum-before-
breakdown ratings, and the Scott is not far behind. So, of all the
amps out there that might be hard on tubes, these seem to be a good
start at that. Yes, I know that even a decent tube-tester is at best a
vague sort of test - but I am still puzzled at the claim that limiting
inrush current to the filaments will add significantly to longevity
all other things being equal. I would think that something that
accounts for local "excess" voltages now more and more common in the
US would be more effective? Further to this, I have seen my fair share
of bad output tubes, but relatively few of them with open filaments -
or is cathode stripping the concern?
Peter Wieck
Melrose Park, PA
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