> From: Rich Measures <measures@vc.net>
To: <amps@contesting.com>
> Date: Wed, 9 Jul 97 15:11:06 +0000
> >Really? Doesn't the TL-922 use a series filament string??
> >
> - Did you measure the filament inrush current in each tube in both
> amplifiers, Mr. Rauch?
My question was"does the TL-922 use a series or parallel filament
circuit?". You stated the only difference was the transformer
"style".
.
> - Does Mr. Rauch's ESR 'explanation' for the 220's minimal fil. inrush
> current wash? It can only be explained by ohm's-law if the total ESR is
> around 70milli-ohms, which would necessarily mean that the open circuit
> voltage from the filament transformer was around 8V---which is
> measureably not the case.
The flux leakage, in a transformer at 50 or 60 Hz, is independent of
load current and considered to be negligable by transformer
designers. The exception to this is when the transformers windings
are spaced apart and a air gap or magnetic shunt are included.
Welders traditionally use an adjustable air gap or magnetic shunt to
control current.
Lamination thickness is selected to minimize eddy currents that limit
frequency response and induce eddy current losses. Lamination STACK
area is selected to allow a certain flux density determined in the
open circuit secondary condition within power dissipation
restrictions due to core losses at full power.
The transformer behaves like a linear resistance in series with a
leakage reactance, and that reactance is negligable without a shunt
magnetic path or air gap between seperated windings at power line
frequencies.
If all resistances in the system are properly accounted for AND the
filament inrush is properly measured the math works just fine. All
one needs to do is use a transformer sized as small as possible to
light the filament, and not use unnecessarily heavy wiring and
filament components, and inrush is no problem. The inrush will
typically be only two to three times the operating current IF you
happen to hit the switch at JUST the correct moment to apply power on
the near-half-voltage upward slope of the line voltage. At any other
point inrush is less. (This is a good reason to NEVER use a zero
crossing solid state switch to turn on a not-step start PA, it
guarantees you are ALWAYS applying power on the upward voltage slope
and that you have a FULL first half cycle of inrush).
During the inrush hysteria of the 70's Buzz Miklos tied the filament
pins of a group of 3-500Z's right across the secondary of a 200
ampere transformer and cycled the tubes thousands of times without
any sign of inrush damage. Inrush damage is mostly thermal in
nature, and is accumulated damage, but Buzz couldn't create problems
even with intentional inrush on 3-500Z's.
But then the needless hysteria let people sell lots of needless
inrush kits.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|