> From: Rich Measures <measures@vc.net>
> >Every amplifier has pluses and minuses Herb. The reason for this is
> >cost.
> >
> I agree, Mr Rauch, cost is a factor. However, there are exceptions.
> Applying more than Eimac's rated voltage to the filament or heater is
> definitely a minus. {DTR-2000, TL-922, some SB-1000s, Sigma, and
> several models of Henry Radio amplifiers} I do not see how this is
> related to cost.
That's right Rich, too much (or too little) filament voltage is
stupid. It cost nothing at all to get the transformer right.
I just measured a new Ten Tec 811 amp, and the filament voltage is
nearly 7 volts with 117 volts of line voltage.
Would I worry about it? Probably not.
The tubes will be gone from hammering the 65 watt anodes long before
the filaments fail.
> Allowing more inrush current to a 3-500Z filament than Eimac's spec. is
> definitely a minus. {TL-922}. The easy fix was to specify a
> current-liniting core for the filament transformer. Cost seems to be an
> unlikely factor here.
You overlook the fact the filament is a series string. The equalizing
tap does NOTHING during start-up, because the tubes cold
filament resistance is much lower than the ESR of the equalizing
winding.
What happens on start-up in the 922 is the thinnest and weakest area
of the weakest tube gets the most start- up inrush power. It's like
turning on one of the old AC-DC five tube radios. The weakest
filament gets all the dissipation, it gets hotter quicker, its
resistance increases faster than the other filaments, and it
dissipates even more power.
Series filaments are problem enough in Christmas tree lamps,
they were problems in cheap-o five tube radios. They are even worse
in an amp with 150-200 dollar tubes.
Series filaments with too much voltage and NO inrush limiting is a
disaster waiting to happen.
73, Tom W8JI
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