On Wed, 12 Nov 1997 09:25:09 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>> 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.
I'll throw in my own 2 cents worth...Trying to reduce filament voltage in
TL-922 by using a smaller wire is not a good idea IMO. The "Kits" that I
have (been instructed to) installed run way too hot for the surrounding
plastic wire and there is no easy way to support #18 without tying it to
the cable harness somehow.
I have also yet to find a TL-922 with over 5.1VAC RMS across the socket
at an AC line of 240VAC.
Reducing the voltage makes a lot of sense when warranted and 4.8VAC would
certainly prolong tube life but IMO the suggested way has a few problems.
I just do what I'm paid for with some customers...others ask questions.
73 Carl KM1H
>
>73, Tom W8JI
>
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