The effect of increasing anode (and cathode) current in an amplifier
tube shouldn't be a source for increasing temperature in the heater.
However, as it may be the result of increasing RF drive, and RF voltage
swing among the elements, then it would be a result of RF back heating.
This can be detected by noting a change in filament current with a CV
power supply, when RF power through the amplifier is increased. A
constant current supply would sense the decrease, and try to raise
voltage to compensate. I believe this is what Steve means below.
Date: Tue, 8 Sep 2015 08:05:20 +1200
From: Steve Wright <firstname.lastname@example.org>
Subject: Re: [Amps] Regulated filament current
In the constant current example this would cause the filament voltage to
further increase which is
the opposite to some of the manufacturers suggestions. I believe for
example that a 4CX250B with a nominally 6v heater should have the
to 5.5v if you are giving the device a good "battering" close to it's
maximum frequency and dissipation rating. A constant current supply
would do quite the opposite.
David Lisney <email@example.com> wrote:>
[....] in directly heated cathodes the temperature would rise as the anode
current and drive rose.
And this is exactly the type of thing I was meaning in an earlier post.
Sometimes, well-meant, and on-the-face-of-it very clever ideas end up
discovering the hard way that tube is cleverer! Oh well..
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