[Amps] Regulated filament current

[John Lyles]

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.
73
John Lyles
K5PRO

> Date: Tue, 8 Sep 2015 08:05:20 +1200
> From: Steve Wright <stevewrightnz at gmail.com>
> To: amps at contesting.com
> Subject: Re: [Amps] Regulated filament current

> David Lisney <g0fvt at hotmail.com> wrote:>
>> [....] in directly heated cathodes the temperature would rise as the anode current and drive rose.
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 
voltage reduced
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.

> 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..