John: Some info. I have an RCA Super Controlfone 450 MHz repeater using an
8122 tube, (same tube as the NCL2000 linear). It has a fancy regulator that
reduces the 13.5 V heater voltage on transmit. I checked the data sheet,
and footnote 1 is:
" Because the cathode is subjected to back bombardment as the frequency is
increased with resultant increase in temperature, the heater voltage
should, for optimum life, be reduced to a value such that at the heater
voltage obtained at minimum supply voltage conditions (all other voltages
constant) the tube performance just starts to show some degradation; e.g.,
at 470 MHz heater volts = 12.5 (approx.)"
This tube has an indirectly heated cathode. --73, Mike, WV2ZOW
On Tue, Sep 8, 2015 at 7:35 PM, John Lyles <jtml@losalamos.com> wrote:
> 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@gmail.com>
>> To: amps@contesting.com
>> Subject: Re: [Amps] Regulated filament current
>>
>
> David Lisney <g0fvt@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..
>>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|