Steve, are you referring to running the filament with 30 KHz AC
directly, or a 30 KHz switcher that provides DC and using DC on the
filament? If it is the former idea, then I suggest talking to the
tube maker if they still exist.
A lot of the larger tubes w/handles have filaments which have
mechanical resonances that are sensitive to audio and LF vibrations.
More modern tubes may have been designed to move these away from
sensitive frequencies, such as 60, 120, 360 Hz. A lot of big tubes
specify DC only for their filaments, for this reason. I was told by a
French tube engineer about one tube which was installed in a
shortwave site, and which kept failing or arcing, when a certain song
was played over the air. True story. The vibrational mode in the
filament would get excited and then G-K short would happen. It took
them a long time to understand what was happening.
Military with 400 Hz supplies had to deal with that frequency for AC
heated tubes.
Up until recently, I have always used AC heaters, with transformers
and a center tap, or used DC from a rectifed three phase mains
supply. This tube is in a line/cavity circuit which has one lead
grounded to the cavity. So i cannot use the old CT transformer to
eliminate hum modulation on the output. I used a rheostat across it 2
years ago, and the CT of that was grounded, providing a center tap
for the cathode DC current that was not modulated by the filament AC
voltage. I am about to test a high power tetrode at VHF, using my
first switching power supply for filament DC source. I am doing this
because coming up with 12 volts at >300 amps is difficult with a
linear supply, and I need smooth DC to not have modulated RF via
control grid modulation. I am worried more about RF effects of
getting back into the switcher than I am of switcher noise getting to
the filament.
John
K5PRO
>
>Message: 2
>Date: Thu, 13 Jul 2006 15:37:48 +0100
>From: Steve Thompson <g8gsq@eltac.co.uk>
>Subject: [Amps] High frequency heater supplies
>To: amps@contesting.com
>Message-ID: <44B65ABC.9080704@eltac.co.uk>
>Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>Does anyone know if there are problems using high frequency (say 30kHz)
>ac to run the heater in a valve with a cathode? For that matter, the
>filament in a valve without a cathode?
>
>Thanks,
>
>Steve
>
>
>If line-frequency (50 Hz or 60 Hz) power can cause a small amount of
>modulation of the RF output, then I would think 30 kHz could do the
>same thing. The output signal would then have sidebands at +/-
>30 kHz from carrier, down maybe 40 or 50 dB. I suspect this
>would be an issue mostly with directly-heated cathodes.
>
>- Jim WB6BLD
>
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