[Amps] high frequency filament excitation TSPA

[John T. M. Lyles]

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 at eltac.co.uk>
>Subject: [Amps] High frequency heater supplies
>To: amps at contesting.com
>Message-ID: <44B65ABC.9080704 at 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
>