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- To: tesla-at-pupman-dot-com
- Subject: Re: Tubes/FILAMENTS
- From: robert.michaels-at-online.sme-dot-org (Robert Michaels)
- Date: Sat, 08 Mar 1997 16:11:50 GMT
- Organization: Society of Manufacturing Engineers
The filament supply for high-voltage tubes (X-ray or trans- mitting) =must= be isolated from ground to a degree equal to the =peak= voltage to be applied to the plate (or anode) of the tube. The official way to to this is with special filament trans- formers which are insulated as between primary and secondary for the high voltage. These can be quite costly. However --- For us experimenters there are two simple work-arounds: 1) - Use a Battery on an insulating stand. 2) - Use back-to-back neon transformers to supply the 120v for an ordinary (no high-voltage insulation) filament transformers. Back-to-back means the high-voltage secondary of the first transformer is connected to the high-voltage secondary of the second transformer. The primary of the second transformer then becomes a source of (say) 115v ac. This 115v source is insulated from the 120v line by an amount equal to sum of the high-voltage ratings of the two neon transformers. -------- In the typical case, both neon transformers would have the same secondary rating. However, disparate trans- formers may be connected (say a 5-kv and a 7.5-kv) to either increase or decrease the 115v ac. The purpose of doing this would be to "correct" the voltage of the filament transformer. For example, to reduce the output of a 12v transformer to 10v (as required by some trans- mitting tubes). In any event pay attention to the power rating of the neon transformers. It must be at least equal to the power consumed by the tube filament, which can be appreciable in the larger transmitting tubes. Robert Michaels, Tough Enough to Live in Detroit, USA T>aaah, this sounds REALLY dangerous. I don't know what circuit you will use T>this in, but usually the filament must be close to the cathode voltage, and a T>battery charger will not have significant breakdown voltage (beyond UL reqmts T>- e.g. around 200 v) between the pri and secondary - filament Xformers for HV T>tubes have high values for this insulation and specify the value on the T>xformer - for example, I had one which specified 20,000 V. If you use the T>battery charger you will at best destroy it or your circuit, and at worst kil T>someone. T>Dr. Resonance wrote: T>> T>> To: Tesla tube experimenters T>> T>> If you need a cheap and reliable tube filament supply for some of the older T>> tubes try a surplus Schaeur battery charger transformer available from Fair T>> Radio Sales, Lima, Ohio Tel. 419-227-6573. They deliver 12 volts at 10-14 T>> amperes (you can also parallel some of the secondary windings). It is very T>> easy to remove a few of the heavy outer windings to get the exact voltage T>> you need to match the tube you are using. The good news is the price -- T>> around $12.00 surplus. If you are willing to remove some windings they T>> also have some oddball voltages around 16 and 20 vac for even less money T>> that will deliver 10-15 amps out. T>> T>> DR.RESONANCE-at-tesla-at-pupman-dot-com T>> T>> ---------- T>> > T>> > The 803's do have thoriated filaments, and the tube manual suggests T>> keeping T>> > them within 5% of their rated voltage, so your set-up should be OK. But T>> it T>> > would be nice to add another turn or so to the transformer to raise the T>> > voltage to 10 volts, since at full TC power, the voltage may drop a T>> little, T>> > depending on the transformer, etc. Current limiting is not required for T>> the T>> > filament supply, since when the voltage is correct, the current will be T>> > correct, based on the hot filament resistance. T>> > T>> > Happy tube coiling, T>> > T>> > John Freau
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