Comments at end. jimlux wrote:
Ed Phillips wrote:futuret@xxxxxxx wrote:I've heard that certain Chinese or Russian 833A's are not that great. Sometimes deals can be gotten on eBay for NOS American tubes. Of course you have to be careful on eBay. Some venders will let you return the tube if it's no good. JohnFor what it's worth here is a new note I got on the Tube Collectors list:"> > Anybody know where the tubes in this list are being made?> > http://www.rfparts.com/tubetran_specials.html <http://www.rfparts.com/tubetran_specials.html>> Chinese, of course! > Figured! All top quality of course? Let me put it this way. If you buy one, and put it in your transmitter, and it arcs over and destroys the power supply (which I have seen happenwith Chinese stuff many, many times),You're saying that the HV power supply design didn't accommodate the possibility of flashovers? Fine for hackers, I suppose, who are willing to accept a rough and ready low cost PS design in exchange for the need for occasional rebuilds.All tubes (even from Eimac/CPI) have the potential for flashovers, not to mention the odd spider crawling in. That's why they have limits in the data sheet for "stored energy" that can be dissipated in the tube. (typically a few Joules).The LANL report "High Power Microwave Tube Transmitters" (or similar..) from William North (available in various places on the web) covers all this stuff in detail. (FWIW, North just recently died, presumably not from HV, although I didn't ask)
Purpose of the note was to say let the buyer beware! The guy who wrote that particular note is in the business of "audio equipment" and tubes therefor [in other words, high power receiving tubes] which most definitely are not expected to arc over or short under any circumstances although equipment using them is usually fused in the input line to protect against catastrophic failures. The same can be said of relatively low power transmitting tubes of the 833 class [< 5 kV plate voltage, < 1 kW plate dissipation]. Such failures would not be expected and equipment designed using them wouldn't have any stored energy constraints. I'm not aware of any higher power tube transmitters which have power supplies deliberately designed to protect against arcing but there may well be some. Several members of the TCA group from which this note came are ex-Eimac and they have often described the great lengths to which Eimac went to pre-condition high voltage tubes to prevent sparking and arcs at voltages well above ratings. Shorts are never expected. I am very familiar with the design of high power MICROWAVE transmitters for radar purposes. Cathode or anode pulsed transmitters are inherently protected against arcing. However, with the advent of high power gridded TWT and Klystron transmitters which may operate continuously with applied HV matters become entirely different and such protection is mandatory. A rule of thumb we used at Hughes [in connection with X-Band radar transmitters] was the "aluminum foil test". The supplies were designed to dump [with crobar switch] HV fast enough in the event of a short that if the output was shorted to a sheet of kitchen foil the size of the resulting hole was very tiny. With VTTC's, particularly those built by enthusiasts without a lot of tube experience, failures due to circuit design can be expected and the consequences are merely monetary if they do get poor tubes. The general reputation of cheapie Chinese transmitting tubes is very poor with respect to gassing and low emission and I was trying to suggest that guys look for something better if they can find it.
Ed _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla