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MV Rectifiers - Ugh!

144.189.40.221

Posted by Lance on February 21, 2003 at 11:34:16

In Reply to: Re: 47'ing the Class A Joplin - Final Results posted by Derick on February 21, 2003 at 04:21:34:

I spent the early part of my engineering career working on very high power (>100 KW) short wave transmitters. At that time (the 1960's), silicon rectifier stacks were just starting to be used to replace the mercury vapor rectifiers that were then in use. I can remember pulling out hundreds of 870B water cooled MV rectifiers (20KV@ 20A, 2000A filament) and replacing them with silicon. I also worked on many smaller transmitters that used 857B, 575A, 872, 866, 816, etc. MV rectifiers. The engineering community breathed a collective sigh of relief when these things went away. They are accidents waiting to happen and the standing joke at the time was not if you were going to have a problem with a MV rectifier but when.

I know they look nice, have low volatge drop and impedance but the negatives are serious. Here are a few:

1) MV rectifiers that are new or have been laid on their side for some reason need to have their filaments run for 20-30 minutes without HV applied to make sure that the mercury is vaporized and out of the filament and other tube electrodes where it can cause internal shorts.

2) Under voltage on the filament (like only 10%) can cause immediate tube failure because of inadequate mercury vaporisation.

3) All MV rectifiers need to have their filaments warmed up for 45-60 seconds before HV is applied to assure that the mercury is vaporized. Failure to ensure adequate warm up time can cause back bombardment of the filament and destroy the oxide coating in a matter of minutes. This means you will need a separate filament supply with a switch or time-delay relay.

4) Mercury vapor rectifiers are prone to "arc-over" or "flash-over" caused by under or over vaporization problems and just random weirdness. If you have never seen an MV rectifier flash over just wait, if you are using them, and eventually it will happen. Tube implosions are very rare (caused by disentegrating and flying pieces of tube elements) but that was one of the reasons MV rectifiers were frequently in cages. Most big transmitters always had a spare rectifier that could be switched in when one of the MV tubes arced-over. It will scare the pants off you when it happens.

5) MV rectifiers, because of the gas plasma, can generate huge amounts of broadband noise that need to be filtered out by special "hash chokes" in the anode leads and by invasive bypassing. This doesn't always happen but it does occur frequently. This was another reason that they were in shielded cages.

6) MV rectifiers are temperature sensitive and need to be operated over a realtively narrow range.

I know an 83 is a long way from an 870B but it still is subject to the same problems on a smaller scale. I have seen some new audio amplifiers using 866A rectifiers and there is no legitimate reason to use them. If you must use a gas rectifier use a 3B28 instead. This xenon-filled equivalent of the 866 was invented just to circumvent all the problems with MV tube.

Don't use MV....they are one family of tubes that deserve to be forgotten.

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