[TMC] Rectifier replacements

Wed Dec 7 19:28:46 EST 2005

Roy, certainly with care and feeding you can somewhat tame these monsters. 
But why, if you have solid state replaceemnts on hand?  Why go thru all the 
trouble and risk, unl;ess you just like that blue glow (and like resetting 
the breaker)..

For an xmitter that isn't used much, I'd be very leery of keeping the 
mercury vapors in there.  As you noted, most of the gear that uses these 
things was designed for 24/7 where they're relatively content.

I had an 866 flash over in a Glob Queen 400 and luckily the hammy hambone 
who had it before me hadn't replaced the line fuse with a 20 amp job or 
worse, wrapped cigarette pack foile around it.  Saved the power xformer 
anyway.

As you say, YMMV :-)

73 John

----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: <TMC at mailman.qth.net>
Sent: Wednesday, December 07, 2005 10:35 AM
Subject: Re: [TMC] Rectifier replacements

>.
>
> John,
>
> Your mileage may vary.  There are some things that can lead to happy 
> relationships with mercury vapor rectifiers:
>
> Larry is working on a TMC-built transmitter. That means that the power 
> transformer is likely way oversized for the job, and that the B+ system is 
> well-protected for overloads above a very conservative level.  TMC stuff 
> was built to run 24/7/365, and, if something did mis-behave, to come back 
> on line quickly
>
> My fairly limited experience with a couple of TMC GPT-750 transmitters was 
> good. These, by the way, are earlier than the one he's working on, I 
> think, and reportedly built on purpose to be "sailor proof" in all 
> respects.  In one case the 872A's flashed over repeatedly, each time 
> popping the B+ or main circuit breaker.  Apparently we finally got enough 
> time on the things so they settled down and worked just fine after that. 
> Note that the GPT-750 has LOTS of fan power, and that may have kept the 
> tubes from warming properly.  In the second 750 I worked on, the 
> rectifiers worked just fine and never gave any trouble.
>
> I've recently been working on a Johnson Desk Kilowatt which has recently 
> been the prime subject of restoration and repair at W0YVA.  The 872A's in 
> that have given no trouble at all.  They have been run in room temperature 
> conditions at least once a week for some time now.  They have also 
> withstood the insults of a faulty HV choke and modulation transformer.
>
> I have made up in the true tradition of early ham radio a breadboard 872A 
> "Cooker."  It is a piece of board from a shipping pallet equipped with two 
> sockets, a 10 volt 5 amp filament transformer and a line cord. (the tube 
> filaments are in series)  I can set a pair of 872A's to warm up and clear 
> the mercury as I work on the transmitter.  The filament heat alone is 
> plenty in room temperatures to evaporate the mercury well down to the 
> bottom of the envelope.  I have a similar cooker for the 866 types.
>
> If you are like the guy we heard about in Montana whose shack is truly an 
> unheated metal hut out in the back yard, I would not try to run a Valiant 
> with 866A's in it, or any other MV rectifier for that matter.  (The 
> unfortunate fellow has orders from the wife unit to keep the house 
> completely clear of all radios.)
>
> Note that the ratings of the 872A are way above the demands of a KW-1, 
> GPT-750, or Desk Kilowatt, so in all these cases there is plenty of 
> reserve capacity in the tubes (and everything else about the high voltage 
> parts, too).  Very likely not so with the 866A's you have used.  The 866A 
> may be pretty much loafing in the Valiant, but in other transmitters they 
> probably are worked much closer to their limits. Also, the 866A is smaller 
> than the 872A by a lot - and the leakage paths that are likely culprits in 
> flashovers are much shorter.  The reputation the 866A has for being 
> difficult may be well deserved. Fortunately, the 3B28 Xenon tubes are 
> fairly easy to find, unlike the 4B32 upgrade to the 872A.  (I'm looking 
> for some at less than commercial prices.)
>
> I have a small number of the predecessors to the 866A. These are type 866 
> or a similar earlier tube that seems to have gone out of production at the 
> time the 866A was brought out. Most of these old tubes are globe shaped, 
> and likely date from the 1930's.  They all show evidence of disaster: 
> clouds of white filament coating flakes inside, bits of filaments rattling 
> around, severely discolored envelope inside surfaces, bent and discolored 
> plates.  I shudder to think what happened to the chokes and HV 
> transformers during the significant events.
>
> I read in an old tube manual that to diagnose difficulties in mercury 
> vapor rectifiers that are flashing over, wrap each plate lead around an 
> iron nail. After a flashover, the nail will be magnetized in one direction 
> for a working tube due to the net DC current, and not for the one that 
> flashed over (or something like that).  Do not try this at home, please.
>
> Happy rectifying to all,
>
> Roy
>
>
> - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
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