[TMC] Rectifier replacements

Thu Dec 8 14:01:06 EST 2005

> what TMC would have done

  "  WWTMCDO ? "

  I like that. That could make a nice bumper sticker!

de KA4JVY
Mark

--- Sheldon Daitch <sdaitch at ibb.gov> wrote:

> One thought in mind on the subject of mercury vapor rectifiers vs
> replacements
> is to consider  - what TMC would have done, if the equipment was still
> in production?
> 
> Without a doubt, those tube rectifiers would have been replaced in
> production with solid state devices long ago.
> 
> I am having to go from memory, but I think in the TMC transmitters that
> we have in Greenville, all of the mercury vapor rectifiers in the units
> were all changed out many years ago, prior to 1979, when I
> first worked there.  The 10KW PAs were installed as part of the original site
> installation and the 40KW PAs were added some years later, and I
> don't know if they came with the solid state rectifiers or we added
> them later.  I don't have TMC manuals here in the office to
> verify.
> 
> The mercury vapor rectifiers were also long gone, by 1979,
> in the early 1950s Continental 420A transmitters.   I have an old
> 420A manual showing the mercury vapor rectifiers, but nothing
> to show when we changed them out.
> 
> I know the GE transmitters also had solid state rectifiers from the factory
> (early 1960s) and the same for the late 1960s Hughes transmitters
> in Tinang.
> 
> >From a commercial application, we had no problems in changing
> out any of the mercury vapor systems with solid state diodes, simply
> looking at it from a system maintenance and reliability standpoint.  Our
> concern wasn't keeping it like it came from the factory, we wanted to
> make sure it worked every time we turned it on.
> 
> 73
> Sheldon
> WA4MZZ
> 
> w3jn wrote:
> 
> > 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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