What MV rectifier is next up from the 673 or 575 /A ?

[Opcom]

What MV rectifier is the next size or two up from the 673?

The 14KV peak voltage is on the ragged edge of the 575/673 which is rated 15KV. Current is OK, about half the maximum average. Also, under light loads and "abusive" stepped loads the reverse voltage can go to 20KV.

No real reason to avoid solid state other than the appreciation of the aesthetic beauty inherent in a gaseous discharge.

A 3CX3000 does not glow that I have seen. Something has to be put up-front to glow so the amplifier will be nice-looking.

What did 'big' transmitters with very high voltages use? There must have been a larger rectifier or did they use them in series?

[W7TFO]

The 857B gets my vote:

http://frank.pocnet.net/sheets/026/8/857B.pdf

Not the biggest, but next in line.

73DG

[Opcom]

Thanks, that's a monster of a tube! That one has huge current, but is 10KV with convection cooling and 22KV with forced air.  I don't understand why precisely but maybe it is like other MV rectifiers in that as the current rises so does the temperature and with increased temp. beyond a certain point the PIV decreases. The 857B might be very safe at 1-2A.

I say this because experienced something like it, where I tried using 872's:
5KV @1.25A @ 20-70 deg C
10KV @1.25A @ 20-60 deg C

but the current was well into the 872 ratings @600mA and had problems at high PIV, and the problems disappeared when I put in 673's:
10KV @1.75A @ 20-60 deg C
15KV @1.5A @ 20-50 deg C

The Machlett 857B is rated:
10KV @10A @ 25-60 deg C
22KV @10A @ 30-40 dec C

Flashovers would be extremely unlikely with that much PIV rating and so light a current load (15% of max) up to a point. The challenge might be to restrict the condensed mercury temperature range to the cooler end of the scale. Mounting could be interesting as the base has leads. Maybe a clamp around the metal base ring?

Those tubes are likely to be hard to find for a reasonable cost but good things come to him that waits. Surplus Sales wants $500. Too much.

[W7TFO]

A little more reasonable:

http://www.ebay.com/itm/250779165358?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

I can smell the fried rectums now...

These look a lot like those in my WECo 405 5kW AM rig.


73DG

[Opcom]

a bit short on volts. Not sure it is the same. Can you see any Hg inside?

Russian Military Rectifier Tube VG-237 (ВГ-237)

Filament voltage - 5 V
Filament current - 22 A
Ignition voltage - 25 V
Maximum average rectified current - 3,5 A
Maximum pulse rectified current - 10 A
Drop in the voltage - 16 V
Maximum permissible amplitude of inverse voltage - 10000 V
warm-up time in the operation - 5 min
Warm-up time after storage - 1,5 h
Period of service - 2000 h

[Opcom]

more pics.

also on the TT-4 for 20KV, but no data sheets:
6894 - 20KV/2.5A @quadrature -$165.00
6895 - 20KV/2.5A @quadrature
these are going to be light on current for a single phase user @ 1.5A


On that Russian tube, It does not look like mercury, the voltage drop is too low to be vacuum, so i bet it is Xenon or?. I asked the seller.
Your message about RARE Russia Military Rectifier Tube VG-237 ВГ-237 1968y has been sent to combat007370. You should expect a response within 1-2 business days.

[KE6DF]

6895's aren't even that expensive.

Here are some used ones on epay:

http://www.ebay.com/itm/westinghouse-6895-vacuum-tubes-lot-/260663519810?pt=Vintage_Electronics_R2&hash=item3cb0c1aa42

[Opcom]

6895's aren't even that expensive.

Here are some used ones on epay:

http://www.ebay.com/itm/westinghouse-6895-vacuum-tubes-lot-/260663519810?pt=Vintage_Electronics_R2&hash=item3cb0c1aa42

Those are pretty cool. Seeing the long pins, I wonder if the short pin base to the upper right might be the 6894, like the 8008 and the 872 have different pins but are the same otherwise.

But still the current on the 6895 and 6894 is probably too low of a rating for the application which is 1.6A with a possible 2A peak. The price OK for a lot of 4 used tubes but after discovering the quadrature requirement to sustain the rated current, they won't do.
(pic of four used 6895 rectifiers)

I have to admit I like the VG-237. A pity about the low 10KV rating, it just won't work even if it is pretty.
(pics of another VG-237)

[Opcom]

Check the differences, here is the 857B / GL-266-B

There is also a AMPEREX 857B TUBE on ebay, the ad says "OR GENERAL ELECTRIC GL-266-B", so that's a lead on a cross.
(It is used and priced $300 which I believe is too much but others may differ.)

So this is a really nice looking tube, the 22KV 10A (average current)  originally suggested. So far it looks like the best candidate. I can imagine the glow in that baby with a load on it. That's the thing- the 673's I use in another TX work great but they don't make much of a glow even at 600-700mA because of the way the cathode housing is shielded. 872's really light up in the same socket but they also blow up unless I crank the voltage down a ways.

[AB9ZG]

The 869 is a bit smaller and good for 20kv, 2.5A.

[KE6DF]

Those are pretty cool. Seeing the long pins, I wonder if the short pin base to the upper right might be the 6894, like the 8008 and the 872 have different pins but are the same otherwise.

That's it according to the datasheet.

The 6894 uses the jumbo socket like the 872a, and the 6895 has the super jumbo like the 8008.

Dave

[W7TFO]

Jumbo is also known as the '50-Watt base"

Super jumbo is also known as the "Westinghouse base"

73DG

[Opcom]

It begs part II of the question because of the scarcity of types for very high voltages:

Has anyone ever placed MV tubes in series or seen it done? Is it a competent thing to do? Why or why not?

I believe yes it can be done and at first look would put equalizing resistors across the stack to divide up the PIV as well as start the tubes at the same time.

I have a good supply of 575A's so why not use them?.


Elmer: What are you doing in the wefwigerator you scwewey wabbit?
Bugs: Just Westing. It's a Westinghouse isn't it?

[K3YA]

MV rectifiers behind  a glass window sure look fine.

However it's quite possible that obtaining a filament transformer for a pair of those big old MV tubes will be more difficult, and expensive than finding the tubes themselves.

I have a pair of 575's in my "Kilowatt" transmitter that will pass an amp on voice peaks.  Due to the tubes construction the blue glow isn't nearly as bright or cool looking as a pair of 866's working hard.  My old Valiant used to memorize me with the light flickering when I talked.
 

[Ed/KB1HYS]

Transformers for MV fils aren't hard. I hand wound mine for my four 866 in bridge configuration.  Granted my B+ isn't monstrously high, but still. the low turns count does make it easy.

[Opcom]

In a pinch, MOT iron with a secondary of 14 ga. outdoor luminous sign wire could be done. It would not be as satisfying as winding the whole transformer but it could get the job done.

There is a new trans. here from PWD that is designed for bridge use having two 10A windings and one 20A, all CT. His products are usually well-insulated.