Rectifier Replacements for 866s

[ka4koe]

Hello again.

My Valiant has the pretty but maligned original 866 rectifier tubes. I received a pair of Taylor 866AS-1 replacements in the deal.

Is anyone familiar with this type tube? I know that some rectifier replacements are not quite "plug and play" due to increased HV.

Thanks

Philip
KA4KOE

[WQ9E]

The voltage drop across the original MV rectifier is very small so unlike when replacing high vacuum rectifiers with SS you don't need to be concerned about adding a resistor to drop the voltage.

[Opcom]

The first time in service, be sure to let mercury vapor rectifiers warm up for 20-30  minutes before switching on the high voltage, and any time after they have been tilted over or shaken.

The mercury can be splattered all over the insides if they are tilted or shaken, and it has to be completely re-evaporated from all element surfaces before they are used. If it is not done, the tubes will be likely to arc internally when high voltage is applied and can be damaged which would be a pity.

Inside the tube, the filament constantly evaporates the mercury, and it condenses on the cooler glass envelope, not on the hotter internal elements. More or less of it will condense on the glass depending on ambient temperature and loading. The remaining mercury is an invisible vapor until current is passed and it glows.

Once this initial treatment has been done, a shorter warm up of 30 seconds is usually fine, refer to the RCA transmitting tube manual for complete details about the care of MV rectifiers. Each datasheet in there will also specify the warmup time for that type. TT-4 can be downloaded here and the MV rectifier advice starts on page 65:
http://www.bunkerofdoom.com/tube/man/tt4.pdf

You will have no reason at all to malign them if you care for them properly. They are very reliable when used correctly. I have used them in every possible socket for 20 years and have never ruined one.

The 3B28, a xenon filled 866 replacement rectifier, needs no real warmup time but it has a less-visible white-ish glow. Not as appealing as the mercury tubes.

[ka4koe]

Yes, they are indeed pretty! Is there any advantage in using a solid state replacement, ie stiffer HV bus? The usual advantages I am aware of are less draw on the filament supply, overall ruggedness, etc.

I assume a stiffer HV bus may result in higher power output?

PHILIP

[WQ9E]

There is no significant difference in output between the MV and SS equipped Valiant assuming the MV rectifiers are in good shape.  The voltage drop across MV rectifiers is low and varies little with current over the rated current range of the tube.

In the Valiant make sure you check the wiring to the 866A cathodes, early production used regular hookup wire and it does fail from the high voltage present.  Later production used proper wire, early can either be replaced or sleeved.  Of course if you use SS replacements just disconnect the 5 volt leads from the sockets and tape them off.

There is no warm-up timer in the Valiant to allow the 866A cathodes to come to temperature before plate voltage can be applied.  If the mic PTT switch is accidentally engaged, the HV switch was placed on, or there is a failure in the PTT circuitry then HV can be applied immediately with power-up and this is a bad situation with MV rectifiers.

I left MV rectifiers in one of my Valiants for looks but one morning when I turned it on there was a nasty buzzing, flash, and blown fuse.  A bypass cap on the PTT line had shorted immediately applying HV to the cold 866A tubes.  Fortunately the plate iron survived but I took that as a wake-up call and got rid of the MV rectifiers.  A few months previously another Valiant owner was not so lucky when he turned his Valiant on with the PTT locked.  He had a cat that liked to "march" on the microphone and apparently the cat had pushed down the locking button on the microphone base while the transmitter was off.  In his case one half of the HV transformer secondary opened up under the load.

[K5UJ]

I've wondered a few times if the Valiant really needed the mercs and if it could have been designed with regular vacuum rectifiers.

[KM1H]

The 866 has a higher PIV than say a pair of 5R4's with sections in parallel that were used in other rigs.
Besides the 3B28 which is an ideal swap for reliability (who the heck looks at an 866 inside a cabinet?) there are a couple of plug in SS replacements that solve all the problems. The voltage drop difference between all of them is about 5V.

Carl

[K5UJ]

Thanks.  The PIV.  I didn't think about that.  I also like the 866 look but would not use them if they're hidden away.  I'd go with Xenon.  I have some idea solid state is a problem with choke input because of "switching spikes" but don't remember much else about it.

[Pete, WA2CWA]

Like Carl, I don't understand obsession with mercury vapor tubes. Go with the 3B28's or solid-state replacements. If inside the cabinet undulating lights are your thing when you are on the air, hang some neon lamps inside the cabinet.

[ka4koe]

Would that be similar to the blue lights under heavily-lowered vehicles, with the very loud "boomp boomp" music?

[W7TFO]

Like Carl, I don't understand obsession with mercury vapor tubes.

I don't understand the obsession with silicon...

73GD

[Pete, WA2CWA]

Would that be similar to the blue lights under heavily-lowered vehicles, with the very loud "boomp boomp" music?

Yep, and just as useful or useless, depending on your motivation.

[K5UJ]

Here's what's to like about da mercs:

http://w3np.com/images/meissner/866glow493.jpg

If you don't dig four 866s then you may as well run a class D rig 

[W2PFY]

Like Carl, I don't understand obsession with mercury vapor tubes. Go with the 3B28's or solid-state replacements.

I agree in part, if you can't see them, use
something that will give you the least amount of trouble. But if you are building something for show, then it is nice to light things up like a Christmas tree.  I think it you were trying to motivate an aspiring ham you would be hard pressed to excite him by saying look at the string of diodes I have in there and then start going through the piv etc. of the devices. OTOH like the picture of my Westinghouse rectifier below, a picture is worth a thousand words!  Some engineer types will not agree

[KE6DF]

836's are a high vacuum plug-and-play replacement for 866a's -- up to 5Kv PIV.

They have higher internal voltage drop than MV or XE tubes, but less than 5R4's.

I've used them way back when I was first a ham, and found them very reliable.

Dave

[ka4koe]

"Some engineer types will not agree"

I am a licensed, certificated/certifiable, elektickel injuneer. I like purdy things.

Philip Neidlinger, PE 25112 (GA)

[W2VW]

If someone fully understands the liabilities of olde rectifiers and opts for the look more power to them.

I always save mine up to give away to those people.

[w1vtp]

Use a pair of these.  Problem solved.  Backlight a pair of unused 866a's with some blue LEDs if you must have the effect.

Al

[ka4koe]

Here are the Taylor's I received. They are labeled "866AS". The boxes are labeled "866AS-1".

Philip

[Todd, KA1KAQ]

The issue with MV rectifiers is their tendency to arc as they age. They're fine when new. But it's easy enough to drop in 3B28s and have a much safer tube that doesn't have the warm up issues when they've been idle for long periods, and also still look nice. More purple than blue. I use 4B32s in place of the 872s in my rig and as soon as I swapped them out, fuses stopped blowing. Until something else went bad. 

[ka4koe]

Moot point as of now. They must have known I was fixing to swap them out with the SS. Both went, heard a zap, smelled some ozone. Very momentary. Nothing appears burned underneath.

Running some checks now. Hope nothing is fried.

[ka4koe]

Power output normal but now almost 200W. Secondary arcing must have been HV due to improper coupling setting.

Coupling with dummy load about 4 coarse, 10 fine. Grid current 7.5 mils, plate current close to 450 mA.

All appears well.

What a way to end a discussion by the lil boogers going teats up!

Philip

[KM1H]

Thanks.  The PIV.  I didn't think about that.  I also like the 866 look but would not use them if they're hidden away.  I'd go with Xenon.  I have some idea solid state is a problem with choke input because of "switching spikes" but don't remember much else about it.

The problem is there also with tubes, toss a scope on any choke PS and it is most visible on CW or PTT. While oil caps are good at self healing those spikes can seriously aggravate electrolytics.

A cure, not 100% but does the job, is a .005 disc across each SS string and another to ground at the output. For tubes and chokes a cap to ground at the choke output helps as well as using more voltage oferhead in the electrolytics.

836's are a high vacuum plug-and-play replacement for 866a's -- up to 5Kv PIV.

But they still have a minimum 40 second warmup time, due to being an indirectly heated tube, which is much longer than a regularly used 866A in a room temp enviroment. I have a pair in one of my HT-9's since the plate switch is right in front of my face.

Carl