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Craig405 3rd February 2006 06:47 PM

Mercury Vapour rectifiers
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i have obtained some really cool mercury vapour rectifiers recently :)

I intend to use them in a tube power amp some day (not yet), they are pretty big.

How long do mercury vapour rectifiers last?
if you cant see much mercury in them is this a problem?

one has its sides covered in mercury but the other seems to have very little in it, (both are still sealed i think / hope).
Any tips on running them up for first time?

here a pic along with the 500 0 500v 70mA transformer that i will run them off, the coin is an english 10p.

(You can see in the pic one is grey inside the other is pretty much see through glass)


anatech 3rd February 2006 06:48 PM

Hi Craig,
Mercury rectifiers need to warm up for about 1/2 hour before use from cold (heaters). They can generate lots of hash in operation, and so are normally mounted inside a metal enclosure. The leads from the supply and transformer need hash filters as well. They are intended to be operated in a vertical plane.

The plus side. A near constant 15 VDC drop from plate to cathode over it's rated load range. So they have better regulation than high vacuum rectifiers.

They are used in industry, and some tube testers (like my Stark 9-66). I am sure you will find them in other places, but not normally in consumer equipment.


Dave Cigna 3rd February 2006 08:52 PM

Specifically, when they've been bouncing around the mercury gets splashed around, so it's important to run the filaments alone for a while - 1/2 hour, an hour, whatever - to allow the mercury to fully heat up. After that, as long as you don't shake them, the mercury will settle harmlessly at the bottom so it's only necessary to let them heat up for a minute or two before you apply HV.

As for how much mercury should be in there, I've seen them with big droplets or hardly any. Don't be surprise if some sticks to the sides or leaves dark splotches. It's nothing to worry about.

You sometimes hear warnings that they produce X Rays. They will if you apply several thousand volts. Actually, pretty much any tube will produce X Rays at that voltage, but not at lower voltages. I've forgotten exactly how much is required, but I'm pretty sure you're safe even at 1500V.

You also hear lots of warnings about noise and hash. I use them and have not been able to detect any hash in the supply (~500V) so I don't know what the fuss is about. I think they're great.

Be careful not to break them because mercury is toxic and once absorbed into your body it's there for ever.

-- Dave

zarniwoop 3rd February 2006 10:50 PM

I have used analytical x-ray instruments with a voltage of 40KV, so I think you won't get any x-rays at a typical audio tube HV potential.

Hg rectifiers seem like kinda of a PITA though, just to see the nice blue glow.

anatech 3rd February 2006 10:51 PM

Hi Dave,
You still need to vapourize the mercury before allowing cathode current to flow. That takes longer than a couple minutes.

They do output hash, not recommended for audio equipment unless shielded.


anatech 3rd February 2006 10:52 PM

Hi zarniwoop,
That might be UV emission.


Dave Cigna 3rd February 2006 11:14 PM


Originally posted by anatech
You still need to vapourize the mercury before allowing cathode current to flow. That takes longer than a couple minutes.
Sure, you need to vaporize enough to fill the tube or it will arc, but with the 866A's that I use one minute is at least twice as long as needed to prevent that. Perhaps at a much higher voltage you might need to wait a little longer, but nothing like half an hour is necessary - provided the tubes haven't been disturbed since the last time they were used.


[B]They do output hash, not recommended for audio equipment unless shielded.
Well, as I said, I can't find the hash even up to 10 MHz or so, and I'm not unfamiliar with that sort of thing. I use them unshielded in audio equipment and I do recommend it; I guess we just see things differently.

-- Dave

Craig405 4th February 2006 01:11 AM

Thanks for replys everyone,

I will give them at least 1/2 hour warm up because i have no idea what they came from or how they have been stored, or even for how long!

It is encouraging to hear that the mercury around the edge of one tube is fine and the lack of mercury in the other is also fine. When i finally get around to implementing these tubes i will watch out for hash, i guess this is in the form of field generated by the arcing? if so it will be at 100hzish for full wave rectified ( i am in uk).

Why would they use a MV rectifier in a tube tester? it seems a bit overkill and exotic.

Still one question remaining how long do they last , i will be drawing a small amount of current through them compared to what they can take i guess (size is large). I hope they will last 10's of years but i have no idea really?

(Hope they look nicer than my regulator tubes when running ;) )


anatech 4th February 2006 01:18 AM

Hi Craig,
What happens if you run them too soon is they may suffer from cathode stripping. They will only arc if you have mercury balls inside the structure, excluding overload conditions.

They use them in tube testers because the load regulation is very good. This maintains the plate voltage for tubes from 6BQ5's to KT88's. Noise is not a concern in this application.

Life on these is extremely long. I bought a spare over ten years ago and the original is still fine. I have a feeling the heater may fail first.


martinab2 4th February 2006 07:35 AM

Your rectifiers look like something from the 872 / 8008 family. Datasheet here which includes a cool little graph of cooking time required for different load currents.

Heater is going to be about 8 - 10 amps at 5v depending on which variant you have - thats 100w for the pair :hot: but then they were designed to pass in excess of an amp at extreme voltages.

RF noise is a much debated topic when it comes to these tubes. I have a few different types and sizes of mercury rectifier tubes mostly because I like the blue glow and watching them warm up!
Although I have never heard any noise, I reluctantly have to admit that my amps sound much nicer with more conventional hard vacuum rectifiers, the MVs introducing a glare and hardness to the sound and some loss of fine detail. (I have experienced similar effects with gas voltage regulators as well) Other people would disagree with me though so give it a try and decide for yourself.

I have a quad of unused 8008a in a box somewhere which were once spares for a transmitter. One of these days I will get round to building a 'party amp' (parallel PP 833's maybe?) Just need to find suitable iron somewhere

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