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Old tubes, and specifically RCA 866-A rectifiers ... any good?

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J. Aaron Holmes

Sun Dec 31 2006, 04:46AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

It happened that I was talking to my father about tube coils the other day, and he reminded me of a rather significant collection of old tubes that he had just sitting in a box somewhere in the house. We searched a bit and finally found it. WOW! Quite a few neat-looking old tubes! Most are NOS in original boxes. Most are RCA brand. In many cases, the boxes gave no indication of what was inside, and I had to pull out the tubes to read the model numbers. There were very few that I recognized. In fact, there was really only one: 866-A.

I knew right away that this was a mercury vapor rectifier tube, as I'd seen these used in some old ham linear amplifiers. Turns out the big box contained FIVE of this particular tube! But while they are clearly unused, I'm thinking they may be no good. Reason: There are tiny beads of condensed mercury all over the insides of the tubes. Hrmph

Best guess is that this is just a sign of exceeding the shelf life of these tubes; some of the boxes (not the 866-A's) had 1950's and 60's dates on them! Any thoughts? Shall I plug one in and run the filament for a few hours and see if the mercury re-vaporizes? Has anybody seen anything like this before? Very strange (to me, at least)... Anyway, I'd love an excuse to somehow use these in a tube coil project, since I remember them looking extremely cool when operating (a cool-looking soft blue mist fills the top of the tube). Unfortunately, the tubes have warnings on them about X-rays

.

There are also a few larger triodes (guessing by number of pins) which were out-of-box and also completely lacking in identifying markings. Might be good for some smallish tube coils, if I could figure out what the heck they are. If anybody wants to try their hand at identifying them by picture alone (I'd be really impressed

) I'll be happy to post a few.

Regards,
Aaron, N7OE

cbfull

Sun Dec 31 2006, 08:33AM

Registered Member #187 Joined: Thu Feb 16 2006, 02:54PM
Location: Central Ohio
Posts: 140

I am not sure about the rectifier tube specifically, but generally the murcury condenses and collects on the filaments after power is cutoff, and if they are stored or moved around, the mercury can shake off of the filament. You can try powering up the filament(s) for a few hours and keep checking on progress.

You might be able to vaporize all of the mercury but it may take a full bake, and I don't know if that can be done to the consumerized product without damaging it. The tubes should be stored carefully, there is likely enough mercury in all of those to require a HazMat cleanup (by modern standards). Plenty of panic out there and most people don't yet know why it's bad to spill.

I would google as much as I could about "mercury" and "lamp", "bead", "bake"

You might find exactly the information you need, if you know what terms to search.

If you post a few pics of the triodes I'll bet a lot of people here might recognize a few for you.

Steve Conner

Sun Dec 31 2006, 02:00PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The condensed mercury is normal, it doesn't mean they've gone stale or anything. i think I've seen these used in tube hi-fi amps, so you could sell them to audiophools

J. Aaron Holmes

Sun Dec 31 2006, 11:17PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

cbfull wrote ...

The tubes should be stored carefully, there is likely enough mercury in all of those to require a HazMat cleanup (by modern standards). Plenty of panic out there and most people don't yet know why it's bad to spill.

I've got a half gallon of mercury in a large glass jar sitting in the corner of the garage. I'll be real upset when I spill *that*

Well, I found enough data to know how to hook up the filaments on these 866A's, so I'll see about baking them a bit. More scrounging in the box last night turned up a couple of NOS 3B28's, which also appear to be rectifiers, though they're called "gas rectifiers", whatever that means (haven't done enough Googling to find out--perhaps they're filled with some noble gas?)

I also found a Western Electric 715A, which is a radar modulator tube (also described in a few places as a "pulse power tetrode") with a 15kV plate voltage rating! The plate dissapation rating is only 60W, though

What kind of tube coil could be made with this odd animal? Small and very HV, I s'pose...

Thanks guys!
Aaron

PS: If an audiophile wants to tell me what he is looking for, I might be willing to part with a few of these for the cost of shipping only, but no guarantees that they'll work! They're awfully old.

...

Mon Jan 01 2007, 12:27AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I agree that beads of Hg are normal. Just get it hot enough to vaporise them and life should be fine.

As to that radar tube, it is designed for very short pulses of high power. Like some type of DRVTTC

But with 60w of plate dissipation you will be limited to a few inches of spark.

J. Aaron Holmes

Tue Jan 02 2007, 08:30PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Aren't all VTTC's "DR" already?

...but certainly liberal use of staccato would be in order for any respectable runs with a 60W rating, I think! There are some beefier successors to the 715-A (i.e.: -C, -D) that look much better, as they have a few 100's of W in the plate rating. I've heard that tetrodes and pentodes, etc., (anything other than tiodes) are a trick to get working correctly in VTTCs though, so being a Toob Noob, I'm inclined to steer clear of these tubes for now.

Regards,
Aaron, N7OE

Simon Barsinister

Fri Jan 05 2007, 04:35AM

Registered Member #116 Joined: Fri Feb 10 2006, 03:19AM
Location: Erie Pa, USA
Posts: 29

Those 715-A tubes are 60 W avg....But what about the 10 amp pulses at 14 KV it will switch. A good candidate for class E.

J. Aaron Holmes

Fri Jan 05 2007, 04:46AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

10A, eh? Where is this information? I searched high and low for 715-A information, but found only tidbits and nothing official. I found a few sites selling them, but mailing the sellers produced nothing I hadn't already found

Regards,
Aaron, N7OE

...

Fri Jan 05 2007, 07:13AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

here is a 715c datasheet... It won't give you the whole story, but it is a starting point...

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