The AM Forum

I recently acquired a pair of NOS NIB Sylvania 866A mercury vapor rectifiers; I believe they are late 1950s-early 1960s vintage.

Upon conditioning these new tubes (30 minutes with filament voltage only, followed by increasing the HV by 200 volts every 5 minutes until the full DC output of 1250 VDC is reached), I noticed that there is some kind of a flaky deposit that is breaking away from the anode structure. This deposit then internally shorts the tube at high plate voltages and it arcs back. It appears to be some kind of manufacturing defect, and I noticed the same thing in a pair of NOS NIB early 1970s vintage RCA 866As as well.

The pair of late 1950s RCA pulls that I use for the HV supply for my push-pull 845 audio driver does not exhibit this problem; the coating on the anode structure is completely intact with zero flaking after probably 50 years.

Now I know a lot of guys say eliminate the problem entirely with these PITA tubes and go with silicon rectifiers, but I like the vintage blue flame of the mercury vapor tubes in the rig. Form follows function, but these MV rectifiers look ultra cool!

Can anyone recommend a brand of 866As that they have had consistently good luck with? As I stated above, the '50s RCAs seem to be fine, and exhibit none of the flaking of the coating on the anode structure that I noticed with the other tubes.

Thanks & 73,

Bruce

Those 866s are becoming useless.  I would switch to 3B28s if you have the space.  The coating on the anode is imperative for proper operation.

any brand will do. Bust out the glass and install a hunk of vector board with a string of 1000 volt diodes. Look at the series Z of a rectifier tube and ask yourself
why have a resistor in series with your B+ supply.
Save power the earth is getting too hot and Al Gore needs $2K worth of juice a month.

3B28's did the trick for my HB rig. None of the problems of 866's and no arc backs.

If you pay the shipping I will send you a whole shopping cart full of murcury vapor tubes. Many of them are new in boxes. You can then perform your zorch study and report back to the group with your findings.
Skip
K7yoo

Is it worth risking an irreplaceable power tranny to use these miserable things?

I agree with others: go with the 3B28s. They do make a nice purple glow (if that's a concern), especially the initial *thwack!* when you key up. Drop-in replacement, eliminates the warm up after non-use issue along with the arcing. I went to the 4B32s in my transmitter after the 872s started to spaz, quieted things right down. So I swapped out the 866s in another rig with 3B28s, same excellent result.

Yup, I agree with you guys. Bear in mind my comments about the coolosity factor with the MV rectifiers and about form following function. I am obviously not heeding my own advice here!

Another rectifier that I have used in lieu of the 866As in my push-pull class A 845 audio driver is the high-vacuum type 836. No issues with these at all, they are very reliable, have the same size coke bottle ST-style envelope, and drop right in. The 836 has a lower PIV rating, but in the 845 driver PSU running at 1250 VDC, this is not an issue. Don't have the blue flame of the 866As or the purple haze of the 3B28s, though.

I thought more guys would be using the MVs to preserve that vintage look, especially in those older rigs that used them originally, but I guess you have to draw the line in the sand at some point when it comes to safety of the equipment, heat load,  and reliablity issues, etc. No argument there.

To that end, I may go back to the 836s if I can't come up with some additional reliable 866As.

Thanks for all of the suggestions!

73,

Bruce

The 866's and bigger pubes are so pretty to watch when you say a long haaaylow. Or play some really good AC/DC tunage . Dummy load, please.
Use Vr tubes on the screens of a mod tube and watch a little flicker too.
The 3B28 is not a good show.
Camawn you AM radio engineers are not going to fess up to looking at the flickering HV tubes while the station played your favorite song on AM???? I bet it was a pretty sight in a 50KW rig. A 20V1 or 2 is nice to watch.
Fred

Thank you, Fred! A man close to my heart! I was beginning to wonder why I was the only one who wants that blue flame of the MVs in the shack.

Not to mention the orange glow of the plates in the 833A modulator tubes varying with the audio!

This is real radio. Can't get this with a rice box.


When I worked as a part-time broadcast engineer during my college years, there was a Collins 21E that another station had where they never replaced the MV rectifiers with the popular Wilkinson silicon stacks. That was the coolest rig to watch during modulation. Same thing when I visited the old 50 KW WKBW in Buffalo in 1973 (I went to school in Rochester, NY, about 60 miles from Buffalo). They had an old Western Electric rig, and the MV rectifiers in that rig looked like big X-ray tubes. Talk about the coolest thing to see for a 19-year radio ham! I think that rig ran about 15 KV on the plates of the final and modulator tubes, and although the transmitter must have been 30-35 years old at that point, it was in literally as-new condition. I think the engineers used to wax the rig in their spare time; just like being in a fire house.


73,

Bruce

See, I can't even view the rectifiers in my transmitter without defeating the interlocks and shorting bar. We did this when trying to track down a zorching issue that ended up being a loading problem, this is how I know the 4B32s do indeed glow purple. I could easily see it with the bright room lights on, so I bet you'd be able to as well. It's not blue like MV, but hey - it doesn't crap out like MV either. :D

Ok, I'll weigh in here...  I too really love to watch the blue glow of MV rectifiers...especially the blue flicker in a modulator  power supply. I've been using a pair RCA 872s and a pair of RCA 866s in my homebrew pp 250TH X pp 810 rig for about 3 1/2 years now with no problems.  I have a time delay relay that pevents me from transmitting until the MV tubes are well warmed up to make it as easy on them as possible.  I'm sure that the risks and problems associated with the use of MV tubes described by the others that have responded to this post are entirely accurate, however I personally choose to accept the risk for the sake of experience the nostalgia that only MV can deliver.  Ultimately it becomes a personal decision but I would advise that to minimize the risk, use only NOS with a liberal break in period. 

Best of luck!

Brian - W8KHZ

I also use MV's in my 20V-3 and I think they look very cool. It uses a pair of 575A's and a pair of 866's (RCA) plus I think a 5U4. The Collins engineers must have thought the same thing because they put those babies front and center in the picture window.