The AM Forum

In the process of replacing the HV contactor in my power supply, I managed to twist the tube off the base on one of the 866A's.

No big deal as I have a whole box of spares I bought for just such an occasion.

Unfortunately, on this occasion, they all seem to have some sort of problem; like junk floating around, loose cap, loose base, gassy look, etc.

After trying a few with no luck, I tried one of the very strange looking tubes and it seems to work.

Instead of the standard 811 sort of envelope, it is pear shaped and looks like something right out of Edison's light bulb lab.  It is marked 866A but I have never seen one like this.

The anode is a dome instead of the more flattened disk on the common ones and it complete covers the top half of the cathode.

Not only is the filament invisible giving no clues but there is no blue glow when it is running.  I ran the rig up to full power and it is as dark as a black hole and the other normal one is a lovely blue.

Anyone know anything about this?

Jack K9ACT

All the original,  early Pre-WW2 866's and 866-A's were shaped that way, just like early receiving tubes like 45's, 47's, 210's, etc.

I have seen some 866(A)'s with a small receiving-tube or 807 sized plate cap instead of the standard large size plate cap, requiring some kind of JS adaptor.

Don't throw away a tube just because the plate cap or base is loose.  I have glued many of them back using JB Weld or "Crystal-clear two-ton" epoxy.  It should work particularly well with 866's, since they don't get extremely hot like power output tubes.  Depending on the type of plate cap connector used, you may not have to do anything about the loose plate cap if the connector is the type where you squeeze the clamp to disengage the connector from the cap, without having to stress the cap.  Just be careful not to twist or pull on the cap. You don't want to break the wire off right at the glass envelope seal.

The same with the base.  You need to glue it back down if you use the twist-lock sockets that employ the bayonet pin.  If the sockets are just the normal plug-in, pull-out type, you can (carefully!) insert the tube and remove it by pulling on the bakelite base instead of the glass envelope.  Another quick fix is to use fibreglass "strapping tape" and wrap it round the top of the bakelite base so that half the width adheres to the  glass.  That should hold for a while until you can do the more permanent fix or locate another tube.

But it's such an easy fix, I'd go ahead and re-glue the tube, unless I needed it immediately for emergency replacement.

First of all, it is an A version and the attached pic from a German web site corroborates this.

I ended up with a great learning experience about how tubes are made.  I resoldered and epoxied the cap on one and did the same with the base on the other.  The rig is now up and running fine.

The older style that did not glow blue apparently had some problem that was not obvious just by making sure the filament worked.

The good news is that the off and on stench of burning Bakelite that has been plaguing this rig for two years is now gone.  It was the contactor in the hv primary.

Not sure what happened to the pic but if you click on it, you will see it.

js

The ones with the completely exposed filament are my favorite. Getting kind of rare.

And lets not forget the 966.
Gary...WZ1M

Just an 866(A) by a manufacturer that decided to use their own non-standard type number.  I seem to recall seeing ads in older magazines by some company that advertised 566's.

The 211 was released as a 311 by some manufacturers.