[ARC5] "Noodling" About Tubes? Re-activating.,..

[Kenneth G. Gordon]

On 22 Oct 2013 at 18:13, wa5jci at flash.net wrote:

> If I had tubes like these I'd rig up a jig and run the filament in
> each tube for a couple days to see if that would do any good before I
> gave up on them. Might not make any difference but maybe it could?
> 
> de Pete  WA5JCI

I agree, Pete.

There are (or were) at least three common methods of reactivating 
thoriated-tungsten filamented tubes. Which method you can use which will 
result in increased emission depends a lot on how the filaments are made.

A little info on thoriated-tungsten filaments, as I know it, although 
undoubtedly there is someone here who can correct and fill in what I don't 
mention: but here goes anyway.

Basically, there is some thorium alloyed (if that is the proper word) with the 
tungsten out of which filaments are made. This thorium vastly increases the 
numbers of electrons emitted by a hot tungsten filament. There is then a 
layer of thorium, about one molecule thick, on the surface of the tungsten 
wire.

(There is another part, or a different kind, of the process in which acetalyne 
gas is used to "carburize" the filament, but I know almost nothing about this 
part of it.)

Almost all present-day transmitting tubes use thoriated-tungsten filaments 
since such a tube can be run at much reduced voltage thereby promoting 
longer life.

After so many hours of running, the thorium that is near the surface of the 
filament, basically, as I said, about one molecule thick, is pretty much used 
up and thus the filament emission drops off, sometimes to pretty close to 
nothing.

If one heats the filament above its normal operating temperature with NO 
other tube voltages applied for a certain amount of time, the thorium that is 
still in the filament, but "deeper" can be brought to the surface, "boiled" to the 
surface, thus "reactivating" the tube.

In my own case, I once read in an old Marine Radio Handbook that certain 
types of thoriated-tungsten filamented transmitting tubes can be reactivated 
by the following process:

1) Apply 2.5 times the normal rated filament voltage to the tube for one 
minute. (For a tube with a 5 volt filament, this means 12.5 V....with 
corresponding current.)

2) Reduce the filament voltage to 1.5 times the rated filament voltage and 
hold for one hour.

3) Reduce the filament voltage to normal and test.

4) If emission has not returned to normal, repeat the process.

5) If after the second run, emission has not returned to normal, discard the 
tube

As I said above, there are basically three methods to accomplish this 
reactivation process, and which method will work depends on how the tube is 
constructed and especially, how the filament is constructed.

I will dig the other two methods out of my files and post those here if anyone 
is interested.

A bit of history of my experiences:

At one time I was given a dozen 304TLs which had been used as 
shunt-regulators in a chemistry research department's NMR. They had been 
used very hard and were completely "flat" exhibiting no plate current at all 
with full 3 KV plate voltage applied and no grid voltage. Much of the glass 
where it could "see" the filament had been "burnt" dark brown.

I used the above process on all of them, and every single one was returned 
to full useful emission. I used one and then two in my own modified BC-610 
"linear" amp when I was running phone patches for AFMARS, and gave 
others to friends. They all had equally good luck with the reactivated tubes, 
and I and several others were still using those same tubes several years 
later.

All of the tubes acted as though they were new or very lightly used.

However, recently I tried the same method with some Chinese 572Bs which 
had gone "flat". The above process did not work with those tubes. However, 
it DID work with a couple of old Cetron 572Bs I had, although in their case, 
the return to full emission didn't last more than a year.

I was then advised by a helpful person at one of the old tube manufacturers 
that with the Chinese 572B, one should raise the filament voltage to 
something like 50% to 75% greater than normal voltage, and hold it there for 
about an hour, then reduce to normal and test.

The way I felt about the 304TLs was that they were flat anyway, so I sure 
couldn't lose anything by trying this rather drastic method. In the case of the 
304TLs it worked stupedously well....but then again 304TLs are noted for 
their tremendous filament "reserve" anyway.

In Dave's case, since one or more of his 211s are flat anyway, using the 
above treatment on them cannot make the situation worse.

In fact, if Dave (or anyone else) would care to send me one or more flat 
211s, I will try any of the three methods I know of, beginning with the 
"kindest" one first, to see if we can reactivate them. It sure can't hurt.

Ken W7EKB