[ARC5] OK Smart People - Mystery 211s

[Kenneth G. Gordon]

On 15 Jun 2012 at 7:16, David Stinson wrote:

> Just finished testing my stash of 211 transmit tubes 
> by putting them in the BC-375.  
> More than half were bad- mostly gassy.  sniff....

Hmmm...from the below, not necessarily....

> Anyways, here's a mystery:  A few would not 
> draw much current or put out much power in the
> PA stage, but they seem to work just fine
> in the Oscillator stage.  If this was low emission,
> I'd think they'd fail in either stage.

No.

>  As a result,
> I've got four tubes marked "OSC ONLY."
> 
> What's up with this?

It COULD be gas, as Jim mentioned, and, again, as Jim mentioned it MAY 
be able to be fixed. As I remember it, his way is about as good as any.

However, in my experience, one way to tell if the tube is gassy is to apply 
normal voltage and whatever current can be produced and look into the tube 
in the area between the plate and the other elements: i.e., INSIDE the plate 
structure. If there is a light-colored blueish glow from there, then the problem 
IS gas.

If there is a darker blue glow outside the plate structure, most often against 
the glass, this is an indication of a very hard vacuum...but you knew this.

The other problem with symptoms very much like those you mention is, 
simply, low filament emission. This most often does NOT effect an oscillator, 
since oscillators, even in the BC-375, are not called upon to output 
substantial power and low emission will most often not be a factor here.

I have seen tubes which were so weak that they would not even register at 
all on a tube tester that continued to function quite well as an oscillator.

If it IS low emission, and IFF the filament was originally "substantial", and IFF 
the filament is a thoriated-tungsten type (which most of those were), then the 
normal reactivation procedure, although varying somewhat, works like this:

DO NOT apply plate or grid voltage!

Apply 2.5 times normal filament voltage to the tube for 1 minute.

Reduce to 1.5 times normal filament voltage and hold for 1 hour.

Test.

If still not up to normal, repeat.

If still not up to normal after this second go-around, discard.

What this process does is to heat the filament to the point that any excess 
thorium within the filament wires is "boiled" to the surface and deposited 
there, reactivating the emission.

I used this method on a batch of a dozen 304TLs which were so flat they 
wouldn't draw even 1 mA at a plate voltage of 3200 VDC with no grid voltage 
at all. When finished, they worked as well as a new tube for years 
afterwards.

There is another way to do this, mostly for triodes like the 572B, but it may 
work with the 211, which method is a bit less "stressful": 

Connect the grid to the plate, and apply normal filament voltage to the tube. 
Insert a millimeter in the plate voltage lead, and apply something like 45 volts 
to the anode, or whatever voltage is necessary to achieve about 250 mA of 
plate current. Watch the meter. At some point in time, plate current will start 
to rise, and may rise very rapidly. At this point, shut off the anode voltage. 
The tube should be ready to use.

Anyway, let us know what transpires.

Ken W7EKB