[ARC5] OK Smart People - Mystery 211s

[Kenneth G. Gordon]

On 16 Jun 2012 at 10:35, Neil wrote:

> > Surely this will lead to severe grid damage?
> 
> Might: depending on the tube. 
> 
> Ken W7EKB
> 
> Ken,
> I'm not sure if your original post was meant to be a tongue in cheek
> way of disposing of a tube that had resisted all attempts to
> rejuvenate it, but for those unfamiliar with tubes, be aware that
> making the grid DC positive with respect to the filament or cathode
> virtually guarantees destruction of the grid.

No. It was not "tongue in cheek". It has worked quite well for 572Bs....BUT 
one must be VERY careful when doing it!!!!

Thus 45 VDC, NOT 2100 VC or even 1250 VDC (211s) on the anode.

In fact, this method would probably work just as well if the grid were 
connected to the filament instead of the plate.

Connecting the grid directly to the plate in a triode was done in the past, 
although most certainly not as a matter of course.

As an example (although this was NOT done with a LARGE RF power 
triode!) in the Heathkit C-3 condenser checker, the grid is connected to the 
plate and the tube, a 1626 power triode, is used as a rectifier at 500 VDC 
output. I have taken such 1626s after they have been used there, and 
plugged them into an ARC-5 transmitter, where they are commonly used as 
a VFO tube, and they worked just as well as a new tube. The grids were 
never damaged.

In fact, I wrote an article for Electric Radio magazine some time ago on the 
restoration of the Heathkit C-3 so I am VERY familiar with that particular 
tester. I have two here and use them regularly.

I only suggested this method as a POSSIBLE way to reactivate the 211s, 
having never yet used it on a 211.

I think it would work very well if proper precautions were taken. 

If one was worried about grid dissipation, then a milliameter could be 
connected between the grid and the plate, the current or current-difference 
could be monitored, voltage measured, and PD calculated.

I don't see a problem.

Ken W7EKB

P.S. All modern 572Bs I have tried to reactivate so far have proven to have 
VERY "iffy" filament reserve: say, little to none. I have used any of several 
methods. None were satisfactory. 304TLs and many of the older power 
triodes had/have filaments with tremendous reserve capacity.