[Amps] 8877 Test

[R.Measures]

On Feb 14, 2005, at 7:38 AM, TexasRF at aol.com wrote:

> Hi alex and all,
> This is a true story: About a year ago I was given an unused 8877 that 
> was
> about 20 years old. After all the warnings about firing up old tubes, 
> I used my
>  newly acquired solid state 0 to 10kv module and trusty Triplett 630 
> VOM to
> test  it for leakage. It showed leakage from plate to grid beginning 
> at about
> 600vdc.

Every tube ever made has leakage at 600VDC, the question is how many nA 
at how many V ?

> I ran the filament only for about a week, testing leakage daily. The
> leakage  voltage slowly increased day by day and after a week had 
> reached about
> 5kv.

Leakage V does not increase or decrease -- leakage current does.

>  I  know really good tubes are better than this but decided to stop the
> burn  in.
>
> At this point it was installed in a 2m PA and fired up at half plate
> voltage, about 2000vdc. It seemed to operate ok at that voltage so 
> after about  an
> hour the plate voltage was increased to 3000vdc and then the normal 
> 4000vdc.
> There was an immediate "incident" that blew the 2A fuse in the B+ 
> circuit. The
> fuse was replaced and the amp has been performing perfectly since then.
>
> I am tempted to retest the tube leakage again but it is just way too 
> much
> trouble to disassemble the cavity to free the tube for a test.
>
> Since then I have tested an old TWT this way and found the same 600vdc
> starting point for leakage. After only one day with the filament only, 
> this tube
> would then go all the way to 8kv before onset of leakage. The tube is 
> rated at
> 4kv so it probably will be ok when I finally get around to testing it 
> with
> rf.
>
> The moral of this? I will always test tubes for leakage, old or new, 
> in the
> future, before use. I can see that it will prevent some of the big 
> KAPOWS!
> seen  in the past.
>
> 73,
> Gerald K5GW
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org