[AMRadio] Testing tubes: Cathode Interface

[DOXEMF at aol.com]

In a message dated 10/2/03 11:11:33 AM Eastern Daylight Time, 
merz.ds at mellon.com writes:


> Could this be a form of cathode interface problem? Cathode interface is a 
> problem sometimes seen when old tubes are used in scope vertical amplifiers 
> with fast risetimes. Something happens to the tubes that causes the actual rise 
> time to deteriorate as the tubes age. The "something" that happens is called 
> cathode interface. I have observed this effect personally on old Tek scopes 
> so I know it is real and apparently widely recognized. But I don't know if 
> anyone has done any work to see if cathode interface has an impact in other 
> tube applications like audio amplifiers.
> 
> Anyway, I just thought I'd throw that idea into the discussion. I am no 
> expert and my ears prefer solid state audio. 
> 
> 73, Don Merz, N3RHT
> 
> 

Don,
Thanks for the reminder on that. I have several of the 500 series Tek
mainframes and have read the advisory notices on cathode interface
problems. Seems it was pointed at the 6DJ8/6922 tubes in particular
in the sections you mentioned.
I forgot completely about that trait and will look at the possibility
that the problem lies there. Makes sense. Reduced rise time indicates
a loss in H. F. response along with the inability to reproduce proper 
harmonic relationships.
Enjoy,
Bill KB3DKS


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