[AMPS] Drake L7 and 'new' Eimac 3-500Z

[Steve Jackson]

Gents:


Just tried to install two new Eimac 3-500Z tubes I bought from two different guys.  One of the tube was new in an unopened Eimac yellow box, with a 1995 date code.  The other was new in an opened box, with a 1993 date code.


Previously, amplifier would not make more than 600W on 40, and went down to 300W on 10 meters.

160 and 80/75 had full output.  Tubes had 1976 date code so I figured they were soft.  But amp worked... well, kinda.  I won't mention the overheating and buzzing power supply!


Now with the newer tubes in it, when I key the amp, the power situation is different.  The amp tunes up (plate dips, loading OK, has gain) but makes funny noises, and still won't put out full power.  Also perhaps notable, the meter lamps dim appreciably when amp is keyed (this didn't happen with old tubes).  According to the schematic, the illumination lamps are across the filament supply.  I did replace the lamps, same type as originals, and non-keyed brilliance is OK.


I ran the new tubes with filaments only -for 6 hours- before applying plate voltage.


Questions:


1.  Do I have a grid-filament short in one of the 'new' tubes?


2.  Does anyone have a full-output, late-date-code Eimac bottle for sale?


3.  Anyone care to comment why the amp might still tune up, have correct idling plate current,     and put power out - if there is such a nasty problem with a tube?



thanks....


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Bay Architecture Lab

Research Triangle Park, NC

(919) 941-1726

sjackson@baynetworks.com

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