>Rich Measures wrote:
>
>> If Pout drops when the amplifier is hot, tank component heating is more
>> likely the reason than a problem with the 3-500Z.
>
>This is a new problem that did not appear before. So I will check for bad
>connections, burned switch contacts etc.
burned contacts would cause a large problem if the amplifier was hot or
cold.
>However, the problem is that I
>can't seem to get the 150 ma grid current with normal drive.
>
There is no Eimac spec. for 150 ma grid current. The spec. is 400mA of
anode/plate current. If the emissive life is on the wane, anode current
drops. . . . . Grid current is not a reliable indicator of tube
performance.
>> - If you are able to get 400mA of plate/anode current with 65w drive,
>> the tube is still good. // Note: The Eimac spec. is 400mA max.
>
>It is harder to get 400 ma than before. The big difference is that it is
>hard to get 150 ma grid current -- with normal loading it seems to peak at
>about 110ma, even when I increase the drive way above 65w. I have a
wattmeter in line
>with the exciter, by the way; swr is reasonable and I can see the drive
level.
>
>I used to run about 410 ma plate and 150 ma grid with an output of about 800
>watts CW on 20 meters (slightly more on lower bands, slightly less on
>higher).
>Now I'm seeng about 700-750 watts at 400 ma plate and 110 ma grid current.
>
>Any comments about which tube to get if I do end up replacing it?
>
There is more than one factory in China that produces 3-500Zs. There
appears to be some variation in quality. . If you are looking for a
tube with the same Mu (130) as the out-of-production Eimac version, I
understand that RF Parts sells a Chinese equivalent. . Amperex still
makes high quality 3-500Zs, however the Mu is approx. 200, so the bias
potential would have to be reduced to maintain the 75mA of needed ZSAC
(zero sig. anode cur.), and the drive requirement would be a bit less to
obtain 400mA of anode current, and the input Z would be slightly lower.
.
- later, Vic
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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