> I am working on a well known amp (tunes 160-10mtrs) and the 80 mtr. portion
>of the coil (160 and 80 are a separate coil) was apparently overheated and
>"bent" a bit , altho it doesn't appear to be damaged. My question is why has
>this happened.? My guess is that maybe the amp was "overdriven" or
>"underloaded", and thus had too much circulating current.!!!
>Can anyone shed
>some light on this problem ??
** Overdriving could be a possibility only if the tube was capable of
substantially more emission than was needed to produce normal RF output.
For example, If a 3CX3000A7 were used in a 1500w amplifier, only about
1/4 of the available emission would be used. Thus, if such an amplifier
was driven to the tube's emissive limit, there would be c. 4x the RF
circulating current in the tank - so the I^2 x R heating loss in the tank
L would be c. 16x as great as normal.
- Since this scenario seems quite unlikely for economic reasons, perhaps
the amplifier was designed by someone who does not realize fully how much
skin-effect progressively reduces the RF current carrying capacity of
conductors. For example, at 60Hz, #12 Cu exhibits barely perceptable
heating at 20A. However, at 30MHz, 5A in #12 Cu produces noticable
heating. Thus, tank conductors need to be forced-air cooled. My guess
is that the amplifier you are working on has no forced air flowing
through the tank. Who manufactured the amplifier you are working on,
Carl ?
- The Heath SB-220 is laughed at by some because of its low-tech fan,
but Heath's engineers did the right thing by using a portion of the fan's
exhaust to cool the 3-500Zs' (critical) filament pins and the tank L.
However, was it serendippity or good engineering practice by someone who
had taken a course in AC circuit-analysis?
>The owner has said he uses a tuner , swr mtr.
>etc to load his antenna on 80 so it wouldn't seem to be a loading problem,
>but I'm just not too sure. !!!
> any suggestions are welcomed. Thanks to all.!!
> carl / kz5ca
>
>
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>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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