First, thanks to all who have offered suggestions on the "low output on
10m" question. I have several very plausible things to try and thankfully
it's a three-day weekend.
Now for another kinda question...fired up this same amp this AM on 80m, got
mostly full output, but what I mostly got was smoke out of the grid current
meter. Meter continued to indicate normally entire time, showing 120mA Ic
(meter is 500mA movement, amp is 2 3-500Z GG in a textbook configuration).
Typical setup with meter running from filament CT to ground and bypassed
with 0.01u disk. But no diode protection...yet!
Plate idle current then went from 120mA @ 2500V to 240mA, telling me at the
least that the zener (7.5V, 50W) went short. Oh well.
Investigation of the meter movement (thankfully still operational, but
really smelly) revealed that the built-in shunt (in the form of a small
coil of resistance wire) had evidently heated to the extent to melt some
insulation on the inner wirings from shunt to movement.
Now why would this be? My guess (and that's exactly what it is) is that
enough 80m energy got into the filament CT/B- circuit loop to toast the
zener and heat up the meter. Even though it's supposedly bypassed.
Two other salient items: with amp idle (keyed to idle plate current but no
excitation) something (the zener probably) generated quite the hash in my
receiver. I seem to recall seeing this mentioned somewhere...should one RF
bypass the zener (across itself or to ground?). Second, the grid current
meter isn't grounded at the meter panel; rather, a shielded two-wire cable
runs from the bottom of the amp up to the meter (bypass is applied directly
at the meter terminals). So it's 20" wire up to the meter, cross the
meter/bypass cap, and then 20" wire down to the place where it's actually
grounded.
One other thing is that in the bias tee, the 1st bypass cap after the
B&W800 choke is one of those 500pF, 20kV red TV-style jobbers that Rich
specifically recommends *against* on his web page. Could this seemingly
poor bypass cap be the true culprit (allowing too much RF voltage back into
the PS circuit?)
Looking to learn! Thanks in advance. This is fun, even with the smoke!
------------
Scott Townley
nx7u@primenet.com
------------
Collector of:
Stoddard Aircraft EMI/RFI receivers and accessories
Big Parts for that Big Linear Amp
70's era RF test equipment HP/GR/Tek
Radio-related technical reference material 1940+
...anything else that will keep me off the streets at night
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