Thanks for all the replies! Must have a dozen or more.
Did some more investigation based on the many suggestions and have
determined that the problem is with one of the tubes. It appears that
the filament has a small piece that has broken off, causing only the
bottom half of the filament to lite. If I look into the tube carefully,
I see a small piece of the filament, about 1/4 of an inch long and
curved, maybe forming a third of a circle, loose inside the glass. The
resting plate current for that tube is about half (60 ma) of the other
tube. Plus, I can see that the top half of the filament is not lit, only
the bottom half. The other tube is lit all the way to the top of the
filament/grid region.
I gather from this that the filament is constructed, not from a single
wire, but from at least two wired in parallel. I even measured the
resistance of both tube's filaments and find that the bad tube reads .2
ohms whereas the other tube reads .1 ohms.
So, looks like I have a bad tube. Also, of interest, as one reply
suggested, the good tube gets more color on one side of the plate than
the other, suggesting that the plate is not perfectly centered around
the filament and grid. Could be the amp took a jolt at some time and
damaged the tubes.
Anybody have a known good pair for sale? Any recommendations on buying
new tubes?
Thanks again for all the replies.
73, Charles, K4ZRJ
Charles Johnson wrote:
> In an L-4B amp I have just purchased, the two 3-500Zs show uneven plate
> glow at idle. Both tubes are Eimac 3-500Zs. Resting plate current is
> approx 180 ma for the pair, but one tube shows almost no color, the
> other quite a bit of color. Does this indicate overall condition of the
> tubes? At 100 watts of drive, I'm seeing about 200 ma of grid current,
> 650 ma of plate current at 2500VDC, and about 1000 watts output. I would
> think that a matched pair (which these probably are not) would show
> about the same amount of color, but this seems to indicate that one tube
> is drawing more current at idle, and probably more current at full
> power. I have not tried them individually. Any ideas or comment? Thanks,
> Charles, K4ZRJ
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