[AMPS] Unruly L-7 Amplifier

[Jeff Weinberg W8CQ]

Hi All,
I am currently wrestling with an L-7 amplifier and need some help.  I
recently rebuilt the power supply and repaired the HV dropping string on the
amp and all seemed fine.

The amplifier was working great on 75m with about 1400 out and slightly less
on 20m.

I sent the amp back to its owner and he has a problem on 20m.

After about 5 minutes of a QSO, the fuse resistor (0.82ohm) in the power
supply keeps blowing.
He also mentioned that the amp doesn't like 20m.  It seems to take off (very
high plate and grid current when tuning up) when tuning up and all he can
get into it is about 35 watts.

I have checked the plate and load caps for previous arcs and grid caps look
good.

Anyone have any ideas why this amp would blow the resistor on 20m?

It has original Eimac 3-500Z tubes in it.  Could a tube be getting soft or
going bad and arc over on 20m?  Any thoughts on parasitics?  The L-4B and
L-7 are usually very stable amps.

I am really struggling with this one!!  All help is appreciated.

Regards,
Jeff Weinberg  W8CQ
w8cqjeff@ameritech.net



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