Jeff:
This really sounds like one of two things to me:
(1.) An intermittant Grid-Filament short from one of the 3-500Z. The low
drive problem mentioned may be a tipoff that the cathode sees a path to
ground and causes a high VSWR for the driver. There might be some showing
of change in idling current and grid current if such a problem were present.
If the tubes were removed from the amp and heater current applied
individually in a test setup, a Grid-Filament problem might be observed.
Using a Hipot test on G-F during this time may identify the problem.
(2.) If there is a parasitic present, the amp might become erratic on one or
more bands. Your older tubes might lead one to assume that possible lower
gm due to their age would make the problem less likely. However, it seems
when things are normal they are capable of full output. It wouldn't hurt
to look closely at the parasitic traps for resistor value change. I have
found amps with suspected tube problems were actually parasitic in nature.
I hope the amp's meters are diode protected against the big bang theory.
Well, that's two ideas for my two bits worth.
.....Merit W6NQ
> 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.
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