I am in the process of repairing a Drake L4 amp that suffered some
lightning damage. The only physical damage was a .001 1400 VDC bypass
cap on one leg of the 230 vac line inside the RF deck. The only place I
can see the 230 goes inside the RF deck is to the filament transformer.
The power supply itself looks fine. No other problems are apparent from
a close visual inspection. The meters appear to function, high voltage
seems to be correct, bias switches and the tubes draw idle current when
biased on. I checked the TR relay contacts, cleaned them and checked the
RF path for continuity.
The symptom is no power out when the amp is in line. Well almost no
power out. It shows about the same output as the exciter drive. No plate
current change when drive is applied, no grid current drawn when drive
is applied. The tune and load caps have no effect on plate current, but
they do have a huge effect on the amp input VSWR. Plate voltage show
just under 3000 VDC. Since I see the input VSWR change when I try to
adjust the tune and load caps, I am assuming the RF path through the
relay and amp itself is good.
I am to the point I am thinking the tubes are bad, but I hate to buy a
pair until I am sure they are the problem.
Any suggestions?
Danny NG9R
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