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[Amps] Strange pi network tuning problem

Subject: [Amps] Strange pi network tuning problem
From: Guido Tedeschi <>
Date: Sun, 12 May 2019 01:33:02 +0200
List-post: <>
The PA of the Yaesu FL-200B that I'm restoring has a strange problem: it
tunes perfectly on 15 and 10m, but on 20, 40 and 80m it doesn't tune
correctly, the power is low, the current is high and there is no dip on
current but only a peak on current when the pi network is tuned for the
maximum (low) power.
Moreover, with the Plate knob tuned for maximum power, the indicator on
knob is outside of the correct areas.
- The 6JS6C pair is not at 100% (only 60W on 15m and 50W on 10m), but it
seems fair and it should give 100W on 80 and 40m
- It is correctly neutralized, because on 10 and 15m the dip coincides with
maximum power
- The drive voltage from a 12BY7A seems fine
- The PI network seems fine, because i tested it with a VNA connected on 50
ohm PI side and a 2200 ohm resistor between plates and ground to simulate
the tubes load with about 600V and 300mA. The PI network matched perfectly
the 2200 ohm load on all the bands and the plate knob indicator matches the
correct area.

Any idea?
Thanks and 73 de Guido, ik2bcp
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