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[Amps] HF2500 - 15 meter problem

Subject: [Amps] HF2500 - 15 meter problem
From: Steve London <>
Date: Wed, 24 Aug 2022 16:51:42 -0600
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I have a HF2500 with a pair of 3CPX800's. It's been a great amp.

Up until now, it's had a minor issue. The tuning/loading drifts of 15
meters as it heats up. Not a big deal - just a small tweak of the tuning
brings it back to full power. No issue with any other band.

Today, while operating, it went dead on 15. Suddenly, only a few watts
output. No drama, bangs, etc. 30 mA grid current, 750 mA plate current.
Tweaking the tuning made no difference, until it was way out of normal,
when it increased to 250 watts and something arc'd. All other bands are

Here's what I did, so far:
- Examined the band switch and contact to the 15 meter tap on the coil.
Looked good and measured good with an ohm meter.
- Examined the tuned input network. No obvious problem. Placed a 47 ohm
resistor across the output of the input network, instead of the tubes. SWR
and Z looked nominal on all bands.
- Bypassed the input network, driving the tubes directly. Still bad on 15
meters. All other bands are good.

So, that eliminates the input network as the problem.

What am I missing on the PA or Pi-L side that would only affect 15 meters ?

Steve, N2IC
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