I use the AL-811 amp on 75 to 20 meters. It has three Cetron 572B tubes. 65 watts in on 75 & 40 meters produces about 800 watts PEP out and the meter is known to be accurate. On 20 meters 45 watts in makes 800 watts PEP out and amp appears to oscillate with 70 watts drive plus I get RF lip burns on mike grill shield. No RF burns with 45 watts drive and amp operates normal. Appears an oscillation at higher power input? This never appeared in past and amp needed the 65 watts drive for 800 watts PEP out on 20 meters. Antenna match is less than 1.5 to 1 SWR on 20 meters. Also, producing a AM carrier I hear a squeal on 20 meters and no squeal on 40 meters.