I designed the AL811 amplifier in 1991 as a replacement for the AL84 sweep tube amplifier. 

Every AL811 three tube amp was supposed to be shipped with a 200-ohm input swamping resistor. This resistor stabilizes the amplifier on 10-meters and makes it take a more reasonable drive power level. This resistor along with a small mica capacitor replaces the missing 811 tube. It also loads the input down, something totally necessary in this amplifier because the neutralizing circuit is left out of the 3 tube models.

Every three tube 811 amplifier was supposed to have this resistor. I kept telling them to add it, they would add it for a while, then they would stop.  (Your amplifier might or might not have this resistor.)      

This is a simple addition. Installation might or might not require drilling one 0.125 inch or 1/8th inch hole in the chassis. Some amplifiers might have the hole, some might not.