[davet753]

I bought this from a friend the other day and have got it on the air. The AL811H is rated at 800 watts SSB, and this one does almost that much. It has just had all four of the 811 tubes replaced, and the 10 meter mod has been done.

I do have an issue when using the amp on 20 meters. If the plate and load are adjusted for peak output power, after about half a minute the receive on my radio goes dead. The spectrum scope on my transceiver doesn't even show atmospheric noise when that happens, but there is some very narrow and spurious band noise that shows up (that looks like oscillation to me). It's like something just wipes it all out. I think the problem is parasitic oscillation in the amplifier, because moving the plate tune off-peak will stop it, or decreasing load capacitance, turning the amp off, or switching bands on the amp will all return the receiver back to normal.

I learned that Ameritron has made some modifications since this AL811H was built in 2007. The parasitic board has been updated with changes to the coils and resistors. I went ahead and ordered a new board from Ameritron, so that will get replaced.

The amp has grounded grid final PA tubes, but they are grounded with an MOV, 4 resistors, and 4 capacitors. Ameritron has since updated this design by directly grounding each tube grid straight to the chassis, and adding a couple of gas discharge tubes in the filament circuit. Supposedly, this should increase stability and result in less damage in case of catastrophic failures. Any improvement in stability ought to help cure my parasitic oscillation issues, too. Since I'll have to have it apart to replace the parasitic board, I figure I might as well do the grid mods while I'm in there.

Has anybody else ever dealt with the 811 amps before? I have heard folks say these amps are not the greatest, but the AL811 has been the best selling amp in the world for years now, so I would think they're probably not so bad. Mine is the 4 tube version, so it offers several benefits over the 3 tube entry-model (especially neutralization).