AL 572 Problem, need help

[K3OFX]

Hello,

Last night I was re-tuning my AL 572 amplifier on 40m into a 40m dipole but I had the band switch on the 20m band. Just as I saw the reflected power go to 200 watts while tuning the antenna capacitor, I heard a faint "tink" sound. It did not sound like a high voltage arc. Operator Error! If I had the band switch on 40m, the tuning would have been very close to where it should have been and would only have needed touched up.

Now I get very little power out. Can someone please give me some guidance on where to start to troubleshoot this amplifier? I have an understanding of electronics but not vacuum tube based circuitry or failures. I realize reflected power and poor SWR are bad, but I don't know what may have failed.

I have a new set of 4 matched 572s but I am hesitant to install them without some testing first.

The only info I have found on the internet is to check diode D117, but I don't understand why this part would be bad. Any technical help would be appreciated.

Thanks

Bob, K3OFX

[N7WR]

If one of the tubes arced (likely) D117 and 2 or 3 other components in the chain can get fried. Ameritron has an "arc suppression modification" (done at the factory) to prevent this sort of daisy chain effect in several of their amps.

[W1QJ]

That "tink" sound pretty well desribes an internal tube arc. Probably one tube crapped out on you. Chances are D117 will need repalcing now. You can easily tell if it does because your grid meter will follow the plate meter exactly in deflection. You'll need to find the bad tube and replace it. Subbing in one good tube for each will turn up the bad one in a few tries. Or replace them all. I doubt you did any more damage than maybe D117. The amp will still work regardless of D117 shorting, it is just the grid meter will now read plate current until changed.

[WB2WIK]

I'm surprised it made any real output under the conditions described. Normally the input match would be so bad (with the bandswitch in the wrong position) that the transceiver would cut back on power output to the point where there was't any -- or very little.

Does sound like a bad tube (or tubes) and a D117 failure, based on the rest of the description.

If I select the wrong band on my AL-80B, the output is usually "nothing" and there's very little to no grid or plate current because there's virtually no drive.

[K3OFX]

Problem fixed, it was D117. Tubes not damaged. Thanks for all the help.

Bob, K3OFX