[Amps] SS Amp questions

[Georgens, Tom]

I will not let my love/hate relationship with ALS1300 end

The ALS1300 has worked flawlessly for me in two CQWW CW contests from 8P5A and reinforced why I bought the unit.  Unfortunately, multiple design, manufacturing, and quality issues have repeatedly put the product on the shelf for 6 other contests.  It has been back to MFJ multiple times for repair, including one set of blown finals.  While the contest duty cycles are high, I do not push the amp over 1KW, the antennas are well matched, and the bandswitching is automatic so the amp has not been abused.  To be fair to MFJ, they have been mostly accommodating.  However, after what looked to be yet another failure of the finals, I intended to give up.

However, I just could not just let it sit there and had to tinker with it some more, just to find out what was wrong.  Indeed, one of the MRF150's was bad.  As it turns out, I have a set of  4 matched MRF150's from a prior project.  I removed the MRF150's from the prior board and replaced the suspect ALS1300 transistor and its parallel partner.  Since replacing the transistors is so tedious, I just stopped at two just to see if the amp would at least function.

My question is how many of the transistors need to be matched?  The design is two boards, each with 4 transistors in push pull.  DO just the two in parallel need to be matched, all 4 per board need to be matched, or all 8 on both boards combined.  I noticed that all 8 of the original transistors are hand marked with a "17."  I am guessing that is the measured gain and that is how they are matched.

Also, even if matched, there will be some variation.  What is the best way to adjust bias?  Do I just match the quiescent currents?  Should I be watching the waveforms at full power and then adjusting?  Should I be adjusting based on a spectrum analyzer and two tone test?

Right now, the amp appears functional and will make power, so the diagnosis was correct.   However, it is not yet usable as the current draws between modules is different and there is a big imbalance in output of the two modules.  I am good with replacing the other two transistors on the board, but not too keen on buying 8 new matched ones.  Either way, I will need advice on the biasing process

Any suggestions appreciated.

tg