Several months ago I purchased a used Cary SLI80 Signature amp that I’m using with Martin Logan CLS2 speakers. The sound was very good as received so I put off experimenting with tube selection or checking bias adjustment. Because, though, I was able to buy a matched quad of Gold Lion KT88 tubes at a very good price, I decided to try them. I switched them with the stock Electro-Harmonix tubes and measured the bias amperage. I was surprised to find that one channel was reading significantly higher than the Cary recommended setting of 75 mA, at 85mA and the other channel displayed a lower value than specified @ 50 mA. I hadn’t measured the bias amperage before making the switch so have no reference values for comparison with the original tubes. Also, I don’t claim to have the best ears on the planet but the difference in sound was obvious. First, I didn’t expect power tubes to impact sound much and, according to things I’ve read, the direction of proposed potential change was opposite of that predicted. Upper frequencies seemed more defined and forward especially with piano music which comes across a somewhat strident. This is most noticeable when set in ultra linear mode. Maybe the overall change is toward improved accuracy and I just need more time to adjust my listening to it but I’m curious to hear what comments that those of you with more experience have. Are these changes the result of different tube manufacturers, bias adjustment or, most likely, a combination of these factors? At this point in my thinking, though, the obvious next step would be to return to the original tubes with proper biasing and reevaluate.
