Once again I have a KT88 power tube failure after only 90 days of use - out of warranty of course. And this batch were the more expensive Genalex Gold Lions. And, you should always replace output tubes as a pair. I think my all tube LCR system is getting too expensive and time consuming to maintain at peak operating condition. Tubes, especially power tubes, have limited life and require monthly bias adjustments to keep at optimum operating conditions. Note that we here demand peak performance of out HTs all the time don't we? And then there's the idea that the amps are constantly degrading as the tubes age over a year.
The Chinese and Russian tubes are not of the legacy 1960s quality from the old American factories. And keeping up my system that uses 12 power tubes and 15 small signal tubes is a bit much. Rack Space, waste heat from the filaments and the required fan cooling, and the intermittent crackles and pops in the speakers that are normal with tube amps. And yes, my tubes are run within spec and not over driven in any way. I even installed AC filament regulating transformers to keep that parameter in spec.
As many here know tube amps have a particular good sound to some people but they are not sonically accurate. I have mine down to 0.1% distortion which is as good as many transistor amps but the frequency response is not ruler flat and changes with power output. This is quite common when using output transformers which are mandatory with tubes (OTL amps excluded).
It was a fun run since 2006 when I built them but as I have quite a few pro amps laying around from various jobs I think it's time for a change. And in light of my new all digital audio system rework, perhaps it's also time for different power amps as well.
NOTE: The picture still shows the tube crossover at the top. I ditched that two years ago in favor of a Behringer UltraDrive unit. Not as bad as some people think of Behringer but it was a bit noisy at low levels.
Edited by Glimmie - 11/26/13 at 12:42pm