This is absolutely true. And i think in the meantime even more so with these kind of single output tube OTLs as there is quite a bit of current through the tubes.Cressy Snr wrote: ↑Sun Dec 09, 2018 10:36 pm ... it has taken a bit of fine adjustment of feedback, tube dissipation and B+ values in order to optimize the amplifier for maximum output with the least distortion into my own particular speakers, and helpfully, optimizing the amp has not actually required giving the output tubes a thrashing. It needs a sensitive approach,.....
I finally installed my prototype protection circuit for the 6C33 over the weekend and had to replace the 2R7 resistors with 10R/2W pots for setting the current and the trigger point for the circuit so that it will shut down the tubes in case of a fault. And, boy, did this pot change the sound again! Fortunately to the better and not to the worse but still this couldn't have been expected. When I replaced the same resistors with stepped attenuators for fine tuning Ip for each 300B in my SE OTL there was no audible difference regarding SQ. Only the "nervousness" was suddenly gone. Resistors and pots are both wirewound.
Another good reason for the pots instead of resistors is that the tubes get weaker over time (the 6C33 dropped 20mA after about 500hrs of use) and can be easily readjusted to the ideal value.