I have had the 7581 tubes in my am for a while. I play through the amp every day, on week ends sessions can last for several hours. Out of the box the 7581 sounded great. Every time I powered up the amp it just kept getting better. Finally go to the point of hearing the full potential of the tubes. It is that point where you find out that you either like them or hate them. The full quad of 7581 were performing well, until today. They turned to the dark side. It was gradual not alarming in any way. At first I thought it may be the walnut guitar I was playing through so I tried a different guitar with more brightness in character (maple and alder). Amp still sounded dark. The clean channel was great in tonal character, channeld 2 and 3 just seemd to be lacking something, but what I find hard to put into words. I had a matched pair of Mesa 6L6 just waiting for abuse so I replaced two the 7581 (primary power tubes V8 and V9). That was a great move. The 6L6 have a bit more top end character in terms of brightness. 10W and 45W modes run the 6L6's for that full chime or shimmer of tone. Of course the cocktail of preamp tubes has contributed somewhat to cure the over bright character of the 12ax7a's originally seated into thier respective sockets at the time of assemby (not sure if Mesa ships amps with the tubes in place). 90W mode is now tailored to my liking. I have retained the brightness and chime of the 6L6 and broadend the tonal character in the other extreme with the 7581 as the secondary power stages. Deep tones from the 7581 with all of the chime of the 6L6 ready to strip life from your eardrums. 7581 after many hours of use tend to hang in the dark zone, harder to saturate and overdrive in to clipping, I would assume that some of the higher order harmonics are reduced considerably causing a softer clip and a darker tone. However, if you use 6L6 as the primary power tube, they saturate nicely and operate very well with the pentode mode of channel 3 and at least generate 4th order harmonics with ease (I have run a signal analyizer at Penn State University in 1994 on 6L6 power tubes to study the soft clipping effects of valves, at that time I was designing a solid state amplifier as a senior design project, the goal was to preserve or replicate valve character with FET's which are similar to vacuum tubes ).
I will see how well or how long this works out, before I change to something else.
I will see how well or how long this works out, before I change to something else.