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209.7.230.2
I'm wonding if I bias my Rogue M150s at 35ma rather than 40 if the tubes will live longer and/or the sound will change for better or worse. Thanks
Follow Ups:
....just like Nigel Tufnel.
Nothing drastic, just up about 5mA but didn't really hear much difference. Maybe that's not enough to make a difference. I did notice that the bias meters were jumping up and down around the bias setting. Sort of like a metronome but it would pause at the bias point for a second before moving. Turning the bias back down the recommended 40mA stopped this meter dance.
that biasing an amp down from 40 to 35 will lead to early distortion or clipping due to underpowering the drivers. That was my fear. Thank you.
It is hard to explain properly in a short amount of words.A tube is baised to have a certain voltage difference between cathode and grid. Let's say that is 50 volts. Now we know our tube could handle up to a 50 volt postive peak music signal but no more assuming claas 1 operation. The question then becomes what happens when it sees that negative 50 volt swing? If the tube goes into cutoff we have a class AB amp. If it stays in conduction we have a class A amp. The sooner we reach cutoff on the negative swing (=biased for less milliamps) the sooner we reach class B. When we reach class B all of a sudden the tube that is in conduction works into a different load. This point of transistion and the working of the tube into a different load may be audible.
To me any talk of bias point needs to take into account the actual voltage the tube sees and from that the actual wattage the tube dissapates can be calculated. For a class AB1 push pull tube amp I'd suggest a bias point of 2/3 to 3/4 of the tube's rating. I would also point out that when in class A DC current cancel in the output transformer.
So no, you would not reach clipping sooner or have less wattage. If anything you would have more! But you would reach class B mode sooner....but if that is beyond your normal listening levels who cares???
Russ
P.S. Wattage sets the limit on what the tube can do but current is what shortens live....So a tube with 450 volts cathode to anode and 50 ma of current is likely to last longer than the same tube biased at 250 volts and 90 ma.
If you don't notice any difference in the sound, run them cooler!
Some amps don't sound good if they are run cooler, but most will be just fine.
Scott 222C, running 7189s at 8 W dissipation instead of spec'd 12 W.
Which usually garner statements that they need to be biased a little hot to sound their best. But I can't tell a difference so I choose to run my tubes cooler. Why not just go ahead and change the bias for a few hours and see if you can hear enough of a difference to merit the shortened tube life yourself? If not keep the bias to spec or lower and preserve your tubes.
I've been way deep into all of the tube specified specs, tube curves, maximum voltages, optimum voltages, recommended amp bias specs....and on and on. BUT the good ole ears have it as the ultimate test instrument when the final judgement needs to be made. The best post I've seen on this came from Joe Rosen (think the name is correct) about bias effects on sonic quality....it put me on the track to know WHAT to listen for at various under and over bias situations. I now (strictly from sonic preference) have my bias set at 45ma vs. the recommended 50ma on my
JJ E34L tubes.....that's the "sweet spot" for my particular amp and set up (but sorry, this really isn't about your your amp....but my experience with my dyna st70). Anyway try to find the post by Joe Rosen on this topic....pretty good stuff overall.
Have read that biasing the tubes a little hotter reduces distortion. Should try to listen and see if I can hear that specifically.
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