The Fet Valve amplifiers have a voltage gain of 24, set by a pair of precision resistors in the feedback loop.
Power output into four ohms will double, and double again into two ohms, limited by the size of the fuses used. If the speaker fuses are too big, then there is no protection for the speakers, and the internal B+ fuses will blow first, a pain in the butt to change. If these are made too large also, then the main supply fuse will blow. We typically fuse the big amps with a 10-15 amp slow blow main supply fuse, 8 amp B+ fuses, and 5 amp speaker fuses, adequate for sustained full power 8 ohm operation.
Note that to understand what nearly 300 watts per channel really is, try the following experiment. Place three 100 watt light bulbs wired in parallel safely wired into a small sealed metal box. Plug the thing in and let it set for a half hour or so. Then place your hand firmly on the top of the box.
While contemplating your resulting third degree burns, try and understand how happy the voice coils of your loudspeakers would be dealing with the same amount of heat.
Tube life is very hard to predict. We use regulated 12V DC heater supplies, regulated B+ supplies, and run the tubes far inside their maximum ratings with good heat flow around them. The tubes should last for several years, but of course individual tubes can fail when they feel like it. A tube replacement set from us is $29 at this time, so it is not a backbreaking expense.
Signs of tube aging are; increasing hiss levels or noise, microphonics (tapping on the cover produces a "pinging" sound, or slow muddying of the sound and reduction in bass dynamics. No adjustments are needed if replacing the tubes in kind.
Frank Van Alstine
