I would like to see the schematics for an MT15, to see what might be different about this amp. For any other amp, considering 400VDC on the Plate, I would set EL34s to around 44 mA each tube, and 6L6s to around 45 mA each tube. Figuring that EL34s as 25 watt tubes and 6L6s as 30 watt tubes.
It seems that the PRS recommended setting is quite cold.
IME, ideal idle dissipation, for no crossover distortion, is a minimum of 50%, ranging up to 70% of the tube max rated Plate dissipation. With a fixed bias amp, for 6L6 (30 watt), I like it around 60%, whereas for EL34 (25 watt), and 6V6 (14 watt), I like it around 70%. Many Cathode biased amps run the tubes at close to 100% idle.
Lots of manufacturers have different ideas about tube bias. For example, Mesa runs their bias pretty cold so that they can use a variety of tubes. Other makers run them hotter. A lot’s going to depend on the designer’s thinking.
Why not check with PRS Customer Service, and get the skinny from the horse’s mouth?
Incidentally, I run my EL34s on my HXDA right at PRS’ recommendation. I like the way it sounds. Someone evidently thought this through.
That's correct. I usually bias class AB amps at 70% of maximum plate dissipation (i.e., 25W * 0.7 / 450V * 1,000 ~= 39ma). However, running tubes that hot sacrifices tube life for tone.
I bought a 100w Archon head and immediately installed EL34's. Upon asking PRS, PRS provided me this specific answer which I will now copy & paste into this post.
"We recommend a bias setting of 30mV, you can go + or - 5mV to your taste. The bias adjustment is the little flat head inside the jack above the contact points."
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