[Rob S.]

Hi guys,
     My Rm200 has the bias set to 33mA (done by the previous owner) .  The manual says to raise it to 40mA.  What does the higher bias do to the way the amp performs and how would the sound change? 

I'm running 4ohm 90db speakers and find it sounds great as it is now.

Rob S.

[Scottdazzle]

Roger personally recommends 35.  You get extended tube life with settings below 40. 40 is actually at the high end of the preferred range.  33's not bad.

[Rob S.]

Thanks..... So the tube runs hotter with higher bias, and has shorter life?   Does the power output increase any as the bias increases?

Rob S.

[richidoo]

More bias current pushes the amp further into class A operation. The farther into class A, the larger the voltage signal can be amplified without any zero crossing distortion. In class A, distortion is lower order and low level. The more bias, the larger input signal can be amplified without crossover distortion. Small signals will also benefit from much lower levels of low order distortion with increased bias - further into class A.   Running in class A puts more strain on tubes and power supply, so you don't want to push it, but the manual says 40ma, you would probably enjoy trying it. It should sound even better. In common PP class AB tube amps, el34s are often biased 30ma, and 6550/kt88 at 75mA. Obviously RM200 is a completely different animal. I think there is a version of RM200 running in pure class A. 

A good article on class A and bias. http://www.passdiy.com/pdf/leaving_class_a.pdf

[Rob S.]

Thanks Rich, very helpful as usual.  I'll be raising mine some and seeing if I can hear any difference in sound.

Rob S.