bias testing points

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bias testing points
« on: 12 Nov 2008, 11:39 am »
After reviewing the RM9ii manual, as well as the numerous posts here, it seems there are two ways to check the bias. One is to measure the DC volts across the 15Ohm/10W resistor for a reading of 1.8-2.0. This way I know how to do. But there is also discussion that for EL34's the bias should be about 30ma, and for KT88/6550 it should be about 40ma per tube. What I can't figure out is: 1) where in the circuit/schematic do you take these latter readings, 2) what should the DVM be set for, ma or mv (dc?)? , and 3) which method of bias checking is preferred? Any insights are greatly appreciated! All the best...
-Alex :? :? :?

Roger A. Modjeski

Re: bias testing points
« Reply #1 on: 19 Nov 2008, 02:11 am »
In the manual we suggest just lighting the LED which results in 30 mA per tube. If you want to use other bias levels you can measure them across the 15 ohm/10 watt resistor which carries the combined cathode current of the 4 tubes in that channel.

I always run tubes cool to get the 10.000 hour life. At full rated dissipation life will be in the 1000-3000 hour range.  Multiply the current per tube by 460 volts. My number is about 13 watts. EL-34's are rated at 25 watt dissipation and 6550/KT88 are 35 watts.

I worked very hard to make the amp sound good at the lower (recommended) bias level so owners could get maximum tube life. In my experience, higher bias on my amplifier does not improve the sound though it certainly reduces the life of the tubes.