I picked up a 1984 JCM 800 w/ EH EL34's. The EH's measured around 25 when I tested the bias readings. I put some new =C= EL34's in and they read around 22.5 without any bias pot adjustment. What is the safe range for EL34's in this amp and what is your preferred bias point?

A safe range would be 60-70% of max plate dissipation (25w).....if your plate voltage is 450 .....60% would be about 33ma and 70% about 38ma. I'd set it to 60% or just a little more. YMMV with different plate voltage. Dave C

What is the formula you're using to figure bias voltage. The plate voltage on my Colonial is 475 volts, how do I come up with a bias voltage?

We're not figuring voltage.....we're figuring current. A generally safe range is 60%-70% of max. plate dissipation which in the case of el34 tubes is 25w max per tube, so 0.6x25=15 then divide that by the plate voltage which in your amp is 475, so 15/475=0.0315789 amps or 31.6 milliamps for 60% of max dissipation. This would be measured usually with a bias probe or xformer shunt method using a DMM. It would actually be lower than what is read due to screen current not factored in but that just leaves you with a little extra safety margin built in. As always ,be safe, if you don't really understand what you're doing have a tech or somebody else who's done it before show you because 475 volts can kill. Dave C

Or use this calculator: http://www.webervst.com/tubes/calcbias.htm In this case 70% translates to a plate dissipation of 17 watts for a 34 in a class A/B amp. 17 watts of dissipation (70% of a 34's 25 watt rating) at a 475 plate voltage is 36.8ma. 60% is around 32ma. I found not to go strictly by numbers. Trust your ear. Just try not exceed 70%.

800's can be biased pretty cold and still sound OK. I found 60% is just fine. 70% gets into mushy territory. Just my experience. I've also found the 800's extremly forgiving.