otto

You actually want to set is at about 55% or so, and that would depend on your plate voltage. All amps are a bit difference. On Marshalls, it is actually better to set them with a scope looking at the crossover notch. If you are ever near Los Angeles, I would be happy to do this for you for free. Thank for your replies Myles, you are very kind. I would like to come to LA, but I am from Italy. Your site is very good, thank you, but I did not find the answer to my doubt. My JCM 800 4210 (50 W, split channel) has 450 V of plate voltage. I read that bias at idle must be set to have maximum 70% of power at idle, minimum 50%. I understood that near 70% you obtain more distortion, near 50% more clean, is it not right? You suggest to me 55%?

Hi Miles (and everybody out there), I have a JCM 800 split channel 50 W (4210), bought few weeks ago. Fantastic sound! Tubed with Groove Tubes EL34 (type 4), 2 or 3 years old. After reading the biasing cathode resistore method, I decided to make a trial to understand if the fantastic sound I hear is obtained at 50% or 70% of maximum power, to have a comparison for a next retubing. I weld on the cathode the 1-ohm resistors (very accurate value, 5 watt). Turn the amplifier on, warming up, then I measure the voltage on the anode: 450 V. OK, on the cathode resistor I should measure something between 38 and 48 (also considering the 10 mA grid current). I read 34 on one tube and 28 on the other!! How is it possible that I love that sound so much if the tubes are completely not coupled and the Bias is so low?! Reply: there is something wrong with my ears! But few weeks ago I played with a fantastic VHT, which is a very good comparison… so I don't understand. Now I've biased the two tubes at 46 and 38 to have one tube near 65% and the other one near 50%. I think it should be the case to retube the amp, but I'd like to understand… - The tubes are unbalanced because they are old? - What a very low biasing (about 30%) involves? Only bad sound or also short life? - My 4210 is a '82 model and clipping is obtained through diodes after the preamp valves. The EL34s were completely unmatched and worked with very low current so I think they were able to colour very little the sound. I compared the JCM 800 with a Valvestate and with a VHT Pittbull and there is no doubt: the sound is that of a great tube amp. The question is: how is it possible to hear a typical tube sound if preamp tubes are not used for distorsion and EL34s were used so cold? Thank you