iv got a dsl401 amp, i changed the valves 2 months ago, last night one of the pre amp valves blew so i put a spare one in, when i changed them 2 months ago i set the bias at 550mv as this is what i had been told marshall recomends for this amp, i decided to check the plate voltage and it was 418mv, i stuck the plate voltage into this http://www.webervst.com/tubes/calcbias.htm to get a setting for the bias, this tells me to set the bias at 20ma or 200mv (i think) on another site a guy (that probably knows more than me) said because the dsl401 has 4 power valves i should multiply that 200mv by 4 and set it at 800mv like he has done with his, can anyone confirm that this is right before i set the bias as i dont want to blow my valves any quicker than i need to?