This thread is lind of related to my Did I measure B+ correctly thread http://www.tdpri.com/forum/shock-brothers-diy-amps/159217-did-i-measure-b-correctly.html I got some advice about measuring voltages in a tube amp and last night I was comfortable enough to measure B+ at the key areas of my used Marsh built 5F1. However I think that the voltages at the preamp tube are high. The reason I started poking around in the amp is because it has developed a low 60 cycle hum after the first few days I've had it (or my ears are just more critical of it now and the hum has always been there). It's not a terrible hum, but it's bad enough to drive me crazy in a small room and the hum will prevent close mic recording. Volume does not affect the hum until about 9 or so on the dial. When compared side by side at idle with a healthy vintage '63 Tweed Champ, my Marsh has more hum. Otherwise the amp sounds fine. Without further ado: 5Y3 Mesa-Boogie Pin 8 = 372V 6V6 JJ Pin 3 = 360V Pin 4 = 342V 12AX7 JJ Pin 1 = 251V Pin 6 = 192V Seems high doesn't it? Yes I triple checked the readings. Any advice is appreciated. Thanks.