Okay, I have a problem here that I cannot cure. A Blues Jr III came to me w the V5 EL84 blown. I checked things out and replaced a bad screen grid resistor. Fired it up....big time 60hz hum. All bias voltages are correct. Plate voltages are correct. I got to looking for AC voltage....whoa!...540VAC on the plate of V4 power tube. There are diodes on the OT primary legs, so I replaced the diode on that leg for V4....no cure. I wondered about the coupling cap on that side of the output. Voltages were weird, so I replaced that cap. All of the Dc voltages are now looking correct there, but there is still 540 VAC on the plate of V4. That voltage is not on the primary of the OT when the OT is separated from the power tubes. All filter caps are filtering the AC, it seems, because I do not find that AC there. Does anyone have any insight as to the source of this high AC voltage? I have not reviewed all of the BillM work on these amps. Here is a link to the schematic. https://schematicheaven.net/fenderamps/blues_jr.pdf any help is appreciated.....this one has me stumped.