My old B-15n experienced a big sag in volume so I sent it out to be retubed and recapped. The volume is back - but I'm hearing a hum beginning at about 1:00 o'clock on the volume dial that becomes extremely prominent when the volume is dimed. The hum control works best at full counterclockwise - won't go any further. I tried swapping back in the old (original) preamp tubes. Doesn't seem to help. I don't remember any humm at high volumes way back when. Is my memory faulty - or could something be wrong?