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I just retubed this head, replaced all bias supply components, replaced coupling caps to pa. Plate voltage is at 462v and I biased JJ 6L6 at 45ma each. The head came back blowing fuses. It was a bad tube... I retubed w/ matched set and all is fine. Head came back again blowing fuses. The owner uses it w/ a Farfisa and another instrument I'm not sure of. They said it is fine for a while then blows the fuse. Here in the shop I've got no problem w/ it. It is not blowing fuses, had it on for hour now, checking voltages on tube pins and monitoring plate current... see no problems. Played through it w/ guitar no problems. I'm at a loss here... will keep playing it and see. If anyone has any suggestions I'm all ears. I suppose it could be a bad filter cap and/or preamp tube.
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I cranked the amp and the fuse blew. After putting new fuse in I monitored bias and pin 5 on one side is -26v, other side is original value of -38v. I pulled the last power tube (same socket that bad tube from last set was in), and the bias went back to the same: -38v on both sides pin 5. I put the tube back in to see if bias would be faulty again, to my surprise the bias is now normal: -38v on pin 5 of all tubes....??? All I can think this could be is a bad socket. I visually inspected it to see what look line very fine hairline fractures but hard to REALLY see in there. Anyone experience this symptom before? Now I've cranked the amp up again to see if happens again... nothing, amp seems fine. I KNOW it's not fine though and the fuse will blow again. I guess I'm left w/ replacing socket. Would still like to see if someone's experienced this before.
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