The bass player in my band is having issues with his ampeg head.
After a proper warm up, one 6550 tube starts glowing orange, and the head automatically cuts off sound, I'm guessing to protect the circuitry.
We have already tried biasing but the first section is always off.
We're bringing it to a tech on friday, but he already said that it will need at least a complete retube of the 6550 power section.
Now a complete retube is a lot of money over here... I'm asking for a second opinion, because this tech guy is the only one we know of.
Any thoughts? Is his early diagnose accurate enough?
More than likely a faulty plate load resistor. Either it's open or, one of the socket contacts, where it's strapped across, isn't making contact with the tube pin. So, you either have to replace that resistor, or repair / replace the tube socket. When I say repair, I mean that a good tech could just squeeze the contacts together, but it will loosen up again eventually and have to be replaced anyway. If you just need to get through a gig than you can just remove that particular tube from it's socket and the amp should still have power enough to get you through.
As far as a complete re-tubing, I really can't say. If it's just the one tube red plating than I wouldn't re-tube it. There's really no failure - that I know of at least - that could cause all the tubes to go bad at one time, so first check all the plate load resistors and replace that tube that was in that socket.
I have an SVT-2Pro. At one point, I had a the same tube keep failing. Everytime I'd replace it, I'd re-bias and bam! the signal would cut off. This model SVT had a nice protection mechanism for when a failure occurs.
My SVT, and most (correct me if I'm wrong)have the power section in two sets of 3 6550's. There are two bias adjustments for each set, which is why matching the tubes is important (but can also be expensive). My problem was that my replacement tubes were some junky ones that I was trying to throw in with nicely matched JJ's.
This affected the bias current, causing problems.
My problem was resolved by buying a new matched set. Like I said though (and you probably know) matched tubes are more pricey, but the SVT is a hell of a machine that needs proper components.
Well, a few months later we were finally able to bring the ampeg head to a trusted tech guy, and strangely it was only a bias problem.
I guess he adjusted it properly (or maybe the bias adjuster was faulty)... we feel a little ashamed of not being able to bias our head, but there you go.
Bias problem. Solved!