Ampegs use their tubes hard, applying high plate votage, and getting long life by paying careful attention to the Plate Dissipation factor. Plate Dissipation is the amount of power that is lost (dissipated) in the plate structure as a result of its resistance, and the current flowing through it. This lost power is given off as heat. A tolerable amount is just part of the cost of using tubes as the active components in a circuit. Too much can lead to what is called 'catastrophic failure'. If you hadn't shut down when you did, the glass bottle might have shattered.
The Plate Dissipation is kept within the design limits by adjusting the negative bias voltage applied to the cathode of the tube. I suspect that since there is no provision for simple adjustment of the cathode bias, that re-tube job six years ago amounted to simply stuffing new tubes into the appropriate sockets and hoping for the best. A fresh set of tubes in an Ampeg amp should last for more than six years. My VT-22 had the original tubes in it when I bought it last year. It was made in 1976!
As a minimum, you will need to replace the tubes in the output section. The fact that only one of them got red-hot indicates a problem in the wiring around just that tube, but while it was dying, the rest of them would have been subjected to stresses that will make it very hard to complete a matched set.
Matching is important in any amp that puts out more than about twenty Watts. The ouput section is usually a push-pull design that depends on having identical tube characteristics on both sides of the circuit. Ampeg amps, in paticular, apply high plate voltages that will destroy lower-quality or mismatched tubes in short order.
High Plate Dissipation (enough to make it glow!) results from insufficient bias voltage at the cathode. You may find a bad solder joint, or a broken resistor on the underside of the socket.
If this is unfamiliar territory for you, PLEASE take your amp to a competant shop for examination by a professional tech. The voltages in tube amps, (especially Ampegs!) are LETHAL!
Lordathestrings
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