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Supro Thunderbolt S6420 (c. 1966)

My customer bought this Supro Thunderbolt online, and was disappointed on arrival that it sounded harsh, thin, and noisy. Luckily for him, there were absolutely no major problems. Someone had already replaced some filter caps, and even though they didn’t use the highest quality parts, they were sufficient for now. What it did need was a replacement of the input cap, and a couple of resistors. Also, one of the 6L6 tubes was making a mechanical rattle. I happened to have a brand new pair of TAD Redbase tubes that had just arrived, and they were perfect for this amp. I replaced the old “lamp cord” with a 3-wire cord, and made sure everything was safe and solid.

This amp has an unused triode in the preamp, which provides us with a nice opportunity to make an improvement. Some people would use it to add another gain stage, but that changes the whole character of the amp. Instead, wiring it so that the first stage becomes a parallel triode pair improves the signal-to-noise ratio, and gives a very slight gain bump, while not altering the personality of the amp at all. Just what this amp needed!

With the circuit optimized, the new TAD power tubes, and that big beefy 15-inch speaker, this amp is a tone monster! Of course, no vintage Supro comes through here without me indulging in some early Zeppelin tones, and this one delivered beautifully.

It just happened to be here at the same time as a 1965 Supro Bantam, so I figured a family photo was in order. Don’t they make a handsome pair?


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