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Re: 6BL8's and the dynaco ST-70

The 6GH8 is a common substitute in the ST70, and was used in several aftermarket PCB's. The Russian 7199 is reportedly nothing more than a re-pinned 6GH8. I've personally never tried it, as I'm unlikely to run out of 7199's in this lifetime!

The 7199 was specificially marketed for audio, and reportedly had a twisty filament to reduce hum. It may have all been an RCA marketing trick, as it also has a unique pinout among small triode/pentode tubes. HH Scott made healthy use of 6GH8, 6U8 and 6BL8's in various models without any problems.

That said, there are several more elegant replacement PCB solutions if you choose that route. Ned Carlson, Curcio, AVA and others offer various Dynaco upgrade products. You could retain the original PCB stock to preserve the ST70 for possible future resale.

With the 7199, adding a small voltage divider to reduce the heater-cathode strain on the tube reportedly extends the life of the tube. I believe there was a short article on that many eons ago in Sound Practice.




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