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Well...

Ok, so here's another opinion. Please take with the appropriate dose of NaCl.

The ST70 bias' the EL34's by measuring voltage across a 15.6 ohm resistor, so that measuring 1.56 volts is the equivalent of 100ma per pair of EL34's. 1.56 volts wasn't an accident, it's the voltage of a fresh carbon-zinc "D" cell, and allowed an inexpensive reference for the user.

Frankly I tend to bias the ST70 a bit "cooler", typically around 45ma/tube. V=IR, so (.090)(15.6) = 1.4v The Dynaco can run pretty hot (especially the power transformer). A small drop in idle current makes a pretty large difference. With well matched output tubes I don't find any significant difference. Well matched (and not just gm, but plate current at full B+) is the key - and many commercial vendors don't match tubes worth beans.

But a few suggestions if you're getting flabby bass. Make sure that your 15.6 ohm resistor is really measuring 15.6 ohms - they do drift with age. Second, make sure your filter caps are reasonable - check the ESR with an appropriate ESR meter or consider replacing them. If you still have the original "leaky" coupling caps swap them out. If you really want flabby bass, try triode wiring the EL34's (I know some folks like this mod, I never have). Make sure the feedback circuit is intact. Make sure you're using the correct taps on the output transformers.

And if you really do find the ST70 sounds best at 50ma/tube go for it! Kico's correct - the Svet's are quite inexpensive. Just make sure the correct fuse is in your ST70 and keep a good airflow around the amp.




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  • Well... - Gary Kaufman 03/25/0314:27:06 03/25/03 (0)


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