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Re: ST-70 Buzz - chokes?

I agree with Charles - and would add - if the choke is buzzin' do look at the filter cap (sometimes the first section has dried out, and the power supply is acting more like a choke input).

If it isn't a mechanical buzz, I'd look at the 7199's. They tend to get noisy if they develop heater-cathode leakage. This reflects a poor design in the ST-70. There was an article in Sound Practice (RIP) about this many years ago. The heater should have been bias'd up about 40vdc which puts much less stress on the 7199's. Pretty easy to correct.

Next take a peek at "lead dressing". Make sure that the wiring matches the Dynaco pictorial.

If anyone has changed the line cord to a 3 wire cord you'll also possibly encounter hum.

Next try - I'd resolder EVERYTHING. Especially the resistors on the PC board and the tube sockets.

IMHO, the ST70's filtering is just fine - and that adding more filtering to the power supply makes them sound constricted and dull. If you do think the filtering is a problem clip lead a 20uf/450v cap across the second section as a test. If the hum doesn't change the filter cap isn't the problem.



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  • Re: ST-70 Buzz - chokes? - Gary Kaufman 02/3/0610:32:48 02/3/06 (0)


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