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In Reply to: RE: Tracked down a problem today... posted by Lee of Omaha on July 05, 2016 at 19:41:18
Lee
The 47uf cap you may want to remove if you have excessive gain..I wire all my boards for 12BH7s for the PS/driver and the voltage amp for a 12AU7.
Those are excellent boards but they can be tricky but playing with different configurations help.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Follow Ups:
The 47uF cap is the input triode's filter cap. In attempting to measure the input load resistor, we were also measuring the filter cap through the grid-to-plate short. Too bad, it was a nice Sylvania chrome dome 6SN7.
Between TubesForHifi's generous on-board filtering and our power supply, out ST-70's power supplies are stiff as a board. If you unplug it while on, it plays for about 20 seconds (until the cathodes cool).
When you said bypass,all I could think of was the 47uf bypass cap that Roy used to put on the ST70 and MK3 boards to set gain and now he has done away with it.I try to keep a few on hand being they are so perfect.I usually buy 10 at a time and it takes a couple years to go thru them.
He does give you a generous amount of filtering.That is the most brilliantly designed ST70 board ever..You have lots of filtering,separate bias pots along with AC balance pots,and you can set it up for different tubes.
Did you ever try a 35 to 40uf film cap off the GZ34 in those? Even in the ones I use Schottky diodes in I use a film cap in the charging stage..It really tightens the bass and cleans up the mids and lower highs.
"
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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