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Has anyone tried regulating the b+ on a Dynaco St-35? I have the Gillespie modification on my st35 clone. I figure some further improvement can be had by at least regulating the b+ for the front end.
Anyone have any input?
thanks.
Follow Ups:
If you have implemented EFB then you already have an automatic adaptation
of the el84 bias to variations of B+. No extra regulation needed.
Regulating the pretube B+ won't give much as they are incorporated in
the global feedback circuit.
Regulating filament could give som stability at large changes in AC
( the tubes properties is to a degree depending on filament current),
thus creating a 6.3V ( or 12.6V ) Stabilized DC would be of value. Use
on all tubes for best result.
You could probably regulate the B+ for the front end. You could literally pile up some zeners on the other side of that 6.8K resistor across the 20uF caps. You may want to reduce that resistor to 4.7K to allow for zener current and maintaining the desired voltage.
Do be prepared to use a string a bit lower than what you're looking for, as high voltage zeners tend to suck down a lot of current to hit their target voltage. (And for goodness sake, don't run a 1W zener at more than 300mW, though 1W at warmup is OK)
Thanks. I was thinking more along the lines of a floating Maida-style regulator to do the entire amp.
So I decided to go ahead and try it. I got one of Pete Millett's Maida style floating regulator pc boards. I inserted this before the B+ takeoff which powers the 12dw7 drivers. The input to the regulator was around 350 VDC. It's output is 330 VDC.
In my system this turned out to be a worthwhile improvement. More space between the voices. Better depth. I like it.
Used with Bill Gillespie's EFB mod, this is a nice sounding amplifier.
One more thing: I placed a Solen poly 40 uF cap directly after the rectifiers and in front of a choke. This too improved the performance.
Dave Gillespie
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