Failsafe Bias for the ST-70

knowledge base for the classic Dynaco ST70

Postby Geek » Sat Aug 08, 2009 2:00 am

mesherm wrote:
I used a trick like that until a fellow pointed out that you don't want stiff voltage there... if the line voltage goes up or down, you want the bias to do the same and "track" the fluctualtions.


In my ST-70 days I retrofitted at lest two ST-70s with CC Chong cap boards. Those boards used a Zener diode to regulate the bias voltage to -56 volts. If what you are saying is right then there are a lot of people with CC Chong modded units with problems =:o


If their line fluctuates, yes, bigtime. The ST-70 is the hottest biased, non-auto-bias-correcting amp I've encountered so far... even all the guitar amps I work on are rarely over 80% Pa. The 50mA per tube will take a stock ST-70 to 96% Pa of an EL34 with an original 117V mains transformer.

Luckily, most areas of North America, Europe and Japan don't have this issue. My line is 126V solid with a mid-heatwave dip of 123.5V when all the A/C's are blowing snow to make us Canucks feel at home (lol)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby davygrvy » Sat Aug 08, 2009 2:27 am

Geek wrote:I'll load it up on the wife's box (she has XP, I use *nix)

Runs fine under WINE. I use *nix as well. I don't know if it is documented in the help file, that isn't readable anyways at first, but you can install xCHM and LTspice will recognize that as loader for the helpfiles.

I love LTspice. I hope you can find it useful as well.
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