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In Reply to: RE: new output tubes for my ST70 driver... posted by KevinC on May 17, 2009 at 07:00:05
Kevin,
I have the dynakitparts.com power transformer and the filament taps have a higher amperage rating than the original. I'm not sure how much more it is than the original, but I'm assuming it may be able to handle a current of around 60-70mA per tube per channel. See link below.
If I wanted to extend the current source's adjustment range so I could set it to around 60-70mA, would I simply be dropping the resistance values of R109/209 and R110/210?
As I wrote this, the KT88s came in the mail. What timing.
Best.
It is, unfortunately, the main high voltage supply winding current rating that determines how much bias current you can draw. For a capacitor input power supply like that in the ST-70, the value for this transformer is 0.63 times the AC current rating (300mA) or about 190mA. Of course, you can push that a bit, but it will raise the temperature of the core to do very much more current than that. I would absolutely not use more than 60mA of bias current. To change the bias current, you adjust the bias pot settings, R105 (205 and R108 (208) while monitoring the voltage drop across R104 (204) and R113 (213)P: 1mVDC = 1mA bias current.
Kevin Carter
K&K Audio
www.kandkaudio.com
Thanks for the assistance Kevin.
The KT88s sound great biased at 60mV. They're a superb sonic complement to your front end and I'd urge anybody who is using your driver to go the extra mile with these outstanding tubes. I can't wait to listen to more music.
Great job Kevin!
Best.
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