K&K Audio / Lundahl Transformers

RE: new output tubes for my ST70 driver...

68.221.31.92

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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

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