Help! Initial ias too high with OE Mullard + DIYTube board?

knowledge base for the classic Dynaco ST70

Help! Initial ias too high with OE Mullard + DIYTube board?

Postby DerekVa » Thu May 16, 2013 10:08 am

I'm firing up a Stereo 70 for the first time after a DIYTube rebuild and I have an issue with the biasing. Specifically, when my wirewound pots are turned completely counter-clockwise, I'm getting a bias reading (no 5AR4) of -41.7VDC on each side whereas the instructions in the driver board manual implies I should be around-55VDC. The non-driver portion of the amp is otherwise stock other than (a) the selenium rectifier has been replaced with a u4007 and the bias caps and resistors have been replaced with 47uF / 100V and 10k / 2W parts, (b) The can cap is an 80/40/30/20 CE Manufacturing, (c) the rectifier has the 'diode mod' to protect the rectifier and (d) there are X1/Y2 caps on each leg of the AC line and a CL-90 thermistor in line with the fuse. Tubes are OE Mullard 12AU7, Tung-Sol 5751 and Dynaco / RCA-branded Mullard EL34. Transformers are original 1963 vintage.

Am I OK to put in the rectifier (sacrificial Toshiba) and move on to the second part of biasing or is my bias voltage too high?

Thank you,

-D
DerekVa
 
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Location: Woodinville, WA

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