yarmo
Junior Member
Posts: 69
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Post by yarmo on Aug 25, 2014 9:33:03 GMT -7
Hi guys, I replaced all the tubes today and with the bias pot completely turned clockwise (the starting point), the reading on the DMM is already at 39.5 mA and nowhere else to go but upward?!! I've waited an hour and the reading is now at 39.3 mA. I usually set the bias at 36 mA. Tubes are JJ EF8O6 gold pin - Gold Lion 12ax7 -- Gold Lion KT66 -- Mullard GZ34 - all brand new. For the heck of it, I put the old rectifier tube along with the new power tubes to see what it would do. The reading lowered to 36 mA but still, the only way to go was upward. I left it there since I have a gig this thursday and after playing the amp for a good three hours, all seemed fine for now. Thank you very much!! Read more: ztalk.proboards.com/thread/63936/bias-high?page=1&scrollTo=711652#ixzz3BQEV3ka6
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Post by DRZ on Aug 25, 2014 11:18:02 GMT -7
Try to remember to request MEDIUM range tubes when ever your replacing output tubes from your supplier. The KT 66's you received are probably high range or higher transconductance output leaving little or no swing on the bias pot.
You can increase the bias resisters value by say 10 or 20k and increase the bias range for those tubes. The bias resistor is the resistor connected to the bias pot.
Or you can leave it alone if you if you like the sound, KT 66's can be safely biased to a max of 44Ma / tube at idle. So 39 or 36 will be OK, I like 32 to 34 mA my self.
DR.Z
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yarmo
Junior Member
Posts: 69
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Post by yarmo on Aug 25, 2014 12:16:02 GMT -7
Thank you very much Godfather!
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Post by thunderstruck(formerly doc001) on Aug 26, 2014 7:56:39 GMT -7
Excellent!
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