New Tubes (6CA7)--now bias too high

Dynaco-Doctor Forum

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Hello:

I have an ST-70 with Joe's restoration board and quad-cap replacements that has been serving me well for several years. I currently have an original (or old) set of Mullard EL34s and a newer set of EH EL34s, both of which function fine and are easily biased somewhere in the middle of the bias pot's rotation. Wanting to try some different tubes, I ordered a matched set of 6CA7s from Antique Electronic supply. I eagerly popped them in this evening and went through the process of setting bias; however, after several seconds the voltage reading at the "biaset" point climbs well above 2.5vdc even with the bias pot turned all the way counterclockwise. I haven't pulled the bottom cover yet to take measurements, but verified that the 15.6 ohm resistors read 1.57 ohms (within reasonable tolerance). Anyone have any recommendations? Does this seem like it could be remedied through Joe's bias expansion mod or are these tubes pulling too much current? Your suggestions are greatly appreciated.

-David

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Topic - New Tubes (6CA7)--now bias too high - daveap 17:15:07 02/08/16 (10)

RE: New Tubes (6CA7)--now bias too high - xaudiomanx 14:24:10 02/15/16 (0)

RE: New Tubes (6CA7)--now bias too high - petercapo 15:49:23 02/09/16 (8)

RE: New Tubes (6CA7)--now bias too high - daveap 17:04:47 02/10/16 (7)

RE: New Tubes (6CA7)--now bias too high - petercapo 17:19:10 02/10/16 (6)

RE: New Tubes (6CA7)--now bias too high - daveap 13:19:56 02/11/16 (5)

RE: New Tubes (6CA7)--now bias too high - petercapo 14:15:10 02/11/16 (4)

RE: New Tubes (6CA7)--now bias too high - xaudiomanx 17:49:02 02/15/16 (3)

RE: New Tubes (6CA7)--now bias too high - daveap 15:04:04 02/16/16 (2)

RE: New Tubes (6CA7)--now bias too high - xaudiomanx 17:32:24 02/16/16 (1)

RE: New Tubes (6CA7)--now bias too high - jsblock 04:56:04 05/05/16 (0)

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