bias question

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Re: bias question

Postby Jimmy74 on Tue Nov 04, 2014 2:34 pm

Ok I've redone the tests:
main filter cap = 438vDC
pins 3 on power tube sockets = 437vDC
reading off 1 ohm resistors = 51 and 52mv

how does that sound?
Maybe a difference in readings due to daytime/ nightime?
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Re: bias question

Postby PeterS on Tue Nov 04, 2014 5:12 pm

Seems like a big variation to be due to the power line (10%).

Anyway 0.052*437 / 30 watts = 76%, so I'd cool it down some. The B+ may increase as you reduce the current.
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Re: bias question

Postby Jack Hester on Tue Nov 04, 2014 5:13 pm

Jimmy74 wrote:Ok I've redone the tests:
main filter cap = 438vDC
pins 3 on power tube sockets = 437vDC
reading off 1 ohm resistors = 51 and 52mv

how does that sound?
Maybe a difference in readings due to daytime/ nightime?


437vDC on the plates and reading 51-52mv means you are running right at 75% current calculated from a 30 watt max plate dissipation. You are still safe, as rule of thumb says 70%. If your bias is adjustable, tweak it down to closer to 41-42mv, which will put you in a comfortable 60% of max dissipation. If not adjustable, enjoy, and know that you are ok.

Jack

Note: PeterS and I replied at just about the same time. He's spot on.
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Re: bias question

Postby Jimmy74 on Tue Nov 04, 2014 5:51 pm

Well thanks for the tips, and yes I think the differences do have to do with the power line, anyways I've taken it down to 41-42mv.

thanks again
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Location: Italy

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