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-   -   Optimal operating point for 7N7/6SN7? (http://www.diyaudio.com/forums/tubes-valves/134862-optimal-operating-7n7-6sn7.html)

 pftrvlr 14th December 2008 08:19 PM

Optimal operating point for 7N7/6SN7?

The typical operating plate current for 7N7/6SN7 looks to be 10ma. But due the limitation of the PS, it can only set to around 5ma. Will the distortion be high?

The 6SN7 is used as the first stage voltage amplification of CD signal.

Thanks.

 Tubes4e4 14th December 2008 08:44 PM

Hi pftrvlr,

never mind - using 6SN7 and brothers at Ia=5mA you definitely can get an extremely linear swing using an RL of at least 27k and a reasonable supply voltage.

More information on maximum expected input swing, needed stage amplification factor and maximum available supply voltage certainly would help to give specific operating point suggestions.

Regards,

Tom Schlangen

 jon_010101 15th December 2008 01:50 AM

For the first stage with a line-level signal, 5mA will be fine! The Williamson amp drives triode KT66s with 6SN7s running about 5-6mA. No problem :smash:

 pftrvlr 16th December 2008 05:41 AM

I re-cal the operating points in the p-spice, and here is what I got

Plate 160v
Cathode 5.5V
Current 3.0mA
Plate reisistor 51K

I need to keep the plate voltage at around 160v, because it direct couple to the next WCF stage.

Will it be better off to reduce the plate resistor and increase the plate current to 5ma? The gain is not important.

Thx.

 kuka_v 16th December 2008 10:25 AM

I would like to suggest:

1:
Ups =400v
Up= 131v
Ip=9.5mA
Ug=-2v
Rpl=28kOm

or

2:
Ups =230v
Up= 107v
Ip=6.5mA
Ug=-2v
Rpl=18kOm

In both cases about 0.00 % distortion....

 Sheldon 16th December 2008 04:43 PM

Quote:
 Originally posted by pftrvlr Will it be better off to reduce the plate resistor and increase the plate current to 5ma? The gain is not important.
Three mA current is plenty, as long as your next stage doesn't load the output too much. I'd stick with the larger plate resistor. Or try both and see what sounds best to you. Easy enough to clip in parallel plate and cathode resistors.

Sheldon

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