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RCA SP-20 circuit question.
RCA SP-20 circuit question.
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Old 18th March 2013, 07:10 PM   #1
donnyab is offline donnyab
diyAudio Member
Join Date: May 2011
Default RCA SP-20 circuit question.

Could someone take a look at this circuit and explain the biasing arrangement of the 6AU6 tubes.
Specifically what is the purpose of the 330k resistors between the 1 MOhm Rg resistor and the 1800 Ohm Rk biasing the cathode.
The fact there is even a resistor between them is confusing me.
Thanks in advance.

Link: http://www.oswaldsmillaudio.com/imag...p%20Manual.pdf
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Old 18th March 2013, 08:29 PM   #2
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
I think this question came up in another thread a few months ago. Did you search?

The cathode resistor, presumably used for degeneration, generates more voltage than is required for valve bias so the grid is attached to a potential divider formed by the 330k and 1M.

Most circuits either ground the grid (for DC) or pull it up to a significant positive value. This circuit does not, it pulls it up just a small amount, so it looks strange at first sight.
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