RCA SP-20 circuit question.

donnyab · 18th March 2013, 07:10 PM

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

DF96 · 18th March 2013, 08:29 PM

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.

18th March 2013, 07:10 PM	#1
donnyab diyAudio Member Join Date: May 2011	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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18th March 2013, 08:29 PM	#2
DF96 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.