Tube DIY Asylum

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RE: Two Questions

Posted by mikeyb on August 1, 2007 at 13:16:09:
"can I omit the plate resistors entirely or put in a much lower nominal value (like 1k) and still have the lower output impedance? Or does this create some other problem?"

No, you cant't do either one. The plate resistor is part of the 'load' seen by the 6SN7s, and shouldn't be set much below a value of 2*Rp (about 15K Ohms).

"Yes and I'd like to eliminate that tube. Is this possible? Does the 120v have to come from a 'pre-driver' tube or can it be supplied from a cap in the PS and have the input signal come from the normal RCA jacks?"

It is possible to eliminate the predriver, but the grid to plate voltage and grid to cathode voltages in the LTP should remain the same. With a 300 volt B+ supply, one alternative would be to connect the plate resistors to +300, tie the grids to ground via a 220K resistor, and move the resistor in the cathodes of the 6SN7s to -115 volts. Yes, that would require an adiitional supply, but that would also allow you to use fixed bias on the 6B4s.