This cap could lower PSU noise if properly sized - sonicboom 19:47:14 08/29/15 (0)
 RE: direct coupled amp schematic question - Palustris 18:48:46 08/29/15 (0)
 RE: direct coupled amp schematic question - Caucasian Blackplate 11:58:14 08/29/15 (0)

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 This cap could lower PSU noise if properly sized - sonicboom 19:47:14 08/29/15 (0)
 RE: direct coupled amp schematic question - Palustris 18:48:46 08/29/15 (0)
 RE: direct coupled amp schematic question - Caucasian Blackplate 11:58:14 08/29/15 (0)
 RE: direct coupled amp schematic question - Palustris 08:26:54 08/29/15 (0)

• Western Electric - Rosetta Stone for Triodes

That said, I rebuilt a friend's 300B "Monkey" last weekend, converting to E180F from 6N23P. This is an excellent sounding amplifier using the Magnequest FS 030 output transformers that has been modified several times in an effort to achieve a clear and open sound without any harshness or high frequency glair. The change from the 6N23P to E180F completed the task. They are very clean sounding amps and run the 300Bs at just over 2A3 power levels; operating very relaxed. Not everyone will agree with the operating points, but when power is not the object, then the operating point can be chosen for excellent sound with long tube life.

You will notice that this schematic requires two fewer resistors, but adds a plate choke. Since the plate choke drops far less voltage than R4 in the drawing above, the driver plate supply can be taken from the cathode resistor of the 300B eliminating the top resistor in the voltage divider. Now, you have not only eliminated a resistor but also the unnecessary three resistor voltage divider. Capacitor C4 in the previous drawing is included as a necessary component in the design. When building a "monkey" one establishes the operating points of both triodes and then works out the cathode resistor voltage divider to set the tubes up with the correct operating points using the laws of Mr. Ohm and Mr. Kirchhoff. I tried a red LED to bias the E180F but the -2V bias was too cold and I didn't have enough current: I was looking for a plate voltage around 125V and between 10mA and 15mA plate current. I next tried two 1N5408 in series and got -1.5V: still not enough current; 1 1N5408 and got the expected -.75V but grid current started about -.6V meaning the driver was distorting at the same time as the output tubes; obvously, we want the E180F to drive the 300B to clipping with no distortion. Finally, I tried a Schottky diode and got -.95V for about 12mA and about 125V on the plate: perfect.

BTW: the E180F is seriously undervalued right now. Buy 5 by the same manufacturer and match them up in the amp; you should get at least a couple that are a perfect match.

The plate choke is by Electra-Print.