In Reply to: RE: Pentode Driver Comparison 6C6, EF37, 717A, etc. posted by lovetube on February 14, 2009 at 05:35:49:
Hi LT,
I used a low plate resistor of 47k mainly to minimize the output impedance of the driver. Also a smaller plate resistor will give you a higher plate voltage.
My B+ is 350V and the plate voltage is 180V. I need +/-50V peak to peak anode swing to drive the PX4 to full output. The screen voltage is about 116V. So the plate will never have a lower voltage than screen. If you look at the plate curves, if the plate voltage is lower than the screen, you run into the very non linear part of the plate curves. I tried to minimize that.
The aim is to prevent the driver clipping before the DHT.
The other factor that affects the plate voltage is the plate current. In a pentode, the plate current is controlled by the screen voltage and grid voltage. So if you want a higher plate voltage, you either lower the plate current, or lower the plate load resistor. Other than that, you need to increase the B+.
The way I did it was select the plate current that would minimize the DHT slew rate limitations first. Fixed that, then calculate the lowest plate resistor that can give me enough gain. I then look at the plate curves and select the screen voltage that would give me that plate current.
I than look the the grid voltage and fixed that. Finally I calculate the B+ I needed to get the plate voltage. The plate plate voltage should be > screen voltage + negative bias voltage of the DHT. The B+ would be plate voltage + (plate current x plate resistor).
Finally check the plate dissipation and see if it is over spec.
Thorsten did a nice write up in diyaudio about how to design a good pentode driver for DHTs.
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Follow Ups
- Plate voltage and plate resistor - rtsang 08:31:45 02/14/09 (0)