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In Reply to: Re: Linestage Design posted by sorenj07 on October 20, 2006 at 22:04:51:
OK, fair enough. But a true grounded-grid input is right into the cathode and therefore the input impedance is low – lower than what you’d typically want for a line stage. I think what you’re referring to is a differential amplifier (long-tailed pair) that is sometimes called grounded-grid because the first triode’s cathode drives the second triode’s cathode. I think it is misnomer to call this a grounded grid, but it’s only semantics. In this case the input is to a grid, and that’s good for a line stage.Try connecting your two 12BH7 sections (one tube) together, cathode-to-cathode. Run a CCS from the connected cathodes to ground (or to a -6.3V or -12.6 V supply used for heaters.). Choose plate resistors to B+ that are several times higher than rp (in the 15K to 30K range). Drive the first grid with your volume control. Ground the second grid (there’s your grounded grid). Connect the grid of a cathode follower directly to the second triode’s plate. Voila. Gain will be in the 15dB to 18dB range, non-inverting, close to ideal for a line stage these days. If you wanted to experiment with feedback, return the output through a resistor divider to the second triode’s grid, but you don’t have much gain to spare with a 12BH7.
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- Re: Linestage Design - BBeck 06:39:01 10/21/06 (0)
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