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RE: Schematic?

Posted by Triode_Kingdom (A) on June 30, 2021 at 15:42:24

In Reply to: RE: Schematic? posted by pictureguy on June 30, 2021 at 11:57:26:

Yes, the 12AU7 has been used in a large amount of gear over the years. As a cathode follower or cathodyne inverter, it provides good service. However, some amplifiers have used it as a gain stage, and they often get away with this - sonically speaking - because A) distortion at the output of the amp equals the square root of the sum of the squares of THD generated within each stage, and B) negative feedback typically reduces the harmonics by a factor of 10. Both of these serve to reduce the damage on paper, but this doesn't mean you won't hear the shortcomings of the tube.

The schematic I posted shows a 5V drop across a 1M resistor. That implies it was measured with a meter having a 22M input impedance. The amplifier is from the '60s, so they probably used a VTVM for the measurement.

Sorry, I had to laugh about the intellectual property concept. We're talking about tubes designed many decades ago and circuits invented before most of us were born. Any manufacturer who thinks a long tailed pair on a Dynaco driver board contains intellectual property is seriously paranoid. The value in that product lies elsewhere.

About the CCS, it's necessary to understand that the RMS characteristics of the LTP are being modified in accordance with the frequency response of the IC, as well as by instantaneous response to the changing values of the tubes. I'm not a fan of mixing SS devices and tubes in the signal path, but if I were, this job would be much better managed by a MOSFET or two. It's also worth pointing out that the purpose of the CCS is to balance the LTP, but that eliminates the AC balance control, so the AC characteristics of the output tubes can no longer be manually balanced. Instead, we have to rely on the application of feedback and its attendant limitations.

I'm no expert when it comes to SS processing, but I did provide technical writing services to a company that manufacturers the fabs themselves. Always had to wear a bunny suit to avoid contaminating the equipment prior to it being shipped out. Spacecraft? Some of the products I helped design and test over the last fifteen years at my last position are now on Mars. I guess that's why I got a chuckle from the intellectual property statement. : )

Good luck with your search, there are many routes to success with tubes, old and new.

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The Cable Cooker

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