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In Reply to: RE: here we go again....... posted by vinnie2 on August 19, 2016 at 11:54:24
First you need to determine the operating points for the output tube. That tells you what your plate supply requirements are and also what the drive requirements are for the driver. A 71 operating at max plate voltage of 180V will require 41V to drive the tube to clipping. A typical source has a 2V output so no preamp should be required; if plan for an input voltage of 1V to drive the amp to clipping, you need a driver with a gain greater than 40.
Some tube manuals have a chart showing tubes grouped together by their amplification factor. I chose the 6K5 because there seems to be something special about these high mu tubes with their cylindrical plate and their concentric cathode, grid and plate. My next amp will be 6F5 driving a EML 320B using K&K CCS.
Follow Ups:
How did you determine that 41V will be required to drive it to clipping? Those are the kind of things I am hoping to learn.
"How did you determine that 41V will be required to drive it to clipping?"
I learned it from my tube manual which has the operating conditions. Notice the operating points Ep = 180V; Ip = 20mA; Eg = -40.5V.
Since the max signal we can input the tube is 40.5V peak; 40.5V peak or 28.6 Vrms should drive the tube to clipping. That's not much of a demand. There are many tubes that can accomplish this with 1V input. If you choose a lower plate voltage the grid bias will be less and the max input voltage will be lower. As has been pointed out above, there is not much power available with the 71A. I had thought to make a tweeter amp with mine, but went a different route.
Thanks for the explanation. I actually had a 71A amp hooked up to these speakers sometime ago and it was adequate for my listening needs. It is also a very nice sounding tube. I want to try some different drivers to see what they sound like and to try and learn some design in the process.
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