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Old 26th September 2011, 01:19 AM   #1
dgta is offline dgta  United States
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Default Grid current operation

Haven't indulged in AB2 much, but I'm about to and have some basic questions.

It is generally advised to increase driver current capability (no surprise there), but I find no data. Where do you find the grid impedance curve in that region? How do you know if a CF is enough or you need a 3A source follower?

Is it trial-and-error or just ancient Chinese secrets passed from father to son?
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Old 26th September 2011, 01:50 AM   #2
SY is offline SY  United States
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Ancient secret. Will be revealed by year's end. But there's a few datasheets that are nice enough to spill the beans.

If you can pump 10 mA comfortably, you can drive 6L6GC (a really excellent tube for this service) well into AB2. I've used MOSFET source followers (IRF820) for this- if you want to be cute, the source followers don't have to run class A...
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Old 26th September 2011, 02:03 AM   #3
dgta is offline dgta  United States
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Thanks Sy. Is that December 31st or Chinese year's end? Or perhaps Rosh Hashana?

Well, it so happens that I have a nice Williamson with 6L6GCs driven by 6SN7 diff pair that might just be able to do that without any followers. Do you think?

That's the frustrating part for us neophites. Should I try this or that? We need a trade union with apprenticeships so we can pass these secrets down...
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Old 26th September 2011, 02:28 AM   #4
SY is offline SY  United States
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December 31.

No, you want followers to get AB2 with any kind of acceptable distortion. It could also possibly be done with a transformer at much greater expense and somewhat lower performance; ditto using drive from the plate of a power tube. Followers are easy, help prevent blocking, and can have impeccable performance. There have been some commercial designs (e.g., Fisher 50A) that used transformer-coupled followers and performed quite well.
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Old 26th September 2011, 03:59 AM   #5
dgta is offline dgta  United States
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Thanks Sy. Followers it is.
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Old 26th September 2011, 09:50 AM   #6
bigwill is offline bigwill  United Kingdom
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Seconding MOSFET followers, they work brilliantly and also have great distortion performance, even under fairly big voltage swings
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Old 26th September 2011, 10:17 AM   #7
SY is offline SY  United States
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I used source followers, but if you're a purist, a cathode follower from a tube that can swing some current will also do well.
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Old 26th September 2011, 10:27 AM   #8
DF96 is offline DF96  England
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To learn about AB2 you probably need to look for information on either serious PA amps or AM modulators, as these were the main applications. Hi-fi usually preferred AB1.
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Old 26th September 2011, 12:04 PM   #9
dgta is offline dgta  United States
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Oops, I just realized the followers need a negative supply, which is not easy on the amp I had in mind.

Any chance I can get away with putting the MOSFET before the coupling cap?
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Old 26th September 2011, 12:08 PM   #10
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by dgta View Post
Oops, I just realized the followers need a negative supply, which is not easy on the amp I had in mind.

Any chance I can get away with putting the MOSFET before the coupling cap?
Sadly no, because the grid current will charge up the coupling capacitor, which will throw the bias out, causing blocking distortion
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