Flea-power class-A2 using a power opamp driver - diyAudio
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Old 23rd November 2010, 01:50 PM   #1
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Default Flea-power class-A2 using a power opamp driver

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I always wanted to try using a power opamp as a class A2 driver. I finally got around to some experiments. I managed to have some sucess, especially using a 1636 (ala Darling amp) to get ~1.2W out. Still experimenting - I need to try push-pull, and some different tubes. Here's the info:

Class-A2 and a power opamp driver

Let me know if you can think of any candidate tubes. The power opamp can only put out about +/-36V @ 50mA, so stuff like 811's are not workable. So far I've tried the 1636, 2E26 (OK but not a lot of advantage over A1), and 6V6/6L6/6W6 (worse than A1). Seems like some of the newer semi-useless 9-pin-mini RF tubes might do well here?

Pete
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Old 23rd November 2010, 03:41 PM   #2
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Small power valves were usually either:
- not intended to take grid current, so the grid may overheat
or
- intended for Class C, so not especially linear

Exceptions are QQV series tetrodes (e.g. 6360 in USA), which have a rugged grid for Class C but are also intended for linear applications (mainly AB1). Another possibility is 5B/254M, but I would expect that to behave like a 6L6 or 807.
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Old 23rd November 2010, 03:44 PM   #3
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Old 23rd November 2010, 06:11 PM   #4
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Looking at the grid drive 'scope picture on your website makes me wonder if the TI amplifier chips would have an issue with asymmetrical power supplies. If the supplies don't have to be symmetrical the supplies could be optimized for the tubes needs.
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Old 23rd November 2010, 07:34 PM   #5
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How'bout 3A5?
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Old 23rd November 2010, 07:37 PM   #6
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I just got a bunch of VT61A dual triodes that have ability to sink up to 17ma of grid current...
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Old 25th November 2010, 06:57 PM   #7
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I am planning to build a se amp with a 1619 dht in triode mode. The datasheet shows the positive grid curves for the pentode mode. I manually checked out the triode connection in the a2 region and it looks promising. A +_25v swing with 2k load goes from 135 ma to 20ma. 180v,0 bias and 70ma quiescent current.

Alfredo
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Old 25th November 2010, 10:26 PM   #8
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Quote:
Originally Posted by Gary P View Post
Looking at the grid drive 'scope picture on your website makes me wonder if the TI amplifier chips would have an issue with asymmetrical power supplies. If the supplies don't have to be symmetrical the supplies could be optimized for the tubes needs.
Good point - the IC doesn't care where ground is, so that would work well.

Pete
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Old 25th November 2010, 10:29 PM   #9
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Quote:
Originally Posted by aardvarkash10 View Post
I just got a bunch of VT61A dual triodes that have ability to sink up to 17ma of grid current...
Is that the same as a DET19 or 2C34? I have a few of those, on my list to try. They seem like they would work great in A2.
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Old 25th November 2010, 10:30 PM   #10
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yeah, close to the 2C34 apparently, although I'm having a difficult time tracking down a useful datasheet.
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