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Old 15th September 2009, 02:37 PM   #1
Cassiel is online now Cassiel  Libya
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Default Hey SY, were you aware of this?

http://www.labjc.com/?p=21

Catching on.
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Old 15th September 2009, 03:23 PM   #2
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Nope. Nice to see LEDs get their due. This is a trick I first saw back in the late '70s, and when used appropriately, it's a great trick. I wonder why this guy didn't bias the output stage with an LED array... There can't be too many LEDs in a project. Nope, can't be too many.
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Old 15th September 2009, 06:46 PM   #3
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Originally Posted by stalker View Post
Linearizing and maximizing output swing from a small signal pentode with a CCS paralleled with a resistor is an old trick. It does take some playing around with screen voltages to minimize distortion, but it can definitely do quite well.

I've seen that before, but can't recall where that was.
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Old 15th September 2009, 07:08 PM   #4
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I've seen that before, but can't recall where that was.
Try Gary Pimms web site. On the home page look about 2/3 the way down the column on the left for "pentode + CCS driver". I tinkered with this idea a few years ago and found that the results are highly tube dependent. Each tube (from a batch of the same number and date codes) needs its own tweak to the screen voltage and CCS current. Some just flat refuse to work, and some work great. The unbypassed screen resistor helps a bunch in the distortion department, and again the value is tube dependent. Oddly, the 6CL6 worked quite well for me even though is was one of his worst performers.

http://www.pimmlabs.com/
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Old 16th September 2009, 05:35 PM   #5
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Originally Posted by tubelab.com View Post
Try Gary Pimms web site. On the home page look about 2/3 the way down the column on the left for "pentode + CCS driver". I tinkered with this idea a few years ago and found that the results are highly tube dependent. Each tube (from a batch of the same number and date codes) needs its own tweak to the screen voltage and CCS current. Some just flat refuse to work, and some work great. The unbypassed screen resistor helps a bunch in the distortion department, and again the value is tube dependent. Oddly, the 6CL6 worked quite well for me even though is was one of his worst performers.

http://www.pimmlabs.com/
Yeah, that was it. I was considering the idea for the design of an 845 SET, and noted that comment about the need for "tweaking" and the highly variable performance and figured t'hellwiddit, in favor of a more conventional solution: a two stage voltage amp based on the 6DR7 -- very nice small signal unit, and a large signal unit that can stand up to some serious voltages.
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Old 16th September 2009, 05:44 PM   #6
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a two stage voltage amp based on the 6DR7
I have played with the 6DR7. It has a brother the 6DE7, and the bully in the family the 6EW7. These tubes all have the same pinout, but slightly different gain. The 6EW7 is about twice as big, but still has a 9 pin base.
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Old 16th September 2009, 06:57 PM   #7
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Originally Posted by tubelab.com View Post
I have played with the 6DR7. It has a brother the 6DE7, and the bully in the family the 6EW7. These tubes all have the same pinout, but slightly different gain. The 6EW7 is about twice as big, but still has a 9 pin base.
Both the 6DE7 and 6EW7 have small signal sections with u= 17.5. The 6DR7's small signal section has: u= 68. You can get quite a bit more gain out of that. The linearity also ain't none too bad.
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Old 16th September 2009, 08:46 PM   #8
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Originally Posted by SY View Post
Nope. Nice to see LEDs get their due. This is a trick I first saw back in the late '70s, and when used appropriately, it's a great trick. I wonder why this guy didn't bias the output stage with an LED array... There can't be too many LEDs in a project. Nope, can't be too many.
but NE2's add that piquant touch.
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Old 18th September 2009, 12:11 AM   #9
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Hmmm, dunno about using NE2s as voltage refs. Out of a bag of twenty I find a firing voltage between 60V and 94V.
They are good in a string at the end of a long aerial (almost half a mile) to drain atmospheric potential (lethal). The last one in a string of twenty or so can have a diode and a piezo sounder across it, it makes a cracking indication for an impending storm. Of course they need to be in a strong case. If there is a lightning strike then all the guts will be blown from everything. A diecast ally case will suffer severely (I know) but it will be the shortest path and save most other kit.
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Old 18th September 2009, 12:30 PM   #10
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Hmmm, dunno about using NE2s as voltage refs.
SY used them in the Impasse preamp to limit the voltage from grid to cathode on the 6922's.

I had heard of the antenna trick -- but it must have been 30+ years ago!
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