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Old 1st July 2006, 07:48 AM   #1
Nixie is offline Nixie  Canada
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Default Question about changing a hybrid

Can I use triodes in place of Q13/14?
http://www.headwize.com/projects/sho...lmore4_prj.htm (hybrid electrostatic headphones amp)
Since that's doing most of the voltage gain, seems to me a good place for triodes.
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Old 3rd July 2006, 08:23 AM   #2
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Come on, surely someone can help me?
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Old 3rd July 2006, 09:10 AM   #3
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2SK214 has 50 Volts (approximate) between drain and source...
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Old 3rd July 2006, 11:00 AM   #4
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Well that could be raised (I could pull the cathode lower; I'm still winding the transformer so getting other voltages is not a problem). Or low voltage tube used.
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Old 19th July 2006, 09:31 AM   #5
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Anyone?
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Old 20th July 2006, 07:16 AM   #6
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You can see below what I was thinking.

Can someone suggest low voltage and power tube I can use instead of the 6DJ8 to get higher gain, while it's still nicely linear? I don't need much current capability since it's driving a MOSFET follower anyway.

Click the image to open in full size.
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Old 20th July 2006, 09:48 AM   #7
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That is one helluva schematic for a tube amp! One reason you're not getting replies could be sheer intimidation. Truthfully, I wouldn't know where to start commenting on your design.

In my experience, tube designers start off with tubes only, then maybe (but not usually) put in some SS devices where they can really do some good, e.g. for constant current sources/sinks, B+ delay circuits or rectifiers. Tube designs tend to be very simple. That's my 2 cents.
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Old 20th July 2006, 10:01 AM   #8
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Umm, not my design. There's a reason I posted a link in my first post. It's an electrostatic headphone amplifier. And the only part that really matters here is the third stage...

And I had a very specific question:
Quote:
Can someone suggest low voltage and power tube I can use instead of the 6DJ8 to get higher gain, while it's still nicely linear? I don't need much current capability since it's driving a MOSFET follower anyway.
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Old 20th July 2006, 11:52 AM   #9
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High gain, good linearity, and low plate voltage don't often happen at the same time

ECC86/6GM8 may work better here than 6DJ8 - it is specifically a low-voltage design - but don't write off the 6DJ8. With a CCS load they can do quite well at low plate voltages, much better than you would guess looking at the curves in the datasheet.

I have a circuit running on the bench now at ~30V plate voltage (48V B+), 2mA CCS plate load for two parallel sections, giving 0.08% THD at ~3V RMS out, ~10V RMS max out at 5% THD.

The same circuit gives more output swing with ECC86, but an Amperex 6DJ8 has lower distortion. A modern JJ 6DJ8 is not as good at this low plate voltage but still works OK.

Beware fairly large variation in characteristics tube-to-tube at this unusually low operating point. Adjustable bias and/or paralleling multiple sections is a good idea.

(I'm working on a simple hybrid amp design).

Pete
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Old 21st July 2006, 11:56 AM   #10
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Er, that tube only uses a 30 V supply. I'm running the 6DJ8 at 130 V in the simulation and not getting enough gain. When I said low voltage, I meant on the order of a hundred volts.
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