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Old 9th July 2004, 02:24 AM   #1
Ralphie is offline Ralphie  United States
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Default tube opamps

does anybody know of any simple yet good tube opamp circuits? i am pretty new to this so i want to find something simple that works good. i want to replace the opamps in a tone control circuit with tube circuits, if thats even possible. i think it is. maybe not? i dont know. help would be appreciated.
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Old 9th July 2004, 02:13 PM   #2
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Pass labs has a little primer on opamps. And as an example
they do have a tube opamp.

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Old 9th July 2004, 02:52 PM   #3
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I think Bob Pease (of National semiconductor) has written an article or two on the early days of vacuum tube op-amps.
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Old 9th July 2004, 08:04 PM   #4
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Take a look at this recent thread.

tube tone control?

A very nice active tone control circuit built around 12AX7s is discussed.

You could "copy" a discrete part FET opamp circuit. The I/P circuitry of opamps is a differential topology that (big surprise ) was 1st developed using vacuum triodes.

IMO, Max Robinson's Baxandall circuitry described in the thread is what you should build.
Eli D.
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Old 10th July 2004, 12:06 AM   #5
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One of the pioneers of tube opamps was Philbrick.



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Old 10th July 2004, 11:41 PM   #6
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Old 11th July 2004, 02:37 AM   #7
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Don't forget the old Tektronix Type "O" plug-in for 500 series scopes, was a fancy tube op-amp. I even have one still, good for plotting xfmr. and inductor hysteresis curves (configured as an integrator).

If it isn't broken, take it apart and mod it.
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Old 11th July 2004, 02:32 PM   #8
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For a classical feedback Baxandall tone control you donít need an op-amp, just an inverting gain stage. If you keep maximum bass boost/cut and treble boost/cut moderate to say + and Ė 10 dB (who needs more?) a gain of 20 to 30 will suffice. At the moment I am working on a new tube pre-amp with tone control. Simulations with a 6DJ8 as mu follower (gain = 30) seems promising so far. But I havenít built anything yet, so I am hesitating putting up a premature circuit here.

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Old 12th July 2004, 12:44 AM   #9
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Old 12th July 2004, 01:35 AM   #10
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Philbrick manual download page,all at once (75MB) or by section

HeadWize Design Series Paper with the Barbour and Forsell tube opamps plus the borbely Hybrid Tube/Mosfet Op Amp
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