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Old 16th August 2012, 11:37 AM   #71
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Join Date: Jan 2008
Originally Posted by vilfort View Post
Sorry for the late reply. Been busy... Yes, it can be applied to the DH500 as well. The only difference between the the DH500 and the DH200/220 is the larger voltage on the output stage and of course the fact that the 500 has 6 MOSFETs per channel.

From what I see in pictures, you have a little extra room in the DH500 for mounting a plate w/ tube sockets and the few resistors/caps needed. I was looking for a 500 on eBay, - but they seem to fetch rather high prices relative to a blown DH200 . So I have not tried yet... But looks very doable.

I have been refining the circuit a bit and changed values of some of the resistors. Broskie published a great article on the white cathode follower here: The Tube CAD Journal, August 1999 pg 1 In it (see page 4) he concludes that the plate load resistor of the top tube in the white cathode follower circuit should be Ra = rp/mu.

That seemed easy enough for the 6SN7 : 7,700/20 = 385 ohm. So a 390 ohm resistor would do nicely. To balance the circuit ala Aikido, I went for 390 ohm on the cathode of the bottom triode as well.

I have attached the updated schematic. Works like a charm and sounds very good too. Lots of detail. I now hear layers of instruments I newer heard before... With a DH500 you would have lots of power too.

Please share your findings as your project progresses.

The value of Ra for white cathode follower depends on what load is the white cathode follower driving. At Tube CAD Journal you can find this formula for Ra: "Ra = (rp + 2RL)/mu"

link: ""

Actually I think this circuit would sound alot better when operated in class A. I have my doubts about performance of class AB open-loop stage with just two parallel mosfets.
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Old 20th August 2012, 05:18 PM   #72
vilfort is offline vilfort  United States
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Join Date: Feb 2010
Default Improved Tube/MOSFET circuit

This post was a little older. The evolution of this amp eventually led to using a bit of (oh no!) global feedback. As you can see from the schematic, this is now a modified white cathode follower running in class A. The power bandwitdh goes to 200Khz with a single pair of MOSFETS and to 60KHz with 3 pair.
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