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Old 11th July 2018, 02:40 AM   #11
mashaffer is offline mashaffer  United States
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Not as high of Q as I was expecting. I suspect a MOSFET would tighten it up a bit.
Now the big question why does it work? FB from plate to its own cathode?
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File Type: png Cathodyne6DJ8out.png (52.8 KB, 58 views)
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Old 11th July 2018, 03:32 AM   #12
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by mashaffer View Post
Thank you. I like the looks of the cathodyne... Nice and simple.<snip>]
This schemo is very much like this one. When you see an arrangement like that, with the NFB connected to one side of a pot and the input to the other, you have a boost/cut circuit. At the approximate midpoint of the pot, the RLC suck-out filters drop out as they'll be connected to the neutral point. The three 12AX7 sections along the bottom form the "L" part of the filter as active "gyrators" to avoid coils and the associated problems of shielding against hum pick-up. You could ditch the three 12AX7's and replace with series coils to make passive filter sections. It would work the same.

Moving the center of the pot towards the input side removes those frequencies from the input (cut) and towards the NFB end removes them from the NFB (boost). If you had a feedback resistor to ground, you would have a boost circuit only, with no cut capability.

Vb is the non-invert bias voltage. since the op-amp is connected from 9.0V to ground instead of a balanced +/- supply, the non-invert terminal needs to be biased to 4.5V. The output stage naturally splits the voltage so the invert terminal already sees 4.5V.
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Old 11th July 2018, 06:11 AM   #13
mashaffer is offline mashaffer  United States
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Thank you Miles, that makes sense. I was piddling with a Mu Follower LTP and that might be a nice way to implement that.

So far this is the best I have come up with. The 30 Hz slider is a bit of a problem as both the coupling caps and the cap on the 30 Hz filter have to be made very much larger to get a 30 Hz center. Oddly if the filter cap (C1) is eliminated (shorted) correct center can be reached with only moderately big couplers.
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File Type: png Cathodyne6N1P.png (102.8 KB, 36 views)
File Type: png Cathodyne6N1P30HzSlider.png (31.4 KB, 35 views)
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Old 11th July 2018, 01:54 PM   #14
mashaffer is offline mashaffer  United States
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The solution seems to be quite simple. It turns out that eliminating the cathode coupling cap puts the center frequency spot on and alows the other coupling cap to be reduced. Of course that puts the filter network at about 100V so that some caps may need to be changed out due to voltage limitations but since the four lowest frequency are apparently polarized they would need to be changed out anyway. Fortunately the board is layed out such as to make those changes easy. I suspect that the coils themselves should be OK at that voltage.
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File Type: png Cath6N1PPartDC30Hz.png (30.7 KB, 10 views)
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Old 11th July 2018, 02:28 PM   #15
mashaffer is offline mashaffer  United States
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Tightens up the Q nicely too.
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