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Old 9th February 2011, 10:00 PM   #1
ben_f_m is offline ben_f_m  United States
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Default Solid State Cathodyne Phase Splitter Device?

I'm working on an amplifier with what appears to be a solid state cathodyne phase splitter. It has +300VDC on one leg and -300VDC on the other, with the signal coming from a triode (12AX7) gain stage. The solid state device cannot be read. It's a TO-220 package or similar. What sort of solid state device would be appropriate in this application? Any recommendations for one to try?
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Old 9th February 2011, 10:58 PM   #2
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a very high voltage one - possibly a MOSFET.
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Old 10th February 2011, 12:27 AM   #3
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IGBT survive the most insane of voltage... What value the resistors?

http://search.digikey.com/scripts/Dk...60hd&WT.srch=1


I guess even a MOSFET will do... Original more likely a MOSFET anyway.
Just cause IGBT is a newer lesser known device, definitely not inferior.

http://search.digikey.com/scripts/Dk...=497-5980-5-ND

Note IGBT is 50cents cheaper and half the capacitance of the MOSFET.

Last edited by kenpeter; 10th February 2011 at 12:57 AM.
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Old 10th February 2011, 12:29 AM   #4
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A 1 KV. rated MOSFET seems to be indicated. You want a part with a low reverse transfer capacitance. At 9 pF., this looks like a reasonable candidate. However, you did say the driving element will be a 'X7 section. I don't know if that low gm/high RP triode has enough "stones". Frankly, the only MOSFET I'm comfortable with in combination with 'X7s is the ZVN0545A and that part can't handle the voltage described.
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Old 10th February 2011, 02:05 AM   #5
SY is offline SY  United States
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With an effective C of 18pF, the f3 from a 12AX7 would be 140kHz or higher. Not tragic.
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Old 10th February 2011, 02:19 AM   #6
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Obtanium: 2.7 pF
http://www.fairchildsemi.com/ds/FQ/FQD1N80.pdf

Un-Obtanium: 2 pF
http://ixdev.ixys.com/DataSheet/98812.pdf
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Old 10th February 2011, 02:37 AM   #7
ben_f_m is offline ben_f_m  United States
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Thank you all for your helpful responses.

The resistors are 23k's.

I had not thought of an IGBT. Does anyone have experience with them in an audio signal path application?

The other options look good too (and I'm always interested in more). The fun thing with solid state components is they're inexpensive and have only three leads, so I can try them all, if I'm careful.
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Old 10th February 2011, 02:43 AM   #8
ben_f_m is offline ben_f_m  United States
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The IXYS mosfets are truly unavailable! Do you think one could hear a difference? It would be fun to try, if it were possible.
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Old 10th February 2011, 02:55 AM   #9
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Obtainium $1.78 qty 1 (1200V 6pf reverse)
Digi-Key - 497-4353-5-ND (Manufacturer - STGF3NC120HD)

Nelson Pass built a very nice amp of IGBTs.
Complimentary pair was once available.
Not sure why P-IGBT was dropped?

12,000V P-Unobtainium?!? Silicon carbide: Go figure...
http://www.scientific.net/MSF.600-603.1187

For your circuit:
600V/(23K+23K) = 13mA max. (probably biased at half this amount)
300V*6.5mA = 1.95W max (and probably biased to idle on this peak).

Wonder what blew the original? Is there a Zenier Diode nearby?

Last edited by kenpeter; 10th February 2011 at 03:24 AM.
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Old 10th February 2011, 03:30 AM   #10
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Regarding 800V Fairchild 2.7pF FQD1N80 as "obtainiumed" by Smoking.
0 quantity at Digikey, 73cent TO-252 SMT at Mouser but no TO-220.

Only need 2W, why we wouldn't opt for a fully insulated package???

Last edited by kenpeter; 10th February 2011 at 03:48 AM.
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