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SS phase splitters?
SS phase splitters?
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Old 22nd July 2013, 05:13 PM   #81
Fenris is offline Fenris  United States
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I did this concertina splitting with a LND150 small MOSFET about a year ago:

http://www.diyaudio.com/forums/tubes...-el84-amp.html

It works just fine and has been in use for a year with no issue. It makes life so simple I don't know why more people don't do it.
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Old 22nd July 2013, 07:49 PM   #82
tinitus is offline tinitus  Europe
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SS phase splitters?
cool...thanks

there are surely better posts to quote from your thread... but I just couldn't resist this one ... (I hope George won't mind)

Quote:
Originally Posted by tubelab.com View Post
I'm sure that this will rattle the purists.........In many guitar amp circuits, just stick it into the 12AX7 sockets.....it works.
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Old 22nd July 2013, 11:02 PM   #83
tinitus is offline tinitus  Europe
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SS phase splitters?
wow...a search on LND150 reveals quite a lot of schematics

and a look at spec sheet reveals lowest capacitance...7.5/2.5/0.5

but a very low current rating of max 3ma
doesn't sound like giving much room for errors ?
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Old 23rd July 2013, 12:02 AM   #84
kenpeter is offline kenpeter  United States
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The PNP .model I used has CJC=150pF of Miller.
I figure from base to collector looks like 2x gain?
And 12AX7 plate resistance into CCS is 62.5K

Gives a corner at 16,985Hz.
This is too low even to be the dominant pole...

LTSpice knew CJC. I wonder why it didn't show
more damage from Miller in the overall results?

If we want 100KHz open loop, we want <25pF
here, or maybe a cascode of some sort...

I picked a rather big transistor, just cause I
wanted the TO220 thermal package, I might
need to re-think that decision...

Last edited by kenpeter; 23rd July 2013 at 12:12 AM.
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Old 23rd July 2013, 12:28 AM   #85
Eli Duttman is offline Eli Duttman  United States
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FWIW, the ZVP0545A is the P channel complementary "twin" of the ZVN0545A. Perhaps the "sand" mavens that have been posting in this thread can make something of the part.
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Old 23rd July 2013, 12:45 AM   #86
kenpeter is offline kenpeter  United States
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Yeah I could definately go with that one. Though I would
wish to trade-in that 450V spec for some more gm or a
higher Wattage. 1.55mA*450V=700mW. Voltage spec is
perhaps overkill, not so useful at a realistic current. But
there may be reason to stand-off that much voltage at
no current.

Its weakest spec is 40mS, only 30x more gm than 12AX7.
Still, ain't nothing in AB1 you couldn't drive with that.

Or perhaps that capacitance is measured with 25V on the
drain, and no mention how that cap might squirm if you
swing below 25V bias? Triodes can't go there anyway...

Last edited by kenpeter; 23rd July 2013 at 12:59 AM.
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Old 23rd July 2013, 01:32 AM   #87
Eli Duttman is offline Eli Duttman  United States
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The wimpiest of small signal tubes can drive the ZVx0545A MOSFETs, with great aplomb. For the really difficult situations, consider cascading FETs. Cap. couple the "little fellers" into big, "honking", TO220 case parts. Small value, highly transparent, film parts are practical as couplers, given the extraordinarily high I/P impedance of the 2nd MOSFET.
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Old 23rd July 2013, 06:04 AM   #88
kenpeter is offline kenpeter  United States
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Driving the ZVx0545A isn't the problem. But 700mW might be a problem
driving finals. It simply can't conduct enough current (without exceeding
wattage) to properly shame a triode splitter...

Can we find a discrete P in TO128 or TO220 thats HV and <25pf Miller?
In through hole package that might exceed plate dissipation of an AX7?

How bout this TO220 at a dollarseventeen?
http://www.fairchildsemi.com/ds/FQ/FQP3P50.pdf

Or this TO126 at fourtysixcents?
http://www.fairchildsemi.com/ds/KS/KSA1381.pdf

Last edited by kenpeter; 23rd July 2013 at 06:34 AM.
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Old 23rd July 2013, 06:13 AM   #89
wrenchone is offline wrenchone  United States
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P Channel parts in high voltage are thin on the ground. Try Fairchild, ST and Alpha Omega, though I doubt you'll find anything close to the 25pF gate capacitance you're seeking. 15 years or so ago, Supertex used to offer their small-die parts in TO-220 packages, but no more (and those clocked in at ~50 pF). If you really want the small capacitance with some dissipation capability, the best option is to take one of the Zetex/Diodes, Inc. E-line parts and glue on a heat sink. Good luck.
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Old 23rd July 2013, 06:37 AM   #90
kenpeter is offline kenpeter  United States
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I don't mind gate to source, or base to emitter cap, those are followers...
Its only the Miller end, swinging the opposite direction, I worry not more
than 25pF for this application.

Last edited by kenpeter; 23rd July 2013 at 06:41 AM.
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