• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

SS phase splitters?

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K

kenpeter

Member
Joined 2007
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...
 
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K

kenpeter

Member
Joined 2007
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...
 
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E

Eli Duttman

Member
Joined 2004
The wimpiest of small signal tubes can drive the ZVx0545A MOSFETs, with great aplomb. :yes: 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.
 
K

kenpeter

Member
Joined 2007
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
 
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wrenchone

wrenchone

Member
Joined 2004
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.
 
wrenchone

wrenchone

Member
Joined 2004
The best option right now for a P-channel part would appear to be a Zetex or Supertex part in a SOT-223 package, which can use PCB copper for a heat sink, or can be jiggered up with a piece of copper foil soldered to the tab. The Fairchild FPQ1P50 would have been perfect, but it's no longer made, though you might find some old stock parts if you're lucky. For some reason, both Fairchild and ST chopped a lot of the smallest parts out of their lineup. For an N-Channel driver, the Alpha Omega part still looks like the best bet for a TO-220 devce, and it's cheap to boot.
 
wrenchone

wrenchone

Member
Joined 2004
Maybe some - it would have to be a really tight fit. You'd be better off glueing a heat sink to one of the Zetex E-line parts, which look like a TO-92 that's been flattened out. Using one of the TO-92 heat sinks referenced by Eli with some loaded epoxy like Arctic silver would work. Keep in mind that heat sinks designed for TO-92 won't work with the E-line parts, as the shape is different.
 
wrenchone

wrenchone

Member
Joined 2004
If you use the Zetex parts, they come in an E-line (read flat) package, so press fit won't work, especially with a round hole. The conventional TO-92 parts will be limited by the junction to case thermal resistance of a conventional TO-92 lead frame and package, which is pretty high. I woouldn't try wringing more than about a watt out of a conventional TO-92, even with help. The normal limit for an unaided TO-92 packake in free air would be about 150-200mW, depending on the temperature rise you're willing to accept.
 
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