"SiC Puppy SE Amp Using Semisouth 085 JFET

The only problem I can see with the vacuum-type beasties is that their hungry, hungry filaments suck almost as much power as what you're trying to deliver to the speaker load.
Having said that., pentodes have their advantages. The jfets and mosfets kinda emulate pentodes with the flatness of drain current vs. drain voltage characteristic, but they still have that pesky Miller capacitance.
 
Stuffed a fair number of the parts for the first version of the amp that will employ a Semisouth 1200V 85 milliohm fet or equivalent device from United Silicon Carbide. I may go for the USC device, as they claim to have improved the junction-to-case thermal resistance to a value more in line with Class A operation. I tested out a couple of my little 1kV IXYS depletion mode fets that will be going in the front end of this monstrosity and got a pair that bias up at around 2.5V reverse bias for 10 ma drain current. That's a little better than I remember than for the Supertex DN2540N5 devices, which have a lot higher gate capacitance, probably meaning they need more reverse bias for 10 mA of drain current.
 
Got a chance during the holidays to chop the heat sink in half,( once I figured out how to re-mount the non-ferrous blade back on my chop saw) spot, drill and tap the holes for the PCB supports, and do preliminary placement for the power semis (cap multiplier mosfet and SiC JFET output device). More drilling and tapping this weekend. I have some 4 mil mylar sheet on order to channel the air from the 48V fan through the heat sinks. It's a modest 80 mm fan with 0.08A draw at 48VDC - I'll be dialing it down to half that voltage for quieter operation, probably by using a couple of series zeners and a filter cap.
 
Here's a pic of the new amp channels sans output transformers. I figure I'll use either a pair of One Electron UBT-3s or a couple of vintage 3K Transcendars (full size) I have floating around. I plan to look at the output with and without global feedback. The HV bench supply I have at work will do to power up the channels for initial investigations.
 

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There is a silicon carbide JFET marketed by United Silicon Carbide/Qorvo that is a fair equivalent, with similar RDSon and a little more input capacitance than the Semisouth 085.I think both Digi-Key and the Mouse have them.
Edit - it's actually just the Mouse - this device has a little lower RDSon than the 085, and supposedly better juction-to-case thermal resistance.
https://www.mouser.com/ProductDetail/Qorvo/UJ3N120070K3S?qs=byeeYqUIh0OCiO%2B%2BGjwWnA==
 
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Woody - the Semisouth device had an RDSon of only 85 milliohms for a 1200V device. An equivalent silicon device would be enormous, with a correspondingly high gate capacitance. The die was rather small for a device of that rating, and had about the same junction to case thermal resistance of something like an IRF130. That made thermal management challenging for the SiC Puppy amplifier.
 
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