Any suggestions for P channel audio mosfets?

[Mark Johnson]

Maybe today it is time to explore series stacks of MOSFETs , as The Old Ones explored series stacks of BJTs, 45 years ago. If Nelson Pass could get that to work reliably in the Threshold amps in 1975, it is a tremendous confidence booster that you will be able to get it to work reliably with MOSFETs today.

 

[Antoinel]

The semiconductor is a follower in the cathode circuit of a tube. A P channel fet or a PNP BJT are the easy choices here. Any N type device would be used in a common source or emitter configuration here it's active characteristics play a large role in the overall circuit's characteristics. Experiments along this route several years ago failed to produce the desired overall triode like curves from the composite pair.



The 20P50 device looks good except for it's pretty large Crss, especially at low voltages. It's little brother, the 10P50 has more manageable Crss, and better SOA ratings in the area of interest.

I ordered some of the Exicon fets, and I will get some 10P50's. Digikey does not have any in TO220, haven't checked Mouser yet, but the Exicons are TO247, so these will be for the modular version of the UNSET which targets BIGGER power levels.

After sending several mosfets to their death over the last 4 days I have traced the issue to a 40 KHz oscillation at full power. The big Hammond 1628SEA is a near dead short at that frequency which sends the tube current into the hunndreds of mA range. Now that it's fixed I have not been able to blow any mosfets in a 20 Watt SE amp on 520 volts. I have not gone higher yet due to the 500 volt filter caps in the board.

Thanks Tubelab_com for your post and clear answer.

 

[wg_ski]

So it was premature to blame the Pchannel MOSFETs and unnecessary to search for different parts?

Possibly. But most of us tube builders have had problems using mosfets as regulators and followers if the VDS is high enough. Running them at 200-300 volts at even 10 watts of dissipation is often asking for trouble. Many switching types will die, taking out expensive parts with them. Having 600 volts at a few mA at switch on before tubes warm up is also quite possible. George has blown them out, I’ve blown them out - so it was natural to assume it was happening again.

In the Unset, it never sees more than 150 volts on it, if it drives sweep tubes. The cathode can never rise above Vg2, and you usually set that around 150 volts. When you do have VDS at max, the current does tend toward zero. The VDS should be below 50 volts when the current does spike upwards of an amp. Good bipolars will tolerate this, as will laterals and older types like the 9640. If you try to build an Unset with say, KT88s and run the screen up at 400 volts, the VDS might become a problem and you’ll have fun getting mosfets (or even bipolars) with enough SOA.

 

[Antoinel]

I spent a good deal of the last year of my engineering career (2013) chasing secondary breakdown in GaN fets used in RF amps......now I'm chasing it again, or running from it.

Hello Tubelab_com. Your grasp of Inorganic Chem. and its solid state applications must have been excellent.

Your customers [then] asked for your help and guidance. May consider seeking solutions from companies who now create P-MOSFETs. I'm thinking of Mr. Pass who asked SemiSouth to create his collection of Pass SIT devices.

Best wishes
Anton

 

[Tubelab_com]

Hello Tubelab_com. Your grasp of Inorganic Chem. and its solid state applications must have been excellent.

Not me. I was one of two people in an advanced research department who actually built stuff. There were 60+ people who spent their days simulating and calculating. They generated tons of paper, patents, slick Powerpoint presentations, and complex circuit and IC designs, but no real working measurable hardware.

Motorola built and sold state of the art two-way radio equipment, not slick presentations. Someone needed to build prototype hardware, test radios, and test fixtures for all the neat custom radio IC's we developed. That job does not get much recognition, but it was what I liked to do, and did for a long time.

In the case of the blown GaN fets, I had designed the first prototype radio boards, and got a batch built in our in house SMD line. They worked, but in the process of taking qualification data we saw some failures on the final RF amplifier part, the GaN fet. I set up a test table with a fancy Flir thermal camera capable of taking high res video of the bare die. I set up a capture card to trigger on the gate current (should be near zero) and save the last minute or so of video. It took months to capture some of these failure events. The real science took place back at the part manufacturers lab, I just gathered the evidence.

The research department, and Motorola in general had been in a downsizing trend for several years, and when I got "the letter" from the CEO, I took the buyout and left in 2014. It took a couple of years to fix the issue, and I'm not sure of all the details, but that radio product is now shipping.

APX 8000 All-Band P25 Portable Radio - Motorola Solutions

I ordered a few different P channel mosfets and some other parts from Mouser today. I also got a shipment confirmation on the Exicon fets from the UK. I will be testing them all in a couple of weeks.

 

[Antoinel]

Thanks Tubelab_com for your post. Your wrote an impressive resume. I am glad to say it evoked pride in me towards you and your stated achievements. I was in the totally different field of Leather Chemicals. Similar demands and needs to deliver a functional product and its viable applications. I sensed you enjoyed your creative work immensely. [Your] product via its link is alive out there.

I hope that you find the unique P-MOS to complete your special VT power amp. It seems to me that you are implementing your past successful approaches too here.

Best wishes
Anton.