Mosfets for Tubelab SE?

[mctavish]

Its probably been addressed somewhere but my searches have been unsuccessful... Digikey is out of 2SK3563Q-ND, is there a suitable replacement that anyone can suggest? I am moving overseas in a few weeks and I am trying to put together a BOM for another build before I leave. This will be my second TSE and I am using 45's again, George this is a fine piece of equipment!!

[boywonder]

2SK2700, 2SK3564

The above are both 900V rated parts; the 2SK3563 is a 500V rated part, so as a bonus you get more voltage margin. The input capacitance is a little lower on the 2SK3563 though (550pf vs 700-750 pf).

[mctavish]

Great! Thanks for the help.

[beeah]

Would the IRF830APbF work?:
http://www.alliedelec.com/Images/Pro...A/503-0356.PDF

[Nikon1975]

I have the following mosfets available:

2SK3563
2SK3742
2SK3564

What would you use it for a 300B with B+ around 390 V ? (Simple mode)
What mosfet characteristics are important for this application ? The maximum ratings are obvious, not the rest (Learning mode)

Additionqlly, I want to have separate PSU for the power drive. What current do I have to make available ? I was thinking of using 150VAC with bridge rectifiere and CRC filter.

Thanks,

Davide

[Nikon1975]

Here are the datasheets of the masfet mentioned

[Nikon1975]

Last one

[rknize]

You want a low gate capacitance (Ciss) that is also constant over the source-drain voltage range that you are operating in. This isn't always specified, but there is usually a plot showing this. Those datasheets are not the complete ones.

[tubelab.com]

You want a low gate capacitance (Ciss) that is also constant over the source-drain voltage range that you are operating in.

In a typical mosfet amplifier this would be true, but the mosfets in the Tubelab SE are used as source followers. This is about the only application where I would use a mosfet in a tube amp.

In a source follower the gate to source capacitance (Ciss) isn't important unless it is ridiculously high. In a follower the Source follows the Gate signal and are at nearly the same AC potential so the Ciss is effectively bootstrapped out of the picture. The Output Capacitance (Coss) appears directly across the output, so it can be important, but the fet should have no problem driving this capacitance. The Reverse Transfer Capacitance (Crss) is the gate to drain capacitance. In a follower the drain is at AC ground so this capacitance appears directly across the input and must be handled by the driver tube. We want this capacitance to be low and constant over the voltage seen across the fet. The Tubelab SE has several hundred volts across the fet. The Toshiba 2SK3563 has constant 7 pf for any voltage above 20 volts, so this one is the one to use here, and its what's in my board.

The data sheets you included do not have all of the graphs. I have the full data sheets, but they are too big to include here. There should be a graph showing the three capacitances VS voltage. Look for a Crss that is low and flat.

[beeah]

Thanks for the explanation, George. Based on what I've read in several of your posts, the 2SK3563 is the ideal device for this purpose.

However, the issue is that the 2SK3563 in no longer available through the larger distributors (Mouser, Digikey, Allied, etc.). I don't think it's a temporary production issue; it appears that it's been discontinued by Toshiba.

I don't believe that anyone is second-guessing your use of the 2SK3563, but rather that we're at a point where we need a reasonable sub, or at least guidelines in choosing one. Your post helps a great deal in that regard.