Mosfets for ZEN derivates

Because there is somerthing still not clear to see for me, I want post my question again, but this time in an own thread: I built a lightning ZEN (part1) an I plan to develop it on with the coming parts. Because its hard to get some of the Mosfets specifyed here, I want´t to diskuss advantages and diadvantages of the alternatives: IRFP150 to 450, IRFPC50

I already posted at the aleph output thread:
" Main differences seem to be IRFP150 :100V, Id 42A and IRFP240 : 200V and 20A. Ok, both i think is enough, more than we need. Output capacitance of 240 and 140 is about 1300pF, the 1900pF of the 150 is not much more (?) and to IRFP240 parallel is much more than one IRFP150. One different value i don´t know to judge about: What s forward tranconductance (Gfs)? its 6,9 S (Siemens?) at the 240 and the double (14 S) for the 150. what does this? "

How much "voltage safety margin" is needed an for which purpose?

Whats the effect of this transconductance?

Is 1900pF to much for single device stages?

Are there disadvantages of the 150 type?

greetings from the dark and rainy north of germany, Till
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
The fact is, practically any N channel Mosfet will work,
and most of them will work quite satisfactorily. The
voltage should be at least 50 volts, or the supply
voltage, whichever is greater.

The newer, higher transconductance parts (IRFP044 for
example) are the standard part. If you use the older,
lower transconductance parts, you will get somewhat higher
distortion because there will be less gain left over for
feedback. The sound of these seems a bit brighter, and there
is somewhat less control and dynamics, but you might find
that you prefer the sound.

If you use the larger parts, such as the 150, you tend to
get more bottom end, but a little less clarity at the top.
Again, you might prefer this.

In all cases, it helps if you have a sensitive load. Speakers
greater than 90 dB are preferred; at home I run parts
around 100 dB/watt. :)
 
I use IRF350 for similar project (you can see my www). IRF350 has input capacitance of 2600pF and gfs (forward transconductance) of 6S. IRF150 has bigger input capacitance - 3700pF (worse compared to IRF350) but a bit higher transconductance of 9S. I prefer IRF350 to IRF150. You can also use IRFP versions of the same MOSFETs. They have plastic case, IRF have metal TO-3 case (lower thermal resistance).