Here is the amp as it stands. I have added the two zener diodes to protect the JFET and increased the voltage from the PSU to 30volts to increase the volume. I may change the JFET source resistor to 5K6 to lower the gain slightly, which may also give me a fuller tone. Do you think I will benefit at all from changing the 10K Pot to a 100K Pot? I should see an increase in volume shouldn't I? Actually I think the preamp will just be stupidly overdriven.

I've already got  a 33K resistor in series with the input before the Zener Diodes so should be ok. I ran it through LTSpice to make sure, great program that is!

I'll probably use the zener option as I know I can definitely get hold of a few 5.1V Zeners.

Also has anyone ever done what this guy has done with the two zeners? He says its to protect the JFET from blowing.

http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp3.htm

I have just changed the jfet source cap to .68uF which has made the amp sound better, as for the distortion I'll have to take it to my next band practice so I can crank it. May try the MPF102 as I'm sure I read somewhere that its a bit more warm sounding, then again different circuits will sound different etc. I'm having fun experimenting though!

Here is the amp now, I have made some of the changes you specified J M, simply because I just tried the additional resistor and capacitor to begin with and that made a world of difference. The amp is so much better now. I need to test it at band practice really though to hear it at a good volume. My only possible quarrel with it is the overdrive is quite harsh but as I understand it (correct me if I'm wrong) this is a characteristic of transistors. I know Marshall have made a very good Transistor amp that has nice soft valve clipping but their amps are pretty complicated compared to this one! Is there any way to soften the overdrive?

Thanks for that J M. Would .68uF be too much on the Emitter of the BJT? As I don't have a cap as low as .1uF.

O well i'll just leave it in there. Tested the amp at band practice, plenty of volume, still bit too much bass so i'm going to try a 0.68uF cap on the JFET source.

Nearly there guys, here is an updated schematic for the amp so far. I'm pretty happy with it apart from there is still a tad too much bass, so I'm going to replace the JFET Source Cap with a 0.68uF Cap later in the week if I can.

Also the overheating issue is now (hopefully) fixed. I have fitted a much better heatsink which has a rating of 12C/W and the difference even at low volumes is extreme. So I shall give the amp another thrashing at band practice tomorrow night to see if it will hold up. I must say the chip I'm using for my output stage runs very cool considering its pushing out quite a bit of volume!

I shall now modify the resistor values to Roly's spec.

But the audio output from that BJT is the same as it's input if i use those values so what is it doing? With my values it increases the audio signal but with yours it stays the same. I understand the reason for using your values but what does the BJT do?

Ignore the last question, downloaded and ran LTSpice, seems that I get increased output due to the lower current consumption of the BJT. However I found that after adopting the values for the BJT that you specified it didn't actually seem to do much at all! Audio Output was the same before and after the BJT. So what will it actually be doing if I adopt the values you specified?

If I change the resistor values for the BJT to ones you have specified won't that lower the output of the BJT?

Also Roly what is that simulation program you're using?

Tried the amp tonight and found that it still has too much bass, so I'll try what you said Roly and lower the 22uF Cap on the JFET Source to 1uF.

Also I found it overheats on full volume pretty easily! So I've just ordered a much bigger heatsink for the Voltage Regulator as the current one has a rating of 21C/W, compared to the heatsink I just ordered which is 12C/W. When the amp is at full volume the regulator will be dissipating 6.65watts, so multiply that by 21 and you get 139 Degrees C, which is above the regulators 125 Degrees limit. With the new heatsink it should run at a maximum of 79.8 Degrees C.

Other than that it sounds good, even my rhythm guitarist liked it and hes very picky!