MOSFET source follower driver project – done!

1/23/2000 9:08 PM

lion

MOSFET source follower driver project – done! I finally got around to wire up the MOSFET SF stage as a output tube driver in my amp project – and I must say:   WHAT AN IMPROVEMENT!     Actually I think this is the single most effective mod, in regard to tone/preformance, I have tried so far.     In a previous thread here – the one that initially got me interested in the grid blocking distortion problem – Randall Aiken and RG supplied the EE explanations, but Carl Z. wrote (what he called a simplified explanation):     >The problem lies in the driver stage. When the signal pushes the grid into a positive region it (the grid) wants current to work with but the driver can't supply it. What happens is that the tube runs out of steam and shuts off..... <     and further     >...here's where the killer tone comes in. When you use a cathode follower to drive the grid you're essentially allowing the power tubes to clip at the plate instead of at the grid. The result here is that you get a lot more signal compression and rounding of the wave form as you approach the rail voltage. When you clip the tube at the grid you get a wicked hard as nails clip that's not terribly unlike transistor clipping.<     For me, as a musician more than a tech, this hits it right on – even if it’s not quit technically correct.     Before implementing the MOSFET driver - with the volume knob less than two thirds up, there really wasn’t more useable power in this amp. Just mush and a rather harsh distortion, the more the volume was cranked. (Thanks to this BB,I know now that this was caused by clamping of the signal and crossover distortion from the output tubes being pushed into class B biasing)     But now - after the mod – it seems as if the power keeps following the volume knob allmost all the way up – and when fully cranked the amp sings with a very nice overdriven tone. The transition from *clean* to OD is very smooth.     The MOSFET adds no hint of solid-state tone what so ever!     I really recommend this mod to everyone who likes to push their output stage - or needs a little more usable power from the amp.     Thanks RG for all the help and support.     I’ll be happy to answer Q’s (the best I can) or supply further info should anyone interested.     Regards   Lion