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MOSFET source follower driver project – done!

1/23/2000 9:08 PM
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:  
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  
>'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.  
1/23/2000 9:45 PM
Randall Aiken

Now that you know my favorite secret, keep it quiet so nobody else finds out, okay?  
Although I'm sad to see that R.G. has convinced you to come over to the dark side of the force (solid-state), I'm glad it worked out well for you. A Class AB2 (or even class A2) stage is a joy to Of course, it would sound even better with a tube cathode follower. :)  
Randall Aiken
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1/24/2000 2:58 AM
Mark Knapp

At my prefered SPL, this might best be done to a single ended circuit. I assume you are implying that the same effect occurs there (hence the "A2" comment)?  
PS: someone I have run across is selling some Altec PA amps with 2 811A's for the drivers and 2 6W6's as cathode follower/predrivers. Supposedly 150 -175W
1/24/2000 4:10 AM
Randall Aiken

No, I was referring to push-pull class A. The same blocking distortion occurs in push-pull class A1 if the PI is AC-coupled to the grids of the output tubes, which pushes the average DC level down and pushes the amp into class AB1. If the output tubes are driven from DC-coupled cathode followers, the bias stays where it should be (class A2), and blocking distortion is eliminated.  
With SE class A, the effect is not as bad, because there is no crossover distortion. You just get a more asymmetrical clip. However, there is still a time constant associated with the recovery from the grid clamping, and the DC-coupled cathode follower will prevent this.  
Randall Aiken
1/24/2000 4:26 AM
Consorting with the Solid State Devil 8-O
Ok, so I did lure an unsuspecting soul nearer the edge of silicon madness... it was with good intentions, as all roads to hell are paved. 8-)  
This is all a little like the alchemists of the middle ages, seeing just how close to the line of using dark forces for their wills they could come without actually signing over their souls.  
On the other hand, I do believe that overall there are mitigating circumstances to my actions. I was the one who posted the "soul-transplant" suggestion to AX84 that one could buy one of the new Korg/Vox Pathfinders for about $100, remove the solid state innards besoiling an otherwise fine-looking and sounding shell and speaker, and fill it with vacuum-state virtue for a pittance.  
I admit being taken with the image that that there should be thousands of silicon slaves making the world safe and serene for our thermionic friends. There's poetic justice there.
1/24/2000 4:51 AM
Randall Aiken

Hehe...I just had to throw that in there. I suppose the fact that I design with high-tech solid-state stuff all day long has affected my thinking. All I want to do when I get home is to settle down with a nice warm glass of electrons.  
I see solid-state stuff die for no reason all the time - just the slightest transient will kill them. Static charges - when was the last time you killed a tube just by picking it up to put it in the circuit, or by touching the front panel after walking across a carpet in the winter?. Every time I see a broken amp that is loaded with opamps, transistors, regulators, and the like, it always turns out to be one of the solid-state components that has died a premature death. Even the "protection" diode strings on the output transformers of some amps short out, killing the very parts they are supposed to protect. I don't want any of these things in my amps.  
Speaking of cheap mod platforms, has anybody seen this:  
That looks like an amazing "all-tube" bargain for $159. Of course, there is no way in hell it is putting out 30W in class A with only two EL84's, but that's another story.  
Randall Aiken
1/24/2000 5:05 AM
Chris Hogan

What is the most you could expect out of a class A amp with two EL84's? 10 watts or so? Isn't 30 watts with these tubes kinda on the high side even in class AB?
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