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MOSFET Mods

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7/18/2000 2:41 PM

MOSFET Mods

Some of you will remember me discussing the use of mosfets to drive power tubes and also preamp tubes.

I have now done the mods, and you may be interested to know that they were quite sucessfull. My EL34's are now driven from an IRF830 source follower, and the main change has been to soften the distortion mainly due to the reduction in crossover distortion. The remaining distortion is probably due to the two sides not being exactly matched (PI balance/biasing).

I have also put a small mosfet on the gate of my last preamp tube. This didn't make a huge difference, apart from the fact that however hard I drive it, it just won't fart!

Overall, the addition of buffer mosfets has tightened the bass considerably, and it certainly does not fart any more.

Hope this is interesting to someone, if you want any details just ask.

Chris

7/18/2000 3:52 PM
Don Symes	Got a place to post schematics?

7/18/2000 4:45 PM
R.G.	MOSFET Mods for ending Flatulence! Hey! Great!! That is the way the thing had to work, but it's really nice to see someone besides me get good results. How close to the descriptions on GEO did you wind up?

7/18/2000 6:11 PM

Firstly, I do have some webspace, but I need to find a good way to draw the diagrams so they are legible.

The design for the power tube mod wasn't far from that on GEO, just a few resistor changes, but more luck than judgement, it works quite well so I'll probably leave it. The preamp mod was altered to suit the application, but the principle is the same.

I've modded another gain stage to the same spec as the first, and it sounds real hot, loads of gain, but it stay controlled, tight and reasonably smooth. Those 12AX7 triodes are being driven to the max, but it sounds good. You lose a little bit of the bite you get when it crunches up, but that can probably be altered. Someone mentioned about varying the grid stopper to alter the hardness of the distortion, so I might try that.

Recommended for anyone who likes high gain sounds, and especailly tight low riffs. I disconnected the speaker from my Marshall Valvestate, and tried that cos it's in a cab, not on the bench, then turned the bass right up and went for some low E fun. The room was shaking, but the sound didn't once sound at all flabby. Note no negative feedback used. It also seems to make the clean cleaner, and when the power tubes do start to crunch up, the clipping is softer. Have a go! Try some SS improvements.

Chris

7/18/2000 10:30 PM

quote:
"The design for the power tube mod wasn't far from that on GEO, just a few resistor changes, but more luck than judgement, it works quite well so I'll probably leave it."

That's the best form of engineering judgement - it works, leave it that way.

quote:
"Those 12AX7 triodes ... lose a little bit of the bite you get when it crunches up, but that can probably be altered. Someone mentioned about varying the grid stopper to alter the hardness of the distortion, so I might try that."

That was me. The lower the drive impedance, the softer the distortion when you drive the grids positive. You can just insert a grid stopper, as you mention, to harden the clipping back up. The advantages of DC drive will stay there and you won't get flatulence at any drive level.

quote:
"I do have some webspace, but I need to find a good way to draw the diagrams so they are legible."

Since this is a good illustration of the concept, I'd be willing to post these as an example at GEO if you like.

7/19/2000 1:22 PM

RG

When I've drawn the diagrams I'll let you know, and you could post them on your site if you want.

I've been playing more with it today, and I've noticed that allthough the higher gain sounds sound better, the sound has changed considerably, it has lost some of the bite and edge which I liked, especially at the beginning of breakup, I might have to add a moderately overdriven 'normal' stage near the start of the preamp to get the crunch sound, and perhaps switch out one of the other gain stages to make two overdrive channels. There's loads to play with anyway.

Chris

7/19/2000 2:27 PM

quote:
"the sound has changed considerably, it has lost some of the bite and edge which I liked, especially at the beginning of breakup,"

That's the effect of the low impedance drive to the grids. Stick a 100K grid stopper into your preamp tube driver and see if it doesn't come back, at least partially.

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