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previous: Chris Harden Alt. methods of driving power tubes -- 7/8/2000 8:34 AM View Thread

Power Drive

7/8/2000 11:22 AM
Ray Ivers
Power Drive
Chris,  
 
Driving a power tube into hard clipping (like we guitarists have been known to do) takes power; sometimes a little, sometimes a lot with multiple and/or large power tubes. I think a large part of what people believe to be power-tube clipping is really driver-tube clipping, since most guitar-amp designs encourage this sort of thing.  
 
Try the Ernie Ball Music Man site, and check out the power amp design on the 2100-RD:  
 
http://www.ernieball.com/musicman/techinfo/amps/index.html  
 
Winnie Thomas has noted that the .pdf and .zip files are different on some of the amps, you might want to download them both. This power amp design is basically a cascode (cathode-drive) arrangement. I have had people tell me that the driver transistors will dominate the 'sound' of this design, and the tubes are simply level-shifters; I disagree. When clipping occurs, it ain't gonna be the transistor that clips. All the same, I think I would use this design for a loud, clean power amp, not an 'amp-on-ten' overdrive.  
 
One thing I've always wanted to try is a push-pull triode-connected Class A 6V6 or EL84 driver stage thru a step-down interstage transformer to an EL34 pentode output stage, giving you the ability to really drive the hell out of those '34's. You could have variable B+ over a limited range on the (cathode-biased) drivers, which could be done simply with a high-wattage wirewound pot due to the near-constant current draw of the Class A design, and would give you some control over the balance of driver/power tube distortion. It would also be cool to have a dual pot (100K or so) connected as a dual variable resistance between the interstage secondaries and the output tube grids; this would have little effect when the output tubes were not clipping, but at soon as the grids started to draw current, higher settings of this control would add compression and soften clipping, allowing more control of the clean/dirty thing at the guitar's volume knob (Peter S.!). This control would not have the same effect with a conventional driver stage, and is NOT intended as a master volume.  
 
I would love to convert the Music Man circuit to use FET's, with a high(er)-impedance input directly to the drivers, eliminating the op-amps; any thoughts, R.G.?  
 
Ray

 
Replies:
Chris Harden Ray,I've checked ou... -- 7/9/2000 2:49 PM