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Re: "Crossover notch" biasing - my final answer!

9/1/2000 5:31 PM
ken gilbert Re: "Crossover notch" biasing - my final answer!
it's also a lot more apparent if you use a notch filter to trap out the fundamental excitation frequency, essentially making a poor man's distortion analyser. sharp peaks (additional harmonics) will be apparent in the waveform as the tubes "handoff" current from one to another.  
also, the approximate value of grid bias to eliminate the XOD can be easily read from the composite characteristic curve (Vg1 vs Ip), as the point along the X axis (the Vg1) which would be intersected by a "best fit" straight line of the curve in question.  
note that this is NOT the plate current family of curves (the traditional Vp:Ip), but rather the mutual conductance (Vg:Ip) family.  
9/1/2000 7:22 PM
Dave Charneski
Nice Paper! Came to same conclusions myself.
I've already introduced myself to Randall, but for those who don't know me, let me start off by saying that I'm an EE with about 20 years in industry and have been a plunker for about 30 years. I never seem to have much spare time at home, so when I get some, I prefer to play music instead of mess with electronics, although I'd LOVE to be able to do BOTH! (Maybe when the kids get older.) In any case, I *have* studied the tube amp bias issue a *little* bit myself and, several years ago, spent an afternoon with a bias probe, a scope, a signal generator, a DMM, a Fender Tone-Master and my ears. (There was also the college training regarding load lines, operating point and amplifier classes, so I understood the general principles involved here in this experiment.)  
Within a couple hours, I came to the same conclusion as you, Randall. The scope method is useless! On the Tone-Master, I could twist the bias pot back and forth, pegging it one way or the other, and never see any crossover distortion on the scope, regardless of resolution! However, the amp did *sound* like poop at either end of the spectrum. Plus, at the high end (i.e. lots of plate current), in a dark room, the plates gave off a nice orange glow, indicating that they were running too hot. The scope told me absolutely NONE of this.  
The only way I could bias the amp so that it both sounded good and ran safely was to:  
1.) measure the plate voltage,  
2.) calculate the cathode current that would  
result in max plate dissipation,  
3.) using the bias probe, increase cathode  
current until the amp sounded good,  
while making sure to stay below the  
max plate dissipation.  
As it turns out, the tone I liked best gave me  
about 65% of the max plate dissipation. (I ignored  
screen current, knowing that it would just give  
me a little safety margin on the calculation.)  
However, at the time, I didn't really know if 65% at idle was too high or not, because I knew the dissipation under load would be higher. So, I turned out the lights, plugged the amp into the Power Brake and cranked it up WAY louder than I'd ever play it and watched the plates to see if they'd overheat and glow. They didn't, so I figured I was cool (pardon the pun).  
So, I made a note of that bias probe reading and used it every time I changed tubes. Simple! I wanted to try to measure and calculate what the dissipation was under load, but found it too difficult with the equipment I had, so I went with the "experimental" method.  
I ended up liking this method so well that I rounded up some 5W, mil-spec, .001%, 1 ohm resistors and stuck 'em in the cathode of each push-pull pair and routed the cathode end out to a stereo plug I mounted in a spare external speaker jack. Drilled a little hole in the cab right above the bias pot, inserted one of those watcha'-ma-call-its in the hole (for looks) and got me a nice long screwdriver from Sears. NOW, I could bias the darn thing with a DVM, stereo plug and screwdriver without removing anything. Ended up selling the amp, but always loved that nice little bias feature! I think all amps should have that.  
Anyways, I know there's a lot more going on under the hood (and Randall's paper is the best I've seen covering it), but for guys like me, who (at the present time anyway) just want to know enough to maintain and tune an amp for best performance, this simple low-tech (but accurate, effective and correct) method of biasing works beatifully.  
My hats off to Randall for digging so deeply and covering the rest of the bases. I was convinced a few years ago, but your paper reinforced in my mind that I have been doing the right thing! Thanks.  
9/2/2000 5:21 AM
Ditto. excellent job, Randall.
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