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aa-32 mod...(not REAL long!)....2nd harmonic theory....

Posted by radiance (M) on July 1, 2006 at 12:31:08

...this amp is a N.O.S. unassembled kit I got from this forum's classifieds, it's for my daughter Hannah.....this is an elegant - or at least simpler - little design from looking at the schematic; voltage doubler PS with RC stages (yeah, I know, far from optimal, but I can't spend money on it and I'll bet this thing can sing pretty good anyway); triodes all the way except the output pentode ECL85's cathode biased in PP with suppression grids to B+ via 470 ohm (all parralelled via single 470 ohm resistor! - that CAN'T be good for reducing intermodulation distortion, but maybe it doesn't make any difference.....); no ultralinear taps (shoot, can't cut it if it ain't there) but NFB loop (yeah, now here's something I can cut. Oh, wait, I haven't put it together yet.....OK, get to leave that out. Yeah!!! :)

seriously, I'm going to put in four relays, two each channel, long remote control cable for switching relays, and be able to switch the amp in mid-stride through the following modes:
1. stock;
2. stock except screen triode strapped to plate via resistor value to be established;
3. stock except NFB cut;
4. triode strapped and NFB cut;
then put on "Fireflies in a Steel Mill" by The Elected and compare instrumental yumminess to vocal clarity heh heh heh

By the way, on THIS mod I'm finally going to get the oscilloscope and signal generator hooked up and be able to figure out whether the cathode bias needs to change in modes 2, 3 or 4, and figure out how to measure the harmonic distortions.

2nd harmonic theory: given hypothesis that some 2nd harmonic distortion might be just fine, have following theory, welcome feedback (positive only - just kidding):

maybe the essential cause of 2nd harmonic distortion in a triode (I'll get to the pentode in a sec) is this (please forgive me if I'm stating the ridiculously obvious; I don't pretend any of this is new, it's just new to ME):

electrons bounced off plate (by secondary emission from impact of incoming electrons) are caught up in the NEXT wave of incoming electrons - THIS would be 2nd harmonic! Now, electrons that are not caught up in the next wave, but are caught up in the wave after the next, are of course the cause of 3rd harmonic, and so on. Add onto this the consideration that there will also be some electrons emitted from the plate from the 2nd harmonic recaptured electrons and the 3rd, and so on, and now you have higher and higher levels of harmonic distortion, all various multipliers of the fundamental.

the big advantage - that is, according to technodogma that all harmonic distortion is bad - of the pentode is that the suppressor grid (closest to the plate and tied to the cathode, so it's plenty negative relative to the plate) pushes these plate-emitted electrons back on quickly. the problem that is obvious to me with this is that there IS a discrete amount of time delay involved in the electrons being pushed back onto the plate, and, depending upon the ratio of distances inside the tube, these tiny delays actually give rise to high frequency distortions, some portion of which are NOT whole number multiples of the fundamental. I don't really know if these small distortions make a sonic difference, but my intuition is that if some people can hear the difference between power diodes, suppressor generated distortions are probably discernable also. It's too bad that the suppressor is internally tied to the cathode in many pentodes (maybe all, I don't know); no way to mess with this.

But grid 2, the screen, CAN be messed with, and therein lies a WORLD of possibilities, for both PP and SET.....still thinking about the screen; may add some modes to my above list and add a relay each side and try some combos.......and there's always the possibility, in THIS circuit anyway, of just rewiring the sockets and switching to triodes in the output.............. :)

all feedback welcome.

"It ain't a comeback until it's left the shop" Jimmy Dunne, the first man to drive a VW Beetle faster than 200 mph, and he has the forehead scar to prove it; I will always honor him for taking a chance on me when I wanted to be an engine mechanic.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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