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Re: Origins of the JFET mu-amp circuit as used in the "Booster"

6/5/2000 12:29 AM
Re: Origins of the JFET mu-amp circuit as used in the "Booster"
"However, I just can't say "Minibooster"  
without saying "Jack Orman". That's just the way it's gonna be."
And you shouldn't.  
As Jack points out in his editorials, the circuit would be nowhere without him. He personally put it on the map. He did all the development, tinkering, and many subtle changes to make an side-page app note into the useful musical circuit. If you'll look, you'll notice that the Minibooster is very, very different from the JFET mu-amp. It's those subtle changes that make all the difference to the sound. I don't think anyone *should* say "Minibooster" without saying "Jack Orman" in the same breath. Jack richly deserves the credit for discovering this circuit.  
I just found the paper, thought it was very interesting, noticed that it said "mu amp" which made me think about "JFET mu amp" which is what the National app note said. When he first brought out the MB, Jack said that he had done lots of research into tube circuits and had never seen a tube circuit like the MB. I just thought he - and everyone else - would be interested in the techno-archeology. I'm certain that it's mere coincidence that whomever titled the National circuit happened to pick a similar name to the tube circuit.  
Happens all the time in the tech and academic worlds - convergent naming. I think this comes from a fundamental limitation of humans. People being people, there are only so many names that really mean anything to us. After all the good ones get used up - and the good ones go early, I mean, 'd'ja notice how it is with domain names? - then the marginal names get pressed into service.  
Hmmph! "mu amp"?? I mean, really, now. You'd think they'd have come up with something better, like GunderhausenFarkenBedunker, or my favorite marginal name, Belchfire MechaBlaster. Now those, those are NAMES. Names a circuit could be proud of.  
'Course, "MiniBooster" is not all that bad, I guess. Not too flambuoyant. OK.
6/5/2000 12:45 AM

"Belchfire MechaBlaster"
I like that name for a pedal! Anyone care to make it??? What sort of goodies could be put into it to make it belch?  
6/6/2000 4:26 PM
How about trying this with tubes?!
The article in TubeCAD is a fascinating read! Thanks for bringing it to our attention. Just in time too, I'm starting to experiment with mu amps in tube circuits.  
The article answered pretty much all my questions, except one: I'm going to try a mu amp as part of the preamp in my modified Dual Showman, but the B+ voltage is about 400V.. too much to safely connect directly to the plate of a 12AX7. Would putting a resistor (say 15K or so) in there to drop down the voltage to a safe level adversely affect the load on the plate? The mu amp will be driving a tone stack. I want this to be a very clean sounding circuit... any design suggestions? I'm trying to build the ultimate clean, loud amp.  
Has anyone else out there tried putting this kind of circuit in a guitar amp?  
6/6/2000 5:46 PM
Scott Swartz

The DC potential at the output of the mu amp will be only about half of B+ (if its designed correctly). The drop across each triode will be only 200V, so its noproblem. The 300V value in tube manuals is the plate-cathode voltage, which will be about 200V in this case.  
I have done A/B comparisons of the mu amp vs gain stage direct coupled to a cathode follower (Fender Bassman pre) and found the difference to be small. The mu amp is a bit brighter and maybe a little less compressed, probably due to the electro bypass cap on the gain stage in the bassman pre type.  
Either one will work fine and provide low impedance drive for the tone stack.  
If you are going for super clean, by all means try the mu amp.  
There were extensive articles in Glass Audio about the circuit (called a mu follower there). I would recommend purchasing some back issues and reading up.
6/6/2000 10:07 PM

Thank you very much. Your info is very helpful.  
6/7/2000 11:47 AM

What's the max. cathode to heater voltage? The upper triode cathode will be at 200V... You may need to add a DC reference to the heaters.

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