Understanding a Fender tone stack - low-pass filter

[jazbo8]

Love the history... How Leo managed to patent a tonestack in the 60's no doubt made for an interesting story, but similar circuits have been floating around even when Williams published his paper (as a part of the famous Williamson amplifier design for Wireless World). Around the same time, there were several other articles in Wireless World by E.J. James and Peter Baxandall (of the James-Baxandall fame). Even back then, James' article elicited a response from Rocke International of Brussels, claiming that as far back as 1939, they were already selling amplifiers using an "exactly similar" circuit design by their designer Michael Volkoff, so "what's old is new again..."

E.J. James' article.
Baxandall's article.

Jaz

[Steve A.]

Thanks for the information!

So what was the inspiration for the 5F6A tone stack? Just wondering... And where does Westinghouse design we always hear about fit in?

Steve Ahola

BTW the linked patent was for the brownface tonestack with the 70k tap on the treble control. Is there a patent for the BF or 5F6A tone stack? Thanks!

[jazbo8]

I was hoping you know Not sure, but the 5F6A tone stack appears to be a descendant of the Williamson-James-Baxandall variety with the the mid control replacing the bottom resistor on the Deluxe tone stack (shown in woodyc's post above). Tone-Lizard also made some interesting comments on the change from the Tweed to Blackface tone stack here, where he also hinted that the Tweed tone stack was taken from hi-fi designs of the day.

Jaz

[woodyc]

Couple of notes...

The Fender patent is irrelevant. You should be able to tell why by reading it -- the tapped pot was a flash in the pan.

To me, the 5F6A is just a variation on a theme. The only reason I used the '63 deluxe for comparison above is that it makes the relationship more evident.

Also for anybody who doesn't know, the .1u in the Fender stack was not intended to have any tone shaping function. It was just to keep dc off the bass pot. So the only real Fender/Williamson difference are the details of the hipass, and if you listen to both you'll see its not a huge difference. They both have the same basic structure with an adjustable hipass and an adjustable lowpass with a significant and broad mid-dip in the higher guitar fundamentals. (i.e. a totally awesome guitar tone.)

This is a subtle thing, but Williamson was using this circuit strictly for compensation, namely to flatten a response that was messed up by the imperfections of rooms and speakers, et cetera. Fender was using the same tool, but he was creating something more inspired and quite different. It might have been patentable on that basis?

Although Williamson doesn't provide a reference for the rise/fall design, he presents it in such a matter-of-fact way that makes you wonder if he also lifted it from somewhere. If anybody knows, I'd be interested.

[trem]

Well, maybe not a Nobel prize, but...

I totally agree that the stack was one of Fender's greatest innovations (among a lot of really great innovations.)

The way it worked its way into so many of his amps clearly indicates that he knew how important it was. So the fact that he didn't patent it suggests pretty strongly that he felt it was already in the public domain.

I've done a bunch of literature searches and made a several trips to the library to try and figure out where it might have originated. The best I've come up with is a D.T.N. Williamson article called Design of Tone Controls and Auxiliary Gramophone Circuits, Wireless World, Oct/Nov 1949.

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By the way, the fact that it came from somewhere else does not in any way tarnish Leo's achievement. Great electronic design is not necessarily about inventing anything, its more about knowing when and how to incorporate existing art to solve problems.

Speaking of winning a prize.....
That is a beautiful bit of electronics wisdom you have penned.