Fender Tapped tone pots

[txbluesboy]

What is the reason Fender used potentiometers that have a 70K tap for the treble pot on the 6G16 Vibroverb. I think its 350K ohm with a 70K tap.

[Structo]

Not sure of the reason but Hoffman carries new ones if you need to replace yours.
Scroll down this page.

http://www.hoffmanamps.com/MyStore/perl ... =246916440

[txbluesboy]

Thanks Structo, that was going to be my next question. I'd still like to understand the reasoning behind them though.

[tubeswell]

My own investigations have revealed the following cobbled together observations, which I have plagiarised and repeat here.

The tapped pots were used for James (Baxandal) tone controls. It shunts a little bit of the high end (from the bass and mid portion of the tone stack) to ground. The James circuit has a flat frequency response with the treble and bass at 5. Increasing the treble and bass resulted in a mid-scoop similar to the Fender/Marshall tone controls.

Apparently Fender got a bunch of these pots from a HiFi amp builder that went out of business. He used them for a little while in the brown amps and then switched back to the standard circuit.

You'd get identical results with a 280K pot (read 250K), and a 70K resistor (read 68K). Connect the resistor on one end to the bass pot/treble pot junction and on the other to a .0033uF cap to ground.

[txbluesboy]

Thanks tubeswell. I had planned on trying a pot and resistor until Structo told me about Hoffman having them.

[Bear]

Good to know. I've been ogling the 6Gx schematics for a while. Oh, and the couple 5Gx variants -- bass and treb to the left of the volume, how utterly bizarrely cool.

And, outside of the tone stack, danged if the 6G3 doesn't look like it has some genes in common with the Express.

[David Root]

I think weber has them too. I got some from them a few years ago. I did a brown Vibrolux w/ BF reverb. Works great!

[txbluesboy]

Hey David, That brown Vibrolux with BF reverb is pretty much what I have in mind.

[alexage]

In the 1960's tapped volume pots were used in many hifi circuits for the "Loudness" function, which increased bass and treble at low volume. This was/is done because humans perceive low and hi frequencies as lower intensity (vs. mids) at lower sound pressures. Google "Fletcher-Munson Curve", and "Loudness Circuit" for explanations.

Fender tone circuits work by attenuating the signal at different frequencies by shunting them to ground through a RC network. I think that the tap shunts more bass to ground at higher volume levels, hence making it "bassier" at quiet volumes to make up for how we perceive bass vs. volume. Could it be that more bass signal is going through the treble wiper to the volume pot when the volume wiper is closer to ground? Not sure, eq circuits sometimes confuse me, but I hope some well educated person here can do the math to confirm or deny this.

I know when I'm at a gig and it's really loud, I need to turn down the bass on my amp. This is especially true with tweed bassman style amps. Since the brownfaces came right after that, I'm guessing this was an effort to correct that situation, and/or to use some of the audio technology that was popular at the time.

It's also possible that Leo got a ton of these pots for a good price, but I am pretty sure he did not go to the trouble of an extra solder joint just because the pots were cheap!

[topbrent]

Tubeswell made a mention of a workaround that can be done using a regular 250k pot.

This is how it is implemented on the Allen Brown Sugar amp.

(It is the 68k resistor in series with the .0047 cap to ground, hanging from the 250k treble pot.)

Interestingly, this schematic has a 33k in series with the treble pot...280k as noted by Tubeswell.

[txbluesboy]

Interesting, The brown sugar is apparently Allen Amps nod to the Brown Fenders. I notice he adds another 250k pot below the mid pot labelled "RAW". Is this sort of a variable PAB?

[alexage]

The pot below the mid pot increases resistance to ground for the entire tone stack, thereby decreasing both its tonal effect AND the attenuation normally created by the tone stack, thus increasing signal to the volume wiper. So yes, it's a variable preamp boost. It also "flattens" the effect of the tone stack as the boost is increased, which in the case of fender tone stacks would almost always mean increased mids. To hear an approximation of what that sounds like, turn your fender treble and bass controls to zero, and middle to 10 (or 12). That's as close to "flat" as you can get on a classic Fender.