Why does a Twin Reverb lose volume when all the tone pots are at 0?

[mort]

I've never noticed this before because I've never thought to turn 'em all down like that. Looking at the schematic it seems like signal should always pass to the volume pot. What am I not seeing?

http://www.thevintagesound.com/ffg/s...a769_schem.gif

[52 Bill]

I've never noticed this before because I've never thought to turn 'em all down like that. Looking at the schematic it seems like signal should always pass to the volume pot. What am I not seeing?

http://www.thevintagesound.com/ffg/s...a769_schem.gif

The tone stack of the blackface Fender amps with a midrange control sends all of the signal to ground when all of the controls are turned to zero. Take and ohmmeter and measure the resistance from the top of the volume control to ground and see what happens when you turn the tone controls up and down.

[Justin Thomas]

An added bonus: you can wind up the volume to 10 with all the tone knobs on 0, then turn one up slowly to taste. I usually will use the mid & treble; the bass is just a fart knob at that level... It's another flavor of distortion, if you will. Some individual amps do it better than others, and it's more the Neil Young type, but try it out - you might like it, and it's free.

Justin

[mort]

oooo that's a cool idea Justin. I like a goosed signal that's reigned back by a single control.

Why wouldn't the signal pass through the back end of the treble pot and on to the volume pot when the treble is all the way down?

[Justin Thomas]

I'll leave that to the engineers... and it's not my idea. I read it on Harmony Central many years ago, doing research on my Prosonic. Then when I finally got a vintage Fender (Bassman 100), I tried it again and it worked. A note of caution: it still might get pretty loud! I humbly duck out now, as I know nothing more.

Justin

[Tom Phillips]

...Why wouldn't the signal pass through the back end of the treble pot and on to the volume pot when the treble is all the way down?

Because, with all the tone controls set fully CCW, the top of the volume pot is essentially connected to ground potential as 52 Bill said. In that situation all the signal is shorted to ground. I've actually had people bring amps in for service when the only problem was that all the tone controls were set to zero. An easy fix and I never charged any of them for the fix. That situation demonstrates why it is a good idea to sound test an amp while the customer is dropping it off at the shop.

[Alan0354]

It is normal to be like this. The only reason those BF and SF Fenders that only treble and bass still have sound even when you crank both to minimum is because they replace the mid pot with a resistor inside that set to a little over half way.

[mort]

So if the input is shorted to ground but there's still a path to the volume pot then it must be a 'path of least resistance' sort of thing and the signal will simply not go toward the volume pot?

[mort]

IN this situation my neighbor brought me his Twin because during a visit from a potential CL buyer, the normal channel wouldn't work at all and there was also some crackle in the normal channel. The crackle was in the tone stack caps and was easily resolved. And there was no real issue with the volume having dropped out as I discovered that both channels would do that with the tone pots all the way down. He had never turned those pots down and we speculated that the buyer may have done it possibly on purpose in attempt to devalue the amp during negotiations. Slick little trick to try and gank someone

[frus]

So if the input is shorted to ground but there's still a path to the volume pot then it must be a 'path of least resistance' sort of thing and the signal will simply not go toward the volume pot?

no, the volume pot would be shorted to ground
imagine in that schematic that the middle pot is shorted (which it really is if it's at zero). then the "bottom" of the bass pot is shorted to ground. but the bass pot is also shorted, so the bottom of the treble pot is shorted to ground also. and if the treble pot is at zero, then its wiper is connected to its "bottom" which is shorted to ground, hence the "top" of the volume pot is at zero and you have no signal

[g1]

There is no zero on those amps/knobs.
I say that a little jokingly, but there must be some reason Fender went to the trouble.

[Tom Phillips]

By the way. Most customers do not know how the tone controls function electronically and they don't understand the concepts we have been discussing in this thread. Therefore I just explain to them that, on a Fender amp that has Bass/Middle/Treble, if you set all the tone controls to minimum you have no bass, no middle and no treble and that means "no sound." They get that.

Well...there is some sound left but it's just a little bit and does make it sound like the amp is broken.

[mhuss]

A way to 'fix' this is to unsolder the middle tab of the Middle control, and move the wire coming from the .047 cap to that middle tap. This is way Marshall did it. It is functionally the same, except for the case where all the tone controls are turned down, where it still allows some signal through.