Modifying Weber Mass 100

[Chuck H]

Survey says! 4.2 ohms.

Ok. Cool, but...

Moot now (see post #44).

Unless I'm seeing it wrong you appear to have the two black wire leads at the bright switch reversed in your drawing compared to what I see in the gut shots. This means the treble boost cap is never parallel to those resistors so any adjustment to their value won't change the boost effect.

Interesting that they implement those resistors for the treble boost mode though. Probably an impedance adjustment relative to implementing the treble boost circuit.

FWIW I don't see how bridging two of the switch terminals would reduce the effect either. And now, in this case with the switch draw incorrectly I believe it would result in a less than ideal impedance load for the amp at the boosted frequency at some attenuation settings.

[Boy Howdy]

I've drawn maybe three or four schematics in my whole life, so it is certainly possible that I drew it wrong. But dang, Chuck, I just can't see it. I know it makes no sense that it totally bypasses everything, but it sure looks to me like that's what it's doing. Here's a better photo. Hopefully, you can see that the top of the switch goes to the end of the resistors closest to the tone stack, and the middle of the switch goes to the end closest to the output jack.

[Chuck H]

Oh, okay, you're right. I was somehow confusing the aspect of those parallel resistors because they're drawn the other way in the schematic and I was only looking at a slice of the photo for closeup. But yes, the capacitor DOES bypass the full circuit and those parallel resistors are shorted when the capacitor is removed. Good. So...

Try shorting the resistors at the switch as you proposed (top two switch terminals bridged). But what I would do first is test this possibility by just using an alligator clip lead across those resistors (easier and less invasive than soldering). That removes 4.2 ohms of load that the boost cap is bypassing. It may be enough reduction in the effect. If the boosted frequency knee rises too much you could try a 3.3uf, a 4.7uf or just another cap (1uf or 2uf) added in parallel with the existing cap.

If there is TOO MUCH reduction in the effect you could replace those parallel resistors with a 5 ohm rheostat to make the boost user variable. It looks like you could fit a smaller rheostat where the switch is now. Mouser offers a 12.5W Ohmite model that would surely fit. I think 12.5W should be fine since 4.2 ohms isn't doing the heavy lifting in the load compared to the other resistors and the speaker motor.

[Boy Howdy]

I think it's gonna be a good day. I'm not crazy after all. Well . . . that's a discussion for another time. And I've got all kinds of usable/doable options to play with.

Question though, if I parallel another cap with the existing treble boost cap does it need to be the same bipolar type (but of different or same value)?

Thanks, Chuck.

Edit: Wow, those rheostats are $80!

'Nother edit: But cheaper on ebay.

[Chuck H]

Yes, cheaper when you can. It's all coming from the same place after all (east Asia, likely the same manufacturer!)
And I would recommend non polar caps. It's an AC signal across a low load so there is bound to be flyback voltages from peripheral load circuits. In this application it would be ideal if the caps were as non polar as the speakers. Not possible, but we do the best we can.

[Helmholtz]

Speaker signal is ACV. ACV destroys normal polarized ecaps.

[Boy Howdy]

I tried all possible combinations of the switch (using an alligator). It didn't make any noticeable difference. On to the next thing . . . whatever that might be. TBD.