Master Volume - am I doing it wrong?

[Lynxtrap]

I recently did some mods on an already modded Fender amp. It had a crossline MV and no NFB.

The mods I did now included putting back a NFB loop and so I wired in a different MV with one objective being to keep the bias balance. I was also in a bit of a hurry to get the amp ready and did not give the MV very much thought among all the other things I did on it.

The MV is seen in the schematic below.

The only problem with it is that I'm loosing quite a bit of treble with the pot at low settings. Is that inevitable with this circuit compared to the Type-2 MV usually seen? I guess an easy solution would be to put some caps across the pots. Or is there something wrong with the circuit that I don't get?

MV.png

[pdf64]

What’s the purpose of the 33k resistors on the track wipers?

[maxkracht]

I was wondering the same thing. No idea why you would be losing highs. You would lose lows overall if you started with .1uf caps.

[Lynxtrap]

What’s the purpose of the 33k resistors on the track wipers?

They are supposed to isolate the caps from the pot, to avoid interaction. I can't recall at the moment where or by whom that was recommended. And looking at the schematic now, I'm not sure it makes sense

[romberg]

The only problem with it is that I'm loosing quite a bit of treble with the pot at low settings.

Perceiving less highs (and lows) is a common and expected thing with all volume changes. It is not the circuit. It is your ears. The human ear's frequency response changes with volume levels. At lower volumes you hear highs and lows less well.

https://www.fabfilter.com/learn/science ... d-loudness

This effect is why brite caps are put across many volume pots. It will increase the amount of highs at lower volume settings to compensate for the human ear.

Mike

[Lynxtrap]

Yes, maybe so. I thought perhaps it also had something to do with the signal being shunted to ground as opposed to the crossline or the version where the bias supply runs through the "ground" lugs of the double pot.

[pdf64]

What’s the purpose of the 33k resistors on the track wipers?

They are supposed to isolate the caps from the pot, to avoid interaction. I can't recall at the moment where or by whom that was recommended. And looking at the schematic now, I'm not sure it makes sense

I can’t see that they bring any benefit. And whilst they won’t be causing a muffled tone, they’ll be pushing things in that direction.

But, are you sure that the feedback loop is negative? eg does the power amp gain increase if a leg of the 1k2 resistor is lifted out of circuit?

[Lynxtrap]

I can’t see that they bring any benefit. And whilst they won’t be causing a muffled tone, they’ll be pushing things in that direction.

But, are you sure that the feedback loop is negative? eg does the power amp gain increase if a leg of the 1k2 resistor is lifted out of circuit?

Googling around again, it seems I got the extra resistors from a version of this MV by Bruce Collins/Mission Amps. That schematic recommends 33K-100K for the resistors. I had doubts about them but thought they wouldn't hurt so I put them there for good measure...

The feedback loop: There's no howling, and according to the original schematic (this was originally a Bassman 10) the loop is to be tapped from the positive lug of the speaker output.
What makes you think it might not be negative? I have not tried lifting the resistor, I will open it up when I get some time next week and could try it then.

[pdf64]

Howling (free running oscillation) is the extreme case of positive feedback. If the degree of feedback in a positive feedback loop is low, free running oscillation won’t occur. Consider that we we turn up the gain on a microphone high enough, the PA will squeal, howl, ie go into free running oscillation. Turn the gain down a tiny bit and it might stop but not sound good.
Note that your schematic has a lower than normal degree of feedback.
But the frequency response will be affected; as the amp’s output impedance will be increased, hence the impedance characteristic of the load will be reflected to a greater extent on the amp’s frequency response than in open loop operation. So with a speaker, the treble and bass resonance will be increased.
But when the master volume is reduced, the gain that drives the feedback loop’s operation reduces, so less treble boost.