" Cut " control ?

[Michaell]

Saw this control listed on Dr. Z amps

What does a " cut " control do?

thanks

[muchxs]

It's functionally different from a normal guitar amp tone stack in that it's not part of the preamp, it's part of the power amp. It feeds EQ'd and out of phase signal decoupled from one PI plate to another. The PI plates "don't care" but the power tube grids just downstream do.

[Michaell]

[QUOTE=muchxs;1503430]It's functionally different from a normal guitar amp tone stack in that it's not part of the preamp, it's part of the power amp. It feeds EQ'd and out of phase signalQUOTE]

So it's a tone control?

What does it do to the tone?...add bass? add treble ?

[muchxs]

Originally Posted by Michaell

So it's a tone control??

Sort of. When implemented it interacts with the "tone" control or controls and makes a wider palette of tones available.

Originally Posted by Michaell

What does it do to the tone?...add bass? add treble ?

It depends on the RC network used. As you can guess from the term "cut"... it doesn't add anything. It's a passive network, it can only subtract. But you can roll off the "cut" control slightly and raise the volume of the preamp slightly.

[Michaell]

Originally Posted by muchxs

Sort of. When implemented it interacts with the "tone" control or controls and makes a wider palette of tones available.



It depends on the RC network used. As you can guess from the term "cut"... it doesn't add anything. It's a passive network, it can only subtract. But you can roll off the "cut" control slightly and raise the volume of the preamp slightly.

thanks

[Wayne Alexander]

The most common "cut" control wiring, like in a Vox AC15 or AC30, which is also used in some amps that are based on those amps, like some Matchless and Dr Z designs, puts a pot and a cap between the two wires which are the input to the power tubes. The signals on those two wires are out of phase. With the cut control turned to zero, nothing happens. As the cut control is turned, the pot mixes more and more of the signal from one of those wires, through the cap, to the other side. The signals on both those wires are out of phase, so you get phase cancellation of the frequencies above the value that the cap passes through (like in a tone control, bigger value cap gives a darker sound and smaller value cap gives a less-dark sound when the cut control is in use). So the net effect is that a cut control is a tone control on the power section, that rolls off highs as you turn it more, and does nothing when you turn it to zero. As an aside, the cut control is one step away from one of the main types of post phase inverter master volumes - if you just have the pot there and no cap to control which frequencies get cancelled, you've got a master volume control like the ones used in current Voxes and Matchlesses - with the control to zero nothing happens, but as you turn it, more and more of the signal is cancelled because you're mixing two out of phase signals, thus reducing the sound.

[Michaell]

Originally Posted by Wayne Alexander

The most common "cut" control wiring, like in a Vox AC15 or AC30, which is also used in some amps that are based on those amps, like some Matchless and Dr Z designs, puts a pot and a cap between the two wires which are the input to the power tubes. The signals on those two wires are out of phase. With the cut control turned to zero, nothing happens. As the cut control is turned, the pot mixes more and more of the signal from one of those wires, through the cap, to the other side. The signals on both those wires are out of phase, so you get phase cancellation of the frequencies above the value that the cap passes through (like in a tone control, bigger value cap gives a darker sound and smaller value cap gives a less-dark sound when the cut control is in use). So the net effect is that a cut control is a tone control on the power section, that rolls off highs as you turn it more, and does nothing when you turn it to zero. As an aside, the cut control is one step away from one of the main types of post phase inverter master volumes - if you just have the pot there and no cap to control which frequencies get cancelled, you've got a master volume control like the ones used in current Voxes and Matchlesses - with the control to zero nothing happens, but as you turn it, more and more of the signal is cancelled because you're mixing two out of phase signals, thus reducing the sound.

Thanks for the explanation.


Seems like I'll have to find me a an amp with a " cut " and try it out!

[ruger9]

Originally Posted by Wayne Alexander

As an aside, the cut control is one step away from one of the main types of post phase inverter master volumes - if you just have the pot there and no cap to control which frequencies get cancelled, you've got a master volume control like the ones used in current Voxes and Matchlesses - with the control to zero nothing happens, but as you turn it, more and more of the signal is cancelled because you're mixing two out of phase signals, thus reducing the sound.

Do I understand you right, a "cut" control w/o a cap is the same thing as a PPIMV?

[teleamp]

Originally Posted by ruger9

Do I understand you right, a "cut" control w/o a cap is the same thing as a PPIMV?

Yes, a cut control without the cap would be the same as one type of PPIMV.

[tele_jas]

On my Z's.. The cut control is like a Presence (the ultra highs) control, the more you turn it up the more Presence you get.

On my old Vox, the more you turned it up the more it "cut" or reduced the top end (still presence).

[ruger9]

Originally Posted by teleamp

Yes, a cut control without the cap would be the same as one type of PPIMV.

So, let's say you have an amp wired with a cut control they way you described... if you simply removed the cap, you would then have an amp with a PPIMV instead of a cut control?

I wonder if this is how Dr. Z's are wired... most of his amps have a cut control, and almost none of them have MVs... very interesting...

[Wayne Alexander]

To turn a cut control into a crossline master volume, you need to replace the cap with a wire. If you just remove the cap (or lift one lead) the control will do nothing.

[junebug]

Originally Posted by ruger9

So, let's say you have an amp wired with a cut control they way you described... if you simply removed the cap, you would then have an amp with a PPIMV instead of a cut control?

I wonder if this is how Dr. Z's are wired... most of his amps have a cut control, and almost none of them have MVs... very interesting...

Yes, that's it, quite simply.

The key to the cap in the cut control circuit is that it won't pass the lower frequencies to be phase cancelled on the other side of the PI output. The PPIMV works by phase cancelling the ENTIRE signal.

[ruger9]

I wonder why the Doc (Z) hasn't utilized this on many of his models? Very very interesting indeed... and a trick I may try on my next Dr Z (which will be a Stangray or a Stangray GT when it's released.)

[junebug]

Originally Posted by ruger9

I wonder why the Doc (Z) hasn't utilized this on many of his models? Very very interesting indeed... and a trick I may try on my next Dr Z (which will be a Stangray or a Stangray GT when it's released.)

Good question. I'm sure he has his reasons. Part could be the negative perception some folks have of a master volume control. But that's just a guess.

[teleamp]

I think Z builds his amps with gigging players in mind. I find a cut control much more useful than a MV.

[golfnut]

Originally Posted by teleamp

I think Z builds his amps with gigging players in mind. I find a cut control much more useful than a MV.

I don't find an MV useful at all. When I owned the Maz Sr the MV was always full. On my Stangray I don't miss it at all.

[jrfrond]

A cut control simply cuts highs after the PI by mixing them out-of-phase through a capacitor. The higher the cap value, the lower you shift the corner frequency of this simple filter, and the more apparent high-cut you have when the control is turned down. It is NOT a presence control, or even anything like it. Presence works in the negative feedback loop, and essentially disables the feedback loop at high frequencies, which causes a rise in high frequencies through the power amp. We are talking NEGATIVE feedback here, which is why this reads ass-backwards. If you CUT THE HIGHS in the NFB loop, you cause a BOOST IN HIGHS at the output. This is a Presence control. If you CUT THE LOWS in the NFB loop, you cause a BOOST IN LOWS at the output. This is usually referred to as a RESONANCE control.

The cut control, to me, makes a superior one-knob tone, because it doesn't change the gain structure in the preamp. You get all of the preamp "snarl", which can then be tamed later on in the audio chain. The cut control does NOT work well with a post-PI MV control, because the loading of the MV ruins the response of the cut control at lower MV settings. In order to make this setup effective with a post-PI, you need to use a dual 250K pot, replacing the bias-feed resistors (usually 220K) with the pot resistive elements and the pot wipers feeding from the PI caps. This maintains a constant load for the cut control to work properly.

[Wayne Alexander]

John, to make a crossline master volume work together with a cut control and maintain loading so both controls work right, you can isolate the cut control with 10K resistors on both sides, as it's done in the AC30CC and AC15CC series.