Frequency range of presence knobs on 2:90/2:Fifty?

Triaxis, Quad Preamp, Stereo Power series, Recto Preamp

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slybird
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Frequency range of presence knobs on 2:90/2:Fifty?

Post by slybird » Tue Mar 18, 2014 6:55 pm

I have a 2:fifty, I am wondering what the presence knob does. Does it cut or boost, and what frequency range does it effect? Thanks.

JMMP
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by JMMP » Wed Mar 19, 2014 1:54 pm

In Mesa amps, I believe it affects the negative feedback loop in the power amp. That may just be in the Rectos, or that may be every amp but the rectos. In general, I find that it changes the extreme high end not unlike a 'tone' knob on a guitar, but shifted up a few kHz. I recall it being a very drastic control on my 20/20 and 2/90 when I had them
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Jackie
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by Jackie » Wed Mar 19, 2014 6:14 pm

JMMP wrote:In Mesa amps, I believe it affects the negative feedback loop in the power amp.
Indeed - it affects specifically the negative feedback of the higher frequencies.

I would say it affects the frequencies around 8kHz, but that's just a guess/approximation. As JMMP stated, it's quite a powerful control on the 2:90.
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JMMP
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by JMMP » Thu Mar 20, 2014 10:09 am

Jackie,

Do you know if the modern mode on Rectos behave this same way? I recall that modern mode comes from completely bypassing (or completely running through) the NFB loop, and in that mode, the presence control becomes a "regular" tone control affecting the frequencies above the treble knob. If that much is true, would the modern mode on a 2/90 or 2/100 operate the same way?

Your 8kHz guess sounds right for the frequency. To me, the cool thing about the presence knob is that it doesn't interact like the TMB knobs do, so changing the presence doesn't require readjusting the other EQ knobs.
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Jackie
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by Jackie » Thu Mar 20, 2014 2:42 pm

Yes, the Recto's Modern mode does indeed completely bypass the NFB loop. The manual states that
2:90 Manual wrote:Derived from the presence shift switches found on our Mark IV amplifier, and the "Red" or Modern Channel of our Dual Rectifier Solo Head
so I think you're right.

Here's a tech's explanation of what lowering negative feedback does:
When you decrease the amount of negative feedback two primary things happen. 1) the amp gets louder at lower volume settings because the power amp is not "limiting" itself. 2) The decreased feedback lets the power tubes break up sooner and more gradually - this is the important part.

When there's a lot of negative feedback the power tubes either don't break up at all or they do... sort of like on or off with little grey area in between. This is because the tubes have to be pushed hard before they reach that threshold where the feedback rather suddenly cascades itself out of significance. So you can play power amp clean or power amp distorted with not much wiggle room in between.

When there's little to no feedback (some amps use none, like the more famous Voxes [or our Recto Modern mode]) the power amp can distort gradually. So you can control the dynamics of the power amp much more dynamically by how hard you pick or with the volume control on your guitar. Instead of being a sudden point at the onset of power amp distortion there's a sooner and more gradual onset of power amp distortion which you can interact with through your picking. So the amp feels much more dynamic and responsive.

Tonally, reducing the negative feedback causes a natural filtering off of the very high and very low frequencies, giving a relative boost to the mid-range frequencies out of the power tubes. In fact, the volume increase you hear when reducing negative feedback is actually only in roughly the 70Hz to 11kHz range, with the largest boost happening in roughly the 200Hz to 6kHz range.
I guess that would mean a brighter, more cutting tone.
To me, the cool thing about the presence knob is that it doesn't interact like the TMB knobs do, so changing the presence doesn't require readjusting the other EQ knobs
Thankfully yes, and that is consequently the only knob I have been twisting for about a year now :lol:
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crane
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by crane » Fri Mar 21, 2014 1:38 am

According to the schematic, the modern mode on the 2:90 doesn't open the NFB loop as in the Rectos, it just changes its response (probably by lowering the NFB).

And there's another tech explanation. Not sure if it really goes in the same way :?
Overdriving the power amp causes the output transformer secondary to produce a clipped output signal, which represents a lack of output response to a changing input signal. The output transformer provides the source voltage for negative feedback to the phase inverter, so clipping reduces negative feedback. This creates more closed-loop gain, which drives the amp further into an overdriven state, producing even more clipping. The net result is that negative feedback from the output transformer to the phase inverter accelerates the power amp's transition to an overdriven state.
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talltxguy
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by talltxguy » Fri Mar 21, 2014 8:11 am

I really enjoyed soloing with my TA-15, but I could not stand soloing on the modern mode of my Roadster.

I'd be interested in reading some more comments about this topic.
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Jackie
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Re: Frequency range of presence knobs on 2:90/2:Fifty?

Post by Jackie » Fri Mar 21, 2014 12:23 pm

crane wrote:According to the schematic, the modern mode on the 2:90 doesn't open the NFB loop as in the Rectos, it just changes its response (probably by lowering the NFB).
This does make more sense since I have an amp that has variable neg. feedback and the change from high fb to low fb doesn't really sound or feel like engaging/disengaging Modern on the 2:90.

It would be interesting to see if the response of the Presence knob changes depending on whether the Modern is engaged or not. If it lowers neg. feedback in the higher frequencies then Presence would be what... less effective? I know on my other amp the Resonance and Presence controls (that control the NFB in the lower and higher freqs respectively) lose effectiveness when I lower negative feedback.
GUITARS: heavily modded Epi Goth Explorer; Jackson KVMG Pro; Yamaha SG2000; Gibson SG '61 Reissue; Squier CV Tele
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