5e3 - what makes the bright channel bright and the normal channel normal?

[d. spree]

I wondered if someone can explain what makes the difference between the bright and normal channel on the Tweed Deluxe circuit. I have built 3 but do not understand this question. In the preamp stage the guitar inputs seem to run through identical circuits with each input using one half of the V1 tube. The volume circuits seem identical except for the 500pF cap between the bright channel volume pot and the tone pot. So how does this all work to make them sound different?

[loudthud]

The 500pF shunts some of the highs around the volume control. When the volume Is all the way up, it doesn't do anything, but the amp was never intended to be played all the way up. When the tone is turned down, the .005uF cap shunts the highs to ground on both channels. The Deluxe was a low cost amp when it was made in the 50's so the circuitry was minimalist. No extra effort was made to separate the channels.

[Enzo]

The bright channel spends more time studying...

[d. spree]

The bright channel spends more time studying...

Nice one!! It didn't help me get brighter.

[d. spree]

The 500pF shunts some of the highs around the volume control. When the volume Is all the way up, it doesn't do anything, but the amp was never intended to be played all the way up. When the tone is turned down, the .005uF cap shunts the highs to ground on both channels. The Deluxe was a low cost amp when it was made in the 50's so the circuitry was minimalist. No extra effort was made to separate the channels.

I get how the tone control sends highs to ground (and that understanding helped me fix a tone problem and poor grounding with the first amp I built). But when you say it "shunts around" the highs - how does that cap make the bright channel sound different (I'm not saying brihjter as Enzo will post a pithy remark!!!) Does it remove some of the lows?

[Koreth]

This is an oversimplification, but imagine if you will that a cap acts like a resistor who's resistance changes depending on the frequency of AC you put to it. At DC (0Hz), a cap should theoretically be an infinite apparent resistance, as it is made up of two conductors separated by an insulator, and insulators aren't generally in the habit of allowing current to pass. But as frequency goes up and DC becomes AC, the apparent resistance (the correct word here is impedance, by the by) goes down. At high frequencies, the impedance of the cap is low.

This is what lets a cap be used as a bright cap or as a tone control, it's the same mechanism. In a tone control, the higher the frequency, the more signal gets bled to ground. In a bright cap on a volume control, the higher the frequency, the more signal gets bled around the volume control. With high frequency being bled around the volume control, instead of getting reduced along with the rest of the signal, the tone now sounds 'brighter'.

[Chuck H]

Download Duncan TSC here:

TSC

Look at how the frequency moves as the knob is rotated. Sub different values. All will become clear.

[Enzo]

yeah, what Koreth said.


IN the normal channel all the signal goes through the volume control, and the wiper adjusts how much of that comes out. That little cap acts like a wire to high frequencies - treble - but doesn;t readily pass low frequency - bass. SO by wiring it across teh volume control like that, most of the signal still goes through the control, but the very treble parts take the path of least resistance through that cap. SO when the control is half way up, most of the signal is going through there and winds up at half volume. But those pesky trebles are going right past that control through the cap. Sort of a special VIP entrance for treble. And the treble sounds are not turned down by the control then. SO relatively speaking, at half volume on the control, the treble stays at higher volume compared to the rest, and so the sound at the end comes out brighter.

[d. spree]

Thanks, I do now understand.