Bright Channel

[Harry]

You know how early amps would sometimes have a seperate bright/treble input channel? Were there any sort of standard or generic high-pass filter values that was used in these?

[Seljer]

The 4 input Marshalls only had a 100pf to 470pF capacitor letting more treble through the first 2 lugs of the gain control pot on the bright channel. If you dimed the amp there wouldn't be any difference between the channels (however, those amps didn't have any master volume control, and doing that would be very very loud)

[Threefish]

Similary, the various Fender amps had bright channels or bright switches, which had a small value bypass cap around the volume pot. In the case of the amps with a bright channel, the cap was permanently in the circuit to allow higher frequencies to bypass the volume pot. With a amps that had a bright switch in a "normal" channel, the cap could be switched into the circuit with a simple single pole switch.

In the Bassman circuit on this page : http://www.pentodepress.com/summary.html (click on the 12AY7 preamp link on the left), you can see the identical inputs for the normal and bright channels. If you scroll down that page to look at the the second 12AX7 stage, you can see where it says "bright in" - the volume pot is bypassed by a 100pF cap. In the bright switch amps, there's just a switch that opens and closes the two ends of the cap across the amp. If you look at the Bandmaster Blackface circuit here : http://www.ampwares.com/ffg/ (scroll through the list on the left of the page), look to the right of the big "7025" on the scan, past the treble and bass controls to where the volume is - see the switch with the 120pF cap?

As with what Seljer said - the bypass cap is negated when the volume is on "11", as the middle and bass frequencies catch up with the treble frequencies that were being let past the volume pot by the bypass cap.

[d95err]

These are some methods used to make tube amps more (or less) bright:

* Small coupling cap (Marshall uses 0.0022 instead of 0.022 on some models)
* Smaller cathode bypass cap (e.g. 0.68uF) instead of 47uF)
* Bypass caps on volume
* Bypass cap on series resistor
* Small cap across the plate resistor (on the non-bright channel to make it less bright)
* Highpass filter between poweramp inputs (Vox Top Cut)
* Conjunctive filter across the output transformer primary

[stm]

This is a great summary to keep in mind when designing and tweaking pedals!

[Harry]

This is a great summary to keep in mind when designing and tweaking pedals!

That's what my intentions are.

I've been looking at different treble booster pedals and they usually use a high pass filter with a cap of .001uF-.0068uF and a resistor of 22k-80k.