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Tube FX loop?
Tube FX loop?
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Old 24th November 2011, 05:55 PM   #1
escopunk is offline escopunk  United States
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Default Tube FX loop?

I need some help with Tube FX loop design. I getting lost in the details. This for a modified Plexi amp.

Thanx!
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Last edited by escopunk; 24th November 2011 at 05:59 PM.
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Old 24th November 2011, 10:58 PM   #2
Minion is offline Minion  Canada
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Neither of those FX loop schematics are going to work properly ......

Im not good at explaining stuff but I"ll try , .... You need a shorting 1/4 in jack ......

look at the FX loop in this schematic , and pay attention to the FX output .....

http://www.paia.com/prodimages/siabsch.pdf

the shorting jack makes it so the audio is diverted through the FX loop when there is a patch chord plugged in but lets the signal flow to the rest of the circuit when the FX loop isn"t in use .....

My latest amp I just had an output from the preamp and an input for the power amp and just put my effects inbetween and use one of those 6 inch patch chords to connect the two when not using effects , not elegant but it works .....

Last edited by Minion; 24th November 2011 at 11:25 PM.
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Old 24th November 2011, 11:34 PM   #3
Minion is offline Minion  Canada
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Also your cathode bypass caps seem way to small , with 0.1uF caps you are bleeding off all your bass which won"t sound good unless you like a tinny sounding guitar .....

The same with the Cathode follower stage , 0.1uF is WAY too small for a cap coming off a cathode follower stage , they are low impedance so you need an extra large cap there , 4.7uF is a good value that won"t destroy your bass .....
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Old 25th November 2011, 08:36 AM   #4
Nigel Goodwin is offline Nigel Goodwin  England
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Quote:
Originally Posted by Minion View Post
The same with the Cathode follower stage , 0.1uF is WAY too small for a cap coming off a cathode follower stage , they are low impedance so you need an extra large cap there , 4.7uF is a good value that won"t destroy your bass .....
While I don't completely disagree with the sentiment (0.1uF is probably too small), your reasoning is incorrect.

A low OUTPUT impedance has no need for a large capacitor, it's the impedance of the following INPUT that decides the value of the capacitor.

The reactance of 0.1uF is about 30K at 50Hz, so any input impedance over 100K would be perfectly fine with 0.1uF.

Incidently, the reactance of 4.7uF at 50Hz is only 680 ohms.
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