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Old 8th October 2011, 06:57 PM   #1
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Default How to combine 2 signals from 2 gain stages

This is for a guitar amp input.

I will have 2 different gain stages, one a 12AX7 input and another a EF86 pentode input.

These are in parallel from the input but i need to be able to be continously variable on output. So, thinking of using a potentiometer. One end is triode, middle is pentode / triode mixed and other end of scale is pentode. Continously variable.

So how do i connect this on outputs of the triode and pentode? The following stage is another 12AX7, then tone and vol then output tube.

I also want to do this passively and not add another gain stage to combine the signals.

Thanks!
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Old 8th October 2011, 08:47 PM   #2
DF96 is offline DF96  England
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Quote:
So, thinking of using a potentiometer. One end is triode, middle is pentode / triode mixed and other end of scale is pentode.
That is how you connect it. One end to each. Take output from the slider. The value would depend on the circuits. You might need to equalise output impedances.
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Old 8th October 2011, 09:06 PM   #3
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Then you reference the ouput of the slider to ground using say a 500K resistor?

Thinking about this though, as its set up one side or another cannot ever be at grond potential. Therefore there will always be some leakage from one to another.

so EF86 at say 125gain and 12AX7 at 70gain... just guessed on gains... both need say a 500K following stage load resistor.

Use a 250K pot for mixing and a 250K resistor to ground? of course cap couple before all of this.

make sense?
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Old 8th October 2011, 09:19 PM   #4
DF96 is offline DF96  England
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Yes, with one pot you would get some leakage. The alternative is to use a dual pot, wired as two volume controls but the opposite way round so one goes up as the other goes down. This would stop leakage.
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Old 10th October 2011, 12:28 AM   #5
jjman is offline jjman  United States
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Output from the mixer pot could be referenced to ground via the grid resistor on the 12ax7 that follows. I use a 500k to mix a parallel effects loop back in with the "dry" signal. There is leakage but it's not a problem for my application.
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