Input sensitivity switch?

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psychepool
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Joined: Fri Apr 18, 2014 8:29 am

Input sensitivity switch?

Post by psychepool »

I want to switch input grid stopper resistor in my diy amp.
Originally, it was 68K. I want to switch it 33K by add 68K parallel.


I measured it after installed a resistor and a switch. It was measured as 68K / 33K as desired.
However, when I turned on the amplifier and operated the switch, I could not feel any change.


I installed it like below.
input.jpg
Theoretically, I do not think there is anything different from the high / low inputs commonly used in vintage amplifiers and why does not it work?


Please let me know if you have any problems with switching in this way.
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sonicmojo
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Re: Input sensitivity switch?

Post by sonicmojo »

There is a difference from your wiring in the vintage amps. For example in a Twin Reverb AB763,it is essentially dividing the signal to ground with the 2 68K resistors when you plug into the #2 jack.
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Bryan
psychepool
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Re: Input sensitivity switch?

Post by psychepool »

Oh, you're right!

Then, is the result like this?
lohi.jpg
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Littlewyan
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Re: Input sensitivity switch?

Post by Littlewyan »

Not quite. When you plug into the low sensitivity input on a Marshall 1987 or Marshall 1959 (I think most Fenders are the same) the wiring puts the 68K to ground after the grid leak resistor forming a voltage divider.

http://drtube.com/schematics/marshall/1959-01-60-02.pdf

Here is a nice clear schematic so you can see how the input wiring works.
psychepool
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Re: Input sensitivity switch?

Post by psychepool »

Is it right?
lohi_.jpg
Can I operate it with DPDT switch? I want to make it with DPDT push-pull potentiometer.
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psychepool
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Re: Input sensitivity switch?

Post by psychepool »

Dose it function?
input jack hi-lo switch.jpg
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Littlewyan
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Re: Input sensitivity switch?

Post by Littlewyan »

No that will put the 68K to ground before the grid stopper, it needs to come after it. Sorry I don't have time at the moment to draw anything up for you
psychepool
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Re: Input sensitivity switch?

Post by psychepool »

input jack hi-lo switch.jpg

Maybe this is right. But there seems to be a loud pop noise.
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Colossal
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Re: Input sensitivity switch?

Post by Colossal »

How about this? In the High position there is 68k || 68k for 34k/1M. In the Low position there is 68k / 68k.

HI LO Z Switch.png
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psychepool
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Re: Input sensitivity switch?

Post by psychepool »

Oh, you re genius! Thanks!
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