Input jack schematic question

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YMI5150?
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Input jack schematic question

Post by YMI5150? » Tue Nov 20, 2012 4:32 pm

Very confused by the theory behind the schematic in the 12 series. It appears that the zig zag buss wire sends signal to ground, assuming path of least resistance. And why link the positive posts and send to parallel resistors.

I can share that by changing one of the two 68k input resistors I've created a crunch channel with one hole and a nice lead channel in the other hole. The only thing that appears to be different is the path to ground, which was there before I changed the resistor. Witchcraft!
I also get a boost in highs when I insert a jack into the second hole. Nothing changes except the spring contact touching the jack housing.
Thoughts??

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Re: Input jack schematic question

Post by rgorke » Wed Nov 21, 2012 10:26 am

It seems that with the 68K input resistors in parallel, the signal is only getting 33k ish resistance. So, I have seen some single input amps with 33k input resistors.

Or, does it work to provide the top input a 33k resistance and the lower input a 68K resistance? :scratch:

I have heard the bright and normal channel having both a "HI" and "LO" inputs, maybe that is the way it works? :what:
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Re: Input jack schematic question

Post by YMI5150? » Wed Nov 21, 2012 10:38 am

I found this. not sure how accurate it is.

http://torresengineering.stores.yahoo.n ... uitar.html" onclick="window.open(this.href);return false;

I need to verify design of these jacks. i didnt realize there were normally closed switches. that helps make sense of all this.

when i changed the right resistor only to 28k, the top hole got a bit more sizzle but just about right, and the lower hole became more of a crunch channel. very nice actually. Eric can chime in. He stopped by last night and checked it out.

I bought parts to make a foot switch that would select either hole but the problem is they're dependent on each other so when you put cables in both jacks, they dont behave the same. My new project is isolating them, or putting the requried resistor in the pedal to drop the input down to crunch, and leaving the amp stock. either way, i like the idea of having a selectable crunch and lead channel out of a plexi.

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Re: Input jack schematic question

Post by vanhalen5150 » Wed Nov 21, 2012 1:34 pm

So the jacks are a closed circuit with no plug?
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Re: Input jack schematic question

Post by rgorke » Wed Nov 21, 2012 1:39 pm

Hah, that article is eye opening. Thanks for posting.

So, the top jacks are actually 33k and the low jacks are 68k. I can definitely hear a difference between the two.

So, without the 1 M resistor, does that mean my "HI" jack will be noisier that the "Lo"? My "Hi" has a bit more noise. I will try adding the 1 M resistor and see if it helps.
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Re: Input jack schematic question

Post by rgorke » Wed Nov 21, 2012 2:39 pm

Here's the best explanation I can find.
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input_jack.pdf
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Re: Input jack schematic question

Post by vanhalen5150 » Wed Nov 21, 2012 4:51 pm

I'm building my 5th amp. Sometimes I feel like I know nothing......
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Re: Input jack schematic question

Post by rgorke » Wed Nov 21, 2012 9:47 pm

At least you're building...

Here is how you can check ( I did this today):

Put cable in top left hole, put DMM at input side of the corresponding input resistor, put the other probe pin 7, it should read 33k or so.

Now put cable in bottom left hole, put DMM at input side of the other input resistor, put other probe on pin 7, should read close to 68k.

I knew there was a reason there were four holes....

I have grounding issue in the Hi/bright input. If I touch the chassis with everything humming, the hum goes away. I poked around and can't find a loose of cold joint...not a bad hum, just an annoying hum.
"If you make a mistake, do it twice and smile and let people think you meant it." Jan Van Halen.

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