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Help me wire in a guitar jack

B

bigred1

Member
Greetings,
Ive been working on a Newcomb amp and need some input on wiring in a guitar jack, schematic included. Ive read some and experimented, got tons of help from others on this forum, and cant get the volume right.

I get sound but at full volume its just really low. The mike input is plenty loud.

So again Im trying to rewire a guitar jack thats labeled the aux input in this schematic through the
12ax7 tube.

I would much appreciate some help on this.

Dustin
 

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I can't read the schematic as posted. Do you have a larger, higher res version?

That said, I speculate the aux in is more of a line level input, as opposed to a guitar which I think will be more like a mic-level input. If the guitar works though the mic input, why bother with the aux input?

Depending on how froggy you feel, you could try to rewire the whole front end to be like one of the preamps on the AX84 site. That's what I'm going to do with a Grommes S20 PA amp I bought. Going to modify the Grommes to use something similar to the AX84 lead preamp (located in the Building Blocks section).
 
how about using a D.I. box and then run that into the aux input? I assume the lack of volume and sound quality is because of impedance mismatching.
 
I don't think using the DI box is going to do what you want. The amp is basically designed for clean reproduction, afaik, and the AUX input is just a higher level clean input.

I think you'll need to rework the amp for more gain/gain stages with controls between each stage and so you can dial in the overdrive/distortion you want. Not to disregard advice that could be received here, but there are several guitar pre amp circuits at the AX84 site you could look at and compare to yours for some ideas.
 
This is how I do it, using a Switchcraft shorting jack:

jackwire1.jpg


The 68K value is not critical; I've used anything down to 22K.
I do mount that resistor as close to the pin (pin 7 in your schematic) as is practical. The 1M grid resistor is mounted right on the jack; notice how one leg wraps around to the shorting pin in the middle. Shielded cable to pin 7 is recommended, grounded at one end only.

Typical Fender input triode for a 12AX7 also has a 100K plate resistor (to pin 6), 1.5K cathode resistor with 25uF bypass cap (pin 8).

I would at least try this before investing in additional gear.

RWood
 
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Put the guitar input back at the phono input, but clip out the two caps in the preamp - looks like 470 pF and .003 uF. That will get rid of the phono equalization. And you'll have more gain than you know what to do with... and yes, 68K grid stopper resistor as suggested above - 47K will do if you don't have that value...
 
RWood I actually saw this pic at the other site and was going to post it and ask some questions. Thanks for the advice as well Tom Im going to mess with this tonight.
 
Guys THANK YOU, Im rocking now. trick was the 7 pin on the tube. I didnt have a 1meg resistor so I hooked up a lower value for testing, and did have a 62K. It sounds pretty good, getting great volume out of the amp. Im going to get a 1m resistor, read about the point of that one. But Im going to play with the other resistor and see what I can get for fuzz.
 
Glad to hear it.

If you want to have some more fun add a 1uF to 22uF electrolytic cap in parallel with the 1000Ω from pin 8 to ground of that same input tube. The + end of the cap goes towards the tube.

You can put the cap on gator clips if you just want to check it out.
 
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