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5e3 input weirdness

4/7/2005 1:07 AM
5e3 input weirdness
I just assembled my first amp, a 5e3 kit from Mission. Sounds amazing, but I'm having some weird problems with the inputs. If I plug into the normal channel 1 I get very low signal. Normal channel 2 sounds fine. Bright channels sound fine, but Bright channel 1 seems to be louder and have more gain than Bright 2. I should mention that I originally had the resistor on normal 1 backwards, with the gold band lead going to the ground. Would that make a difference?
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4/7/2005 1:20 AM
William Jahnkow

It makes no difference which way the resisters go at all .  
You must have the inputs on the normal channel reversed .  
I bright inputs will be different sounding than the normal. also the # 1 bright is supposed to be louder than the #2 bright , and the same with the normal inputs . They will sound different but should work the same way as the bright , one louder than the other .  
This gives you a choice if you use a guitar with a higher output signal or when using a OD pedal for example to have less input signal going in the amp .  
The reason is on the #2 inputs the signal must pass through two 68k ohm resisters and the 1 meg to ground and the # 1 inputs just one 68k and the 1 meg to ground .
4/7/2005 1:51 AM

Thanks, good to know input 1 is hotter than input 2, but I still can't figure out why normal 1 input is so low. Being the novice I am, I checked the resistors with my multimeter and found resistor on normal channel input to be about 3.5, but on bright channel its around 32. Is that normal? Could it be the resistor's bad?
4/7/2005 8:23 AM
Bruce /Mission Amps
INPUT JACKS revisted... boring if you know it.
"I checked the resistors with my multimeter and found resistor on normal channel input to be about 3.5, but on bright channel its around 32. Is that normal? Could it be the resistor's bad?"
3.5 and 32 "what" Fren?  
It just sounds like one of the jacks is miswired and or one of the leads that runs to the jacks or one of the 68K resistors is shorted... etc.  
Also, the previous "sawitching jack" reply explaination is a little nebulous, so, ....  
The low gain jack and high gain jack are set up this way to provide a way to reduce signal to the first preamp as stated. The high gain jack is the one with the 1M resistor on it and the 68K resistors are actually one the normal jack.  
Lets talk about the using the amp as normally used ... for this example, plug into the high gain jack.  
Now, when not in use, (no plug inserted) the unused normal jack has it's contact points closed. That's why they're called a switching jacks.  
Remember, the points are normally closed in the jacks and it's the actual plug that is pushing open the switch.  
When the points of the switch are closed (shorted together) in the unused normal jack, the two 68K resistors form a parallel pair of resistors at around 34K ohms.  
This new "34K ohm" resistance is in series with the signal path on it's way to the grid of the preamp tube but has very little attenuation at audio frequencies, while the 1M is grounded.  
That 1M resistance is the "grid load" of said preamp triode and is the DC path to chassis ground for the preamp tube's bias ground reference.  
This is allows maximum signal to the preamp tube.  
Now, when you plug the instrument/Mic phone plug into the normal (lower gain) jack, the switch opens and the switch is closed in the high gain jack, shorting that entire jack to ground.  
With the normal jack switch open, it allows one 68K resistor to be grounded (it's sitting on top of the grounded jack of the high gain side) and the other 68K is all alone but it's still in series with the signal, but the signal is taken from the middle of the to 68K reistors.  
So, that same preamp tube's grid has a 68K grounded grid load (high gain was 1M) and another 68K series resistor, (high gain was only 34K.)  
The truth is there isn't a "HUGE" difference in the gain between the 1M and 68K grid load but there is a huge difference in available signal when the two 68K resistors form a classic voltage divider.  
Why?, because half the signal from the instrument/Mic plug goes to right ground before it leaves the jack and the other half goes to the preamp tube's grid.  
That's why it sounds almost half as loud... there is now half as much signal voltage as before and on top of that, twice as much series resistance (68K) which does very slightly roll some of the high freqs off.  
This is all drawn out in the extra "INPUT JACKS" artwork documentaion in the MA 5E3 kit.  
You should now be able to follow the signal through those by looking at the artwork and see how it works when one set of points is closed and the other open.  
4/7/2005 10:04 AM

Thanks for the explanation. Makes much more sense to me. So I'm thinking either the 1M resistor on the jack is shorted, or the 68k resistor on the board that goes to that jack is shorted, yes?
4/7/2005 12:19 PM
Dave H
Re: 5e3 input weirdness
Try this to test the wiring. Get a guitar cord and connect your meter across one of the cord jacks. Plug the other jack into each of the ampís inputs in turn. Both high sensitivity inputs should read 1M and both low sensitivity inputs should read 136k (the two 68k resistors in series).  
4/8/2005 4:08 AM
Re: 5e3 input weirdness -Found it
Not the tube, or the 68k, but the 1M on the input jack. I probably overheated it when I was removing it and resoldering it. Thanks for all the input. A few weeks ago I had never played with electronics besides wiring my guitars, and now I'm learning to troubleshoot my homebuilt amp, use my multimeter for more than continuity checks...thanks to Bruce and the rest of you.  
And Bruce, the 'bucker it on a 3PDT?

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