DIY JCM 800 2204 Bass Control Wiring

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DIY JCM 800 2204 Bass Control Wiring

Postby will_quinn on Thu Feb 12, 2015 9:41 pm

Hello, it's been years since I've been on this forum. I have finally made a DIY JCM 800 50w. It is fully working and very quiet (in terms of hum..there is none). I do have a question about wiring up the bass control though. I used Mark Huss' schematic to build my amp. In his schematic he shows the Bass pot wiper (pin 2) is connected to pin 1 (as is 22uF cap and pin 3 from treble pot) and pin 3 connects to pin 1 of the middle pot. Schematic can be found here: http://mhuss.com/MyJCM/JCM800_2204.pdf

Yet in his picture of a JCm 800 control panel the bass pot obviously has nothing connected to pin 1. see here: http://mhuss.com/php/pix.php?pic=Controls.

I thought that since all other layouts show pin 1 disconnected and that pictures from an actual jcm 800 show the same then perhaps his schematic is wrong. I decided to go by the images and only hooked up pin 2 and 3. pin 1 I left disconnected. I found whilst it had a nice tone it was very bright and that the bass and middle controls had no effect (or very little) on shaping the sound.

Second guessing myself I then double checked the schematic and decided I should short pin 1 and 2 on the Bass control together as per the schematic (and all other Marshall schematics for that matter). This did mean the bass and mid controls were much more effective but the sound maybe isn't just as nice, there is more bass but there seems like there is something missing and I can't quite put my finger on it.

Can anyone shed some light as to whether pin 1 on the Bass control pot should be utilised?

I've a video on youtube which hopefully can demonstrate how bright this amp sounds. Notice how when the ppimv is engaged and the master and gain are dimed (at the end of the video) how the tone rounds off nicely. http://youtu.be/0bbEKoHWCqQ
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Re: DIY JCM 800 2204 Bass Control Wiring

Postby gusfinley on Fri Feb 13, 2015 6:59 am

The bass control is fine either way.

A potentiometer can be wired as a potentiometer or a rheostat. Your treble control is wired as a potentiometer - the full signal from the treble cap enters "pin 1" and a proportion of it is sent out the wiper/tap. This is accomplished by the wiper dividing the total potentiometer resistance into two seperate resistances.

The bass control is wired as a rheostat or variable resistor. You can buy rheostats, but its easier to make one with a potentiometer. You simply only hook up your circuit to only one "resistances" of the potentiometer. You just have to choose the right one since one gets more resistance as you turn it up and the other gets less resistance. The schematic shows the other way to hook up a rheostat: short the wiper to the unused portion of the potentiometer. When you do this, you put a zero ohm resistor (wire) in parallel with the unused portion of the potentiometer essentially removing it from the circuit. Or you can just not hook up the unused side - it doesn't matter since it is really doing the same thing.

Here's a interesting an indepth article all about potentiometers:

http://www.geofex.com/article_folders/potsecrets/potscret.htm

By the way the "bright" sounds you are hearing at lower volumes have to do with the way the pots are wired, too!

Amps can tend to sound dull at low volumes so sometimes "bright caps" are added to the gain/volume potentiometers to bleed some high frequencies past the potentiometer and gives some life to the signal. The higher the capacitance the more the signal bleed. When the volume/gain pot is maxed the two terminals of the "bright cap" are now shorted together and it is bypassed allowing the pure sound to come through.

If you are finding the amp to be too bright you could remove the bright caps or lower their values until you hear what you like (or just keep it dimed!).
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Re: DIY JCM 800 2204 Bass Control Wiring

Postby will_quinn on Fri Feb 13, 2015 1:32 pm

Thanks Gus, you've been a great help!
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