Help with JCM800/Plexi volume

Marshall Amp Discussion

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andresound
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Re: Help with JCM800/Plexi volume

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Re: Help with JCM800/Plexi volume

Post by pompeiisneaks »

Holy cow stay safe!
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romberg
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Re: Help with JCM800/Plexi volume

Post by romberg »

martin manning wrote: Tue Oct 10, 2017 11:20 am Sorry to hear that, good luck! Is it possible that there is a strand of shield wire grounding out the signal on one side?
Oh! That could be it! It would explain the reduced volume and funky sound. It is pretty easy to melt through the inner insulation on shielded cable and cause the inner conductor to short to the outer shield. It should be straight forward to test for a short on the conductors to the shield with a DMM.
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Re: Help with JCM800/Plexi volume

Post by andresound »

Thanks guys. Workshop is clear now, so I will replace the cable, and this time with color coded 2 core.
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martin manning
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Re: Help with JCM800/Plexi volume

Post by martin manning »

You can use the shield to carry the DC bias voltage too.
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Re: Help with JCM800/Plexi volume

Post by sluckey »

I switched to RG-316 a few years ago. The center conductor uses Teflon insulation and even the outer jacket is some kind of high temp plastic. No more melted wires.
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Re: Help with JCM800/Plexi volume

Post by andresound »

martin manning wrote: Fri Oct 13, 2017 10:55 am You can use the shield to carry the DC bias voltage too.
That’s what I did with this PPIMV wiring. But I think you guys are on to something here. I installed a PPIMV in my Rocket build and used MYLAR 2-core screened cable with good results. Will use the same. In my business, we use the Mylar cable by the kilometer. I thought the cable I used this time (balanced mic cable I had lying around), looked better 🙄. So much for good looks 😏
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M Fowler
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Re: Help with JCM800/Plexi volume

Post by M Fowler »

Hope all is well despite the bad weather.

Mark
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

Mark, all is well and calm now. Thank you. The damage to my workshop was minor, so thank god for that.
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Re: Help with JCM800/Plexi volume

Post by andresound »

OK rewire with new cable is complete. Carried out at the office this morning. Will tube and check when home. I get 228k at pin 5 and -30v (no tubes installed). Holding thumbs!

Regarding the low B+ of around 360V. I have seen this is expected of old 50w Marshalls. However, the PT primary has multi-voltage taps. I am currently connected to (i think), the 230/240v primary. What if I change that to the 220v tap? Would I not get a higher B+? Just a thought 🤔
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martin manning
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Re: Help with JCM800/Plexi volume

Post by martin manning »

Looks good, hope it works. Did you check for a short to ground before you pulled the previous wiring out?

Yes using the lower voltage primary taps will increase the secondary voltages. The one down side is that your filament voltage will increase. That will probably make the decision for you.
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Re: Help with JCM800/Plexi volume

Post by andresound »

Amp works as it should! The “balls” are back.👍. Thank you everyone for your help.

Unfortunately, with all the commotion and shit going on.. I didn’t check for a short to ground 😒. I am positive it was the cable. I have just checked one section of the cable (the input side) I pulled from the waste bin and it measures 380k to ground 🤔. Think there might be something there?

I will let my buddy take the amp and play it for a while before we “tweak”. I personally dont like the overdriven sound. But hey, Ive never really been a Marshall kind of guy. I have also re installed the 100n coupling caps.
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