Weird treble pot JCM800 build

Marshall Amp Discussion

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sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

Pin 3 goes to the Bass pot and .022uf cap. Pin 1 gets fed from the 500pf high pass cap.
matt h
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Re: Weird treble pot JCM800 build

Post by matt h »

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sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

I see what you are saying, but I believe it is a horse-a-piece as to which lug I hook the Bass' .022uf to...it is either tied in to the Treble 3 or the Bass 2 and then a wire need to be run between the two regardless. Correct? Incorrect? I just hooked into the closest lug to the cap making a shorter wire run.
Anyway, I am completely stumped. This is my 6th amp build and I have somehow been able to fix a few of my musician friends amps as well. This is seriously driving me to the looney bin :shock:

I have even built a little makeshift tone circuit tree and tied directly into the Cathode. Same results! Treble does not do what it is supposed to, bass seems to be lacking as well. It sorta has a good tone but is NOT a JCM800. WTH.
sonofmickel
Posts: 118
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

OK, my brain literally exploded. Quite a mess.
I hard wired the caps to the pots and ran a single lead back to V2 pin 8 Cathode. I thought that by eliminating any possible unseen wiring from the original amp that it HAS to work, NOPE! Are STK .002 Black caps polarized? They have a blue side and a black side...

What can be the explanation for this type of behavior? I even swapped the 500pf again.

I don't think a reverse pot or even log for linear would do this.

No DC on the PI coupling cap.
thejaf
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Re: Weird treble pot JCM800 build

Post by thejaf »

Bypass the tone stack completely to see if you get "full throttle" sound. Temporarily wire a 0.02uf or larger 600V cap and wire it from the cathode of the stack driving follower to the entrance of master volume. If that works, I'd gut and rebuild the tone stack.
sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

Bypass the tonestack brings on blocking distortion(sounds like a big muff cranked up)?

I built a typical cathode follower tonestack on a brass plate off of the amp and spliced it in, no change. I am going to try a few different voltage divider before the the signal hits the tone stack and possibly the volume pot.

This is very confusing to say the least. I seem to be learning a lot though. Thanks for all the help everyone. I just want this amp to rock.
John_P_WI
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Re: Weird treble pot JCM800 build

Post by John_P_WI »

If you have rewired the tone circuit a number of times and have experienced the same results, I would look elsewhere. What are the odds of repeating the same error over and over again? Check the cathode follower circuit driving the tone circuit and the following PI input, something is amiss here.

Not flaming, the lack of house keeping shown on the pic above is a bit of concern. Even experienced guys would have a difficult time sorting this out...
sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

Doing the same thing over and over again is the tell tale sign of insanity. In troubleshooting electronics it seems to be worse.

As far as housekeeping goes...I did not build the amp, have you ever looked inside a BadCat or Gibson amp? But anyway I am just trying to get the tone controls to work correctly.
Smokebreak
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Re: Weird treble pot JCM800 build

Post by Smokebreak »

At this point I would wire it up like a simple 2 stage + CF plexi. That circuit is very forgiving as far as layout goes. If it's still problematic, then you have made a simple wiring error, or a pot is bad, etc. If it gets better then you either either have a lead dress issue, or again, a simple wiring mistake, but at least it's isolated to the first stage.
I put the bass cap directly on the treble pot, too. No need to drag it over to the bass pot, for me.
I hate to ask, but have you switched plate or grid leads to the outputs?

Edit: The Bad Cats that I've seen look very nice. I think John is referring to the fact that based on the pic, and the lack of care shown there, that there's no telling what's going on in places less forgiving than the tone stack.
sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

OK so I plexied the amp. I removed the 2nd gain stage from the JCM800 and removed the 470k voltage divider from the grid. Basically a plexi now. The tone controls still do not work correctly. The treble pot adds gain when turned CCW and cleans up and loses volume when turned CW. Bass pot does almost nothing. Mid pot sort of works.
This has got to be simple problem that someone here has come across?
Stevem
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Re: Weird treble pot JCM800 build

Post by Stevem »

replace the pots with resistor divider networks one at a time starting with the treb pot and see if it fly's!
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sonofmickel
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

OK! Here it is!
Larry has solved the problem, kind of not fair because it was very apparent to him after I revealed that the volume pot after the tone stack was a 25k pot! He recommended that I use the Duncan tone stack simulator and replace the 512k with a 25k and watch what happens to the signal as the treble gets turned up. The simulator clearly shows what was happening to the signal, turned to 0 the whole signal gets boosted by 8-9db. When turning up the treble the mids bass lose db's while only the highest treble was being boosted. Whew! Tone control works as expected now.

The gain channel still sounds like doo doo though(sounds flubby and buzzy).
I should maybe not be too critical of Phaez though, it looks like this was another of his "project" amps that didn't make it to full production. The parts seem decent enough , carbon film resistors, ic 'lytic caps, Mallory and STK caps, 1 vishay resistor etc...
matt h
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Re: Weird treble pot JCM800 build

Post by matt h »

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Last edited by matt h on Fri Mar 27, 2015 5:08 am, edited 1 time in total.
sonofmickel
Posts: 118
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Re: Weird treble pot JCM800 build

Post by sonofmickel »

Thanks for the encouragement Matt.
I actually did build a JCM circuit, I just figured that the preamp was buggered and left the volume control as it was. I should have checked that but I had no idea that the actual volume control had so much to do with the circuit.

So this should be basic knowledge from now on:
If the treble pot acts like a reverse volume control, check the next value pot AFTER the tone stack. If I EVER see this question asked again, I will promptly offer that the Volume pot value should be checked.

By the way, the amp still has an awesome "field effect" from the PT. That is next in line.

Thanks for all the help people. I have the amp now sounding like a buzzy JCM800.
matt h
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Location: New England

Re: Weird treble pot JCM800 build

Post by matt h »

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