10K NFR in my poopy sounding JCM800?

  • Thread starter Thread starter Kapo_Polenton
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Kapo_Polenton

Kapo_Polenton

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This is a 1982 jcm800/2204 combo. I have always felt it was a bit "darker" sounding but for the past 8 years or so I had found a few simple tweaks that I like that made it sound pretty good. I bypassed the what I thought was a 100k negative feedback resistor with a 47k to give me a bit more vintage vibe (bringing it down to what I thought was in the 50 K region) and had the V2a bypassed with a .68 uf like I tend to like in this circuit. It was still smoother sounding but it took boosts amazingly that could be dialed for more bite. Well a week ago I thought, let me put it back to stock as I already run this spec elsewhere and I don't need more of the same. Well, the amp has lost any mojo it had and it sounds like a flubby mess. So I pop it out of the headshell and put it back on the bench to check it out. I start measuring all the resistor values to make sure that with drift, they are more or less where they should be. I know this isn't ideal as you should probably not measure them in circuit but they all check out....except the negative feedback. It has the markings of a 100k resistor but the thing measure 9.98k??! What's with that? How can it be marked 100k and not be that value or anything close to it? that's like an +\- 80 drift lol. Anyone ever experience this? Because that would definitely explain why it flubs out with the preamp up and especially when that master gets up past 6...it sounds like ass. This might also be why the original guy sold it to me for such a good price lol.
 
Eh...Black brown yellow ...Black Brown Orange...
Easy mistake to make if you're just a middle aged house wife stuffing boards down in Milton Keynes for some spare change in 1982....
Wouldn't be the first time some fukked up values got stuck on a Marshall board
Time and / or heat could have caused the color bands to go off as well...
It was probably 10k from the factory.
 
I didn't think you could measure that one in circuit. Some you can and some you can't. One end being connected to the OT secondary means it is essentially connected to ground (thru the OT because the other end of the OT is grounded). The other end of that NFB resistor is grounded thru the PI ground.

It is typically the resistors that are "isolated" by capacitors and tube elements/etc that are measurable in-circuit.

EDIT: And 47k//100k is 32k. If that is off the 16ohm tap (more signal voltage there) then that is a ton of NFB. But if off of the 4ohm tap that's not really a ton but still on the high side
 
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I guess there might be an easy way to test the theory ..if I bypass it again with 47k and it sounds better, I can probably bypass it with 100k next and get it pretty close to the intended " stock" version. Ideally I don't need to lift the board out of there, PCB are a pain to work on if you can't get them totally out.
 
Something else I noticed, my transformer is a Drake 784 139/2 so the leads in theory should be:

16 gray - 8 green- 4 yellow - Mine is a combo so the switch will look backwards when right side up looking into chassis. Looks like the NFR is on the green wire which is the 8 tap. Shouldn't be that huge of a diff as I would have thought this should be tighter than 4 but I am going to try that first and see if it changes. I suspect that SpiderWars is right, the markings on that resistor are def 100k so that is probably one place where I can't measure it in circuit. Or the part is just completely off and mislabeled. I will start with moving the NFR. Also, this was originally setup with 6550 so that 8 ohm tap would make sense out of the factory from what I can see online.
 
And not to ask the obvious, but have the filter caps been replaced? That will cause the amp to sound like you have described as well... A fresh set of F&T's may be in order...
 
mixn4him said:
And not to ask the obvious, but have the filter caps been replaced? That will cause the amp to sound like you have described as well... A fresh set of F&T's may be in order...
Click to expand...

Good point, I had forgotten about that but I did just check and they were all replaced I'd say 5-6 yeas ago as I can see the evidence of my work. I also have not used the amp much because I was playing the 2204 I built more as well as my Randall RM100/ Lynchbox which sounds fantastic.

I hope this isn't the beginning of a transformer going south.. it is an 82 after all.
 
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