So its time to get my JCM900 2500 SLX back up and running

Rebuilding a JCM900 2500 SLX: stock or mod?

  • Keep it stock, Marshall got it right

    Votes: 5 55.6%

  • Mod a few things to correct some factory mistakes

    Votes: 4 44.4%

  • Gonzo this thing and change it entirely

    Votes: 0 0.0%

  • What the hell do I care?

    Votes: 0 0.0%
  • Total voters
    9
Amp Mad Scientist said:
The screen grid resistors in any tube power amp are: current limiting resistors.
They are supposed to blow if the tube is shorted, protecting the transformers.
But put in a larger wattage resistor --- and the protection is now bypassed.

The people who designed these amps were not idiots.
Click to expand...
I've seen this type explanation several times on the internet. For someone trying to learn from the explanation, it is incorrectly informing them.
Screen grid current is drawn from the cathode. The screen grid (series) resistor is external to the tube, so cannot directly limit current drawn by the screen grid. What the screen resistor does do is to drop voltage due to ohms law. As screen current increases, so does the voltage drop across the screen resistor. This means lower voltage on the screen grid & thus a reduction in current drawn.
Cheers
 
ivan H said:
I've seen this type explanation several times on the internet. For someone trying to learn from the explanation, it is incorrectly informing them.
Screen grid current is drawn from the cathode. The screen grid (series) resistor is external to the tube, so cannot directly limit current drawn by the screen grid. What the screen resistor does do is to drop voltage due to ohms law. As screen current increases, so does the voltage drop across the screen resistor. This means lower voltage on the screen grid & thus a reduction in current drawn.
Cheers
Click to expand...
A resistor cannot drop voltage, without a current load.
 
Mitch Pearrow SJMP said:
@Clockworkmike
What era would this be in a residential home ??View attachment 71033
Click to expand...
Wow! Judging by the cloth type insulation and the style of nail on box, it looks at least mid to late 30s, up to around the mid 50s. Thats NM Type wiring but it definitely has to be earlier than the 1960s because at that time they had begun to use plastic and nylon jacketing. Id say by that plaster board wall? Looks really similar to that hospital section i just did and that was about mid 30s
 
Amp Mad Scientist said:
A resistor cannot drop voltage, without a current load.
Click to expand...
Of course a resistor can't drop voltage until there is current flowing through it. In this case it is the screen grid current that causes the voltage drop across the resistor. More screen grid current = greater voltage drop across the resistor = lower screen grid positive potential. I do have a slight grip on ohms law. Cheers
 
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Cadorman said:
I've never seen that kind of plaster backing before. Was this a transition between lathe and drywall?
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Yeah its kinda weird: it itself, is sorta like a drywall/gypsum board ( yet predates drywall, go figure) and it has a pattern of holes that let the plaster pour thru and bond better i suppose. Theres usually a metal mesh screening directly over top of it that allows the plaster to be applied and stick.
 
Clockworkmike said:
Wow! Judging by the cloth type insulation and the style of nail on box, it looks at least mid to late 30s, up to around the mid 50s. Thats NM Type wiring but it definitely has to be earlier than the 1960s because at that time they had begun to use plastic and nylon jacketing. Id say by that plaster board wall? Looks really similar to that hospital section i just did and that was about mid 30s
Click to expand...
Thanks buddy, I later found out that it was built in 49.
We are stripping the exterior for new siding..319B604E-4A2E-4FD2-A979-8B3751ABDD9D.jpeg
 
Clockworkmike said:
By the suggestions, I think it's settled then and decided:
Click to expand...

Let me help you out.

1. You send the amp to me.

2. I’ll use all the suggestions here on the forum and replace all the bad components for you. (You just have to pay for the components! Yay!)

3. I’ll keep the amp for thorough testing and evaluation for about five to ten years.

Deal?
 
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