Gut Pics of my MKII '77 Master 100 watt Lead & Questions

[Jerry2013]

I took apart my '77 JMP Master MKII , and took pics.

I see that there are only two Dale Screen Grid resistors on two of the Filter Capacitors (recently changed to new JJ's).

Should't there be Screen Grid resistors on all 5 of the Filter Caps?

Also, I am going to wire the 8 ohm jack to the unwired switchcraft metal together in Parallel so I can plug in a full stack. Is there any high current in these? I know most probably not, but best to make certain.


Finally, would you tech Gurus take a look and see if you can tell me something about the amp? I know its had the "Bright Cap" removed I think. Any other comments?

I'm running Winged C's and old stock preamp tubes.


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[ampgeek]

Jerry,

Those two filter cap resistors are most likely the B+ (or HT=high tension if using UK vernacular) dropping string resistors. So seeing two on the cap cans sounds correct.

There is usually a power tube screen grid resistor for each power tube if this amp follows the typical (if there is such a thing!) Marshall plan. Those are most likely the black resistors that I see mounted at each tube socket.

Can you tell us which pins those are connected at for confirmation?

Cheers,
Dave O.

[ampgeek]

Forgot to mention above that it looks like there is a crack in the yellow cap on the far right hand side of the board in one of the pictures.

Is that the case?

Dave O.

[Colossal]

The two resistors on the cap cans are bleeder resistors and are typically 56k on Marshalls. They add safety in that they provide a path to ground to bleed down the capacitors when the amp is powered down.

[Jerry2013]

BTW- I found out its a 1979, not a 1977 year amp..

Jerry,



There is usually a power tube screen grid resistor for each power tube if this amp follows the typical (if there is such a thing!) Marshall plan. Those are most likely the black resistors that I see mounted at each tube socket.

Can you tell us which pins those are connected at for confirmation?

Cheers,
Dave O.

Between pins 3 and 5.

The two resistors on the cap cans are bleeder resistors and are typically 56k on Marshalls. They add safety in that they provide a path to ground to bleed down the capacitors when the amp is powered down.

Why are they only on two of the filter caps and not all four?

Forgot to mention above that it looks like there is a crack in the yellow cap on the far right hand side of the board in one of the pictures.

Is that the case?

Dave O.

I'm not certain if its a crack or a overlap in the seams?

[IMG:800:600]http://i111.photobucket.com/albums/n134 ... dc902a.jpg[/img]


BTW- Is there any high voltage across the speaker jack terminals if I wrere to touch tip to tip or sleeve to sleeve?

[Colossal]

The two resistors on the cap cans are bleeder resistors and are typically 56k on Marshalls. They add safety in that they provide a path to ground to bleed down the capacitors when the amp is powered down.

Why are they only on two of the filter caps and not all four?

Since the B+ supply is series connected via dropping resistors, the bleeder resistor serves as a path to ground for the entire string. In a 100W Marshall's case, the PT's center tap connects to the junction of the first pair of caps since it is a voltage doubler arrangement. So they placed the bleeder resistors on the second pair of caps supplying the screens where there is significantly less current demand, lowering their power handling requirement. So probably a cost cutting measure as well as one of practicality. The caps are stacked to double their voltage handling.