Metropoulos Forum

Hi every body
I just bought a 50w lead board kit for my 1975 50W JMP MKII
the cd suplied with the kit got lots of usefull info.
But there is one thing that missing is the tube socket pins layout.
There is Marshall pics showing resistors on tube socket pins
and there is jumper on some pins too.
Where can i find that info.
I would like to know what are the value of those component and on wich pins theys are.
thanks alot !

Here's a couple images that should help. There are layout diagrams on the Metro CD that show these also.

All of the preamp sockets have pins 4 and 5 tied together (red heater wire).

The second preamp socket (V2) has a 100k resistor between pins 6 and 1 and the pin 1 side lead ties to pin 7.

V3 has pin 3 and 8 tied together.

The power tubes have a 5 watt 1k resistor between pins 4 and 6 (large, sometimes rectangular).

Then they have a 1.5k between pins 5 of each pair. You can also put a 5.6k on pin 5 of each socket instead of these.

V1 can also have pin 3 and 8 tied together if you want shared cathode (remove yellow wire going to pin 8 ).

Thanks alot for the fast answer
Its goin help me alot

My amps originaly doesnt have those 1.5K resistors
I even seen some design with a 5.7K on pin 5 of v4 and v5 to the 220K on the board and sometime a resistor on one of the pin to the ground.
I tried to find every layout/scematics i could find on the net.
They all different ! lol

toner wrote: Then they have a 1.5k between pins 5 of each pair. You can also put a 5.6k on pin 5 of each socket instead of these.

What's the difference or advantage/disadvantage of 1.5K vs 5.6K?

Some of the early amps have no resistor there, right?

GUITARmole wrote:

toner wrote: Then they have a 1.5k between pins 5 of each pair. You can also put a 5.6k on pin 5 of each socket instead of these.

What's the difference or advantage/disadvantage of 1.5K vs 5.6K?

Some of the early amps have no resistor there, right?

Some Marshalls don't have those resistors (aka "grid stoppers" or "swamp resistors"). A lot of 50w amps don't use them.

Their main purpose is to reduce parasitic oscillation, which is kind of like a very high frequency feedback above the audible range. Oscillation can cause red-plating in the output tubes and possible noise issues. There may be other purposes too. I'm not sure.

The main disadvantage of the 1.5k pairs in a 100w is that the pair of tubes that have the signal from the output coupling caps wired to pin 5 don't "see" the resistor. In the diagram above, only the outside pair are affected by the resistors.

Larger value resistors will roll-off lower frequencies. I'm not sure when this becomes audible but some people use up to 10k or higher there.