JCM900/4100 Control Grid Resistors

[bobloblaws]

Hello,
I'm going over a Marshall JCM900 head, 4100 model with PCB marked JMP50B. Curiously, the 2K2 resistors are missing from V4 and V5 control grid pins but are in place on V6 and V7. It appears to be factory solder still on all the control grid pins, although I could be wrong about that. Does anybody have any insight into this? As far as I can tell any difference in bias voltage is negligible, should I just leave well enough alone or add the missing resistors? If someone did this deliberately I can't imagine why.

- Bobby

[Enzo]

It shouldn't affect the bias. It wouldn't hurt it to make it like the schematic, but the amp has been playing without them for 30 years so far, and as far as we know it worked well without them.

Your schematic is revision C, while your amp might be an earlier revision level.

[bobloblaws]

It shouldn't affect the bias. It wouldn't hurt it to make it like the schematic, but the amp has been playing without them for 30 years so far, and as far as we know it worked well without them.

Your schematic is revision C, while your amp might be an earlier revision level.

Fair enough, thanks for the reply, Enzo.

[drewl]

Some of the early 60's models were like that
I've always wondered why.

[Helmholtz]

Even without the 2.2ks, this amp still has 1.5k grid stoppers.

Some of the early 60's models were like that
I've always wondered why.

I think because they didn't need to use 4 grid stoppers.

The primary purpose of grid stoppers is to avoid parasitic HF oscillation.

The earliest 50W Marshalls (model 1987) did not have grid stoppers (though their predecessors, the JTM45/50s, used them).
Early 100W, 4xEL34 models only used them on 2 tubes.

The tendency to oscillate is influenced by the amount of NFB, layout, lead dress and OT properties.
It increases when directly paralleling tubes (see e.g. Termans book).
So it seems Marshall considered grid stoppers only necessary for the additional paralleled tubes in their 100W models.

[bobloblaws]

Even without the 2.2ks, this amp still has 1.5k grid stoppers.



I think because they didn't need to use 4 grid stoppers.

The primary purpose of grid stoppers is to avoid parasitic HF oscillation.

The earliest 50W Marshalls (model 1987) did not have grid stoppers (though their predecessors, the JTM45/50s, used them).
Early 100W, 4xEL34 models only used them on 2 tubes.

The tendency to oscillate is influenced by the amount of NFB, layout, lead dress and OT properties.
It increases when directly paralleling tubes (see e.g. Termans book).
So it seems Marshall considered grid stoppers only necessary for the additional paralleled tubes in their 100W models.

When you mention paralleled tubes I'm assuming you mean adding an additional tube to a given side of the OT. But in my case the two power tubes on one side of the OT have the additional resistors while the two tubes on the opposite side do not.

[Helmholtz]

But in my case the two power tubes on one side of the OT have the additional resistors while the two tubes on the opposite side do not.

That's strange indeed. Might actually be an assembly error.

OTOH, the remaining 1.5k grid stoppers might suffice.

Anyway you can't do wrong adding the missing resistors.

[g1]

So all four of the 1K5's are in place?

[bobloblaws]

So all four of the 1K5's are in place?

Yes, they are

[g1]

There was a version where there was a set of 1K5's shared, then an extra set of 1K5's in series for only one pair of tubes, but not just on one half of the OT like you mention.
Other brands as well sometimes do different stoppers among sets both for control and screen grids, but I've only ever seen same done for 'push' side as for 'pull'.

[drewl]

That's strange indeed. Might actually be an assembly error.

OTOH, the remaining 1.5k grid stoppers might suffice.

Anyway you can't do wrong adding the missing resistors.

I agree, sounds like a factory error if only one side is that way.