Marshall JTM30 - help replacing PCB-mount speaker jacks - Switchcraft straight to OT

[Tone Meister]

Have in a buddy's JTM30. The PT is shorted and on order.

In the meantime, both main speaker and extension jacks are broken and need replacing. He asked if I could just wire in some Switchcraft jacks straight to the OT and eliminate that portion of the output PCB board. Makes sense to me since the original Jalco jacks have proven impossible to source, at least to this point.

What would you pros do? I'm inclined to wire in the Switchcraft jacks, but I'll need some help sorting out the configuration. Those jacks will also need to be insulated from the chassis, correct?

Schematic attached.

[nickb]

I don't think there is enough room for a Switchcraft open frame style.

You can get the jacks you want here Guitar Amp Jacks look for Part #J-560 and Part #J-530

[Tone Meister]

If need be, I'd make room by trimming off that end of the output PCB. We're thinking the Switchcraft jacks will mount to the chassis and would be more robust than the originals.

That said, the J-560 looks to be correct, but the J-530 doesn't. The original has 5-pins.

[nickb]

I'm pretty sure the extra pin can just be cut off - it will be an unused switch contact. You can take a stereo full switched version of one of these and cut off pins to make it fit any other one.

[Tone Meister]

Ok, got it. Still, we'd like to panel mount some Switchcraft jacks. He gigs with this amp regularly and uses the outputs to feed a 4x12 cab, so those jacks see a great deal of wear and tear.

[Enzo]

As a pro, I would install the proper jacks. Customers decide that jacks break because they are plastic, but they usually break because they are abused - even if by accident. But while it looks appealing to use metal bushing jacks, the plastic jack insulates the ground pin of the jack from the chassis at that point. And that MAY be important to the grounding scheme of the circuit. Note in teh schematic the star ground system.

As Tonemeister said, there may be spaces for five pins, but the jack uses three of them, the other two won;t be missed.

[nickb]

I have had someone once insist that I replace plastic with metal, but not to a Marshall. I used a dremel tool to cut away a half inch deep area on the PCB next to the panel and used a S1208 & S1209 washer & bush to insulate the socket from the chassis. Not having your PCB in front of me I can't tell if that is possible.

[Pedro Vecino]

It is a relatively complex circuit that allows:
- disconnect cathodes from the power tubes to protect the amplifier when no speaker is connected.
- assign 8 ohms secondary to 8 ohms jack output (exclusive use).
- assign 16 ohms secondary to 16 ohms jack output (exclusive use)
- assign both jacks (for 16 Ohm speaker) to 8 Ohm secondary.

Probably the easiest thing to do is order spare parts, but those original jacks are rather fragile for speaker connections.
Disconnection of cathodes is effective in some way, but not if nothing is connected to the other side of the speaker cable.
The other thing that would be lost using simple Cliff jacks connected to each secondary, isolated to respect the ground connection assigned to the central point is the assignment of both jack outputs (for 16 ohms) to the 8 ohm secondary. Classic Marshall Cliff jacks also would require enlarging slightly the hole.

[Tone Meister]

Points taken NickB, Enzo, and Pedro. Those jacks are fragile, especially when the player is using a heavy speaker cable out to another cabinet. I'll order some spares to keep on hand.