Marshall JCM 900 4100

[goldtop5]

I have a Marshall JCM 900 (model 4100) that's arcing on one of the tube sockets. Could one of the socket pins be shorting against something else? Or could it be a bad component(s)?

[Mick Bailey]

The socket will need replacing for a start. You may never find the reason why it arced in the first place - it can be from surface contamination, or initially a loose connection with a tube pin. Make sure to check around for any other damage as well.

[soundguruman]

I have a Marshall JCM 900 (model 4100) that's arcing on one of the tube sockets. Could one of the socket pins be shorting against something else? Or could it be a bad component(s)?

There is no choice but to replace the socket.

The reason it usually arcs is:
1. Wrong speaker impedance- sets the amp on 16 ohms, and runs it into 8 or 4 ohm load. Speakers may be wired wrong. Verify this, very important.
2. Bad speaker, blown speaker, voice coil scraping against magnet.
3. Bad speaker cable, or using shielded instrument cable instead of a real speaker cable. Most often, defective plugs on speaker cable. Replace plugs w/ Switchcraft.
AVOID ALL Chinese speaker cables- defective plugs on cable. YOU have been warned. BUY USA cables with switchcraft plugs.
4. Junk speaker attenuator, produces wrong load for amplifier.

Almost always - load related, improper speaker load or speaker connection causes it to arc.