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javier pintos · 07-02-2009, 11:16 AM

hi bob, i do not have a 900 amp or 1960 cabs, but by reading the manuals i would do the following

when using both cabs, switch them to mono, and connect the speaker cable (not an instrument cable) to the 16 ohm socket of the cabs, then connect both cables to the sockets of the amp, and set the amp to 8 ohms since it is wired in parallel (2 - 16 ohm cabs in parallel behave like a single 8 ohm load), i have a 4x12 wired as 2 2x12 at 16 ohm to a DSLhead, and is pretty much the same thing here with yours

always set the amp impedance when it is NOT powered, and never turn on the amp whith the incorrect impedance selected or no cabs connected to the amp

when using a single cab use the same 16 ohm socket of the cab and set the amp to 16 ohm.

The stereo config of your cabs make it work internally as 2 arrays of 2 speakers of 8 ohm each array, but if you connect both sockets like that to the amp, it will behave like a 4 ohm load since the amp outputs are wired in parallel, so you´ll end up frying your Output Transformer

If i´m wrong please someone correct me

from the manuals on the marshall site

http://www.marshallamps.com/download...4100%20Hbk.pdf

6. Impedance Selector Switch
Two-way switch for matching the amplifier to
speaker impedance. Amp heads feature 8 or 16
Ohm operation. Combo versions feature 4 or 8
Ohm options.
7. Loudspeaker Output Jack Sockets
Parallel wired jacks for linking speaker
cabinets. Ensure that the speaker system is
easily capable of handling the full amplifier
power and that the impedance (item 6) is

correctly selected. If in doubt consult your dealer.

http://www.marshallamps.com/download...Cabs%20Hbk.pdf

1) Mono/Stereo Selection Switch.
Selects either Stereo or Mono operation mode.
2) Right* input
When switch (1) is set to:
i) Stereo, this provides connection to the right* side pair of speakers.
Rated at 8 Ohms.
ii) Mono, this provides connection to all four speakers. Rated at 4 Ohms.
3) Left* Input
When switch (1) is set to:
i) Stereo, this provides connection to the left* side pair of speakers. Rated at 8 Ohms.

ii) Mono, this provides connection to all four speakers. Rated at 16 Ohms.

07-02-2009, 11:16 AM	#2 (permalink)
javier pintos Senior Member Join Date: Sep 2008 Location: Mexico Posts: 323	Re: jcm900 hook up to speakers hi bob, i do not have a 900 amp or 1960 cabs, but by reading the manuals i would do the following when using both cabs, switch them to mono, and connect the speaker cable (not an instrument cable) to the 16 ohm socket of the cabs, then connect both cables to the sockets of the amp, and set the amp to 8 ohms since it is wired in parallel (2 - 16 ohm cabs in parallel behave like a single 8 ohm load), i have a 4x12 wired as 2 2x12 at 16 ohm to a DSLhead, and is pretty much the same thing here with yours always set the amp impedance when it is NOT powered, and never turn on the amp whith the incorrect impedance selected or no cabs connected to the amp when using a single cab use the same 16 ohm socket of the cab and set the amp to 16 ohm. The stereo config of your cabs make it work internally as 2 arrays of 2 speakers of 8 ohm each array, but if you connect both sockets like that to the amp, it will behave like a 4 ohm load since the amp outputs are wired in parallel, so you´ll end up frying your Output Transformer If i´m wrong please someone correct me from the manuals on the marshall site http://www.marshallamps.com/download...4100%20Hbk.pdf 6. Impedance Selector Switch Two-way switch for matching the amplifier to speaker impedance. Amp heads feature 8 or 16 Ohm operation. Combo versions feature 4 or 8 Ohm options. 7. Loudspeaker Output Jack Sockets Parallel wired jacks for linking speaker cabinets. Ensure that the speaker system is easily capable of handling the full amplifier power and that the impedance (item 6) is correctly selected. If in doubt consult your dealer. http://www.marshallamps.com/download...Cabs%20Hbk.pdf *1) Mono/Stereo Selection Switch.* Selects either Stereo or Mono operation mode. *2) Right input** When switch (1) is set to: i) Stereo, this provides connection to the right side pair of speakers.* Rated at 8 Ohms. ii) Mono, this provides connection to all four speakers. Rated at 4 Ohms. *3) Left Input** When switch (1) is set to: i) Stereo, this provides connection to the left side pair of speakers. Rated at 8 Ohms.* ii) Mono, this provides connection to all four speakers. Rated at 16 Ohms.