Fender manual TGA-3100 Watt Guitar Amplifier User’s Guide, Speaker Combining

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TGA-3 100 Watt Guitar Amplifier User’s Guide

Speaker Combining

A word about speaker combining: as mentioned earlier, there are 3 speaker outputs on the rear panel, labeled 4, 8, and 16 (ohms). When connecting one speaker cabinet to the amp, you can choose one of the three jacks for proper impedance matching. When connecting a “full stack”, or multiple cabs, you should first determine the impedance of each cab, then calculate the total impedance of the stack. This is very simple to do when using two identical cabs with the same impedances. Simply divide the cab’s ohms by 2, and use the next smaller valued jack for the second cab, or plug one cab into another (if you can) and go into the 4 ohm jack only.

Example: If you have 2 Marshall-type 4X12’s at 16 ohms each, plug one into the 16 ohm jack and one into the

8 ohm jack. If you’re running two Fender-type 2X15 or 2X12 cabs, or equivalent, at 8 ohms each, use the 8 ohm and 4 ohm jacks, or plug one into the other and use 4 ohm.

Impedance Formula

With differing impedances, or more than two cabs (not recommended), it is a bit more complex. The formula for calculating parallel resistances works in this situation, and is as follows:

RT = ____1_______________
__1__ + __1__ + __1__ + . . . .
R1	R2	R3
or,
____1____ = __1__ + __1__ + __1__ + . . . .
RT	R1	R2	R3

Or you can take my word for it, this means if you combine more than two speakers with low impedance, the figure becomes too low to use with this amp. Examples: 4 8-ohm speakers, when chained together have a total impedance of 2 ohms, and 2 ohms is too low. Conversely, a 16- ohm Marshall cab combined with one wired for 4 ohms equals about 8 ohms. Wire cabs together and plug into 8 ohms, or plug one into 16 ohm and one into 4 ohm. Two twin-12 or twin-15 Fender cabs, usually 8 ohms each, equal 4 ohms. Plug one cab into the 8 ohm jack and one into 4 ohm, or chain together and go into 4 ohm.

Loading

Note: this amp should always be connected to a load, even if you are using it as a “preamp.” You can come out of the slave amp output jack (on the rear panel), but only if this amp is connected to a speaker, or what is known as a “dummy” load. This is a large resistor, 8 or 16 ohms, mounted in an insulated box with a heat sink, which has a wattage rating of 100 watts or better. A “power soak” (speaker emulator) or “power attenuator” is not always a dummy load. Although not recommended, this scheme will work. Remember, power attenuators, which work with the amp fully “cranked”, will shorten the life of your tubes.