Demeter Amps TGA-3 - Manual (Page 2)

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 t wo
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 t wo 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 t wo 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 t wo 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 t win-12 or t win-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.
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