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TGA-3 100 Wat t Guitar Amplifier User ’s Guide
Tube Type Switch (Autobiasing)
Returning to the rear panel, to the left of the speaker jacks, you will notice a switch labeled tube type. This
switch has two positions for three different kinds of output tubes. The Demeter has a state of the art feature
known as auto-biasing, which allows you to get even more variation of sound by changing the output tubes (the
big ones). There are three popular tubes presently in use for this amp. The 6L6 (5881) of the large Fenders, with
it warm harmonic richness; the 6CA7 (EL34) of the vintage Marshall, with its smooth sustained distortion; and
the 6550 of the ‘American’ Marshall and SVT, with extra power and Heavy-Metal ‘Crunch’. When the tube type
switch is in 5881 position, the Demeter will automatically select the proper bias of 5881 or 6550 tubes. The
other position is for 6CA7’s (EL34). Incidentally, this amp does not require matched sets of specially graded tubes
for best performance, just make sure the tube type switch is properly set when you change your tubes: usually
once every six months to once a year, depending on use.
1/2 Power Switch
To the right of the speaker jacks there is a switch marked 1/2 power/full power. This cuts off two output tubes
to obtain full output and distortion at a lower level (turn off amp before switching this).
Output Balance
To the left of the speaker jacks is a small adjustment screw labeled output balance, To adjust, turn the amp on
with nothing plugged in but the speaker, and listen to the amp “idle”. With a screwdriver, rotate the control
clockwise and counter-clockwise until you find the spot where the hum disappears. This fine- tuning will insure
the best signal to noise ratio, and should usually be performed when changing or installing new output tubes, or
after several month’s usage.
Effects Loop
The effects loop of the TGA-3-100 is tube-buffered. This means that it will probably match the impedances of
your effects. Looking to the right of the 1/2 power switch, we come to the stereo effects loop jacks. This loop is
designed for a mono send, and a stereo or mono return. If you are using effects, the following will apply. Note:
since there is little standardization of minimum and maximum signal levels between manufacturers of semipro
and professional effect units, this amp has a very wide range of effect-loop gain. Make sure you don’t have your
effect’s input level set too high (in the red). Also, if you are using guitar “stompboxes”, which generally do not
have level controls, it is usually necessary to run the effects send control at a very low level. If you have boxes
which are still overloading, try putting a 10dB pad on the effects send or the input of your effect.
15730 Stagg Street | Van Nuys, CA 91406 | 818-994-7658 | http://www.demeteramps.com/