Tech_Notes_-_Grounded_Emitter_Circuit.pdf

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TECH NOTES
Fender Musical Instruments Corp.
7975 North Hayden Road Scottsdale, Arizona 85258
TECH NOTE #
TN 98-4
ISSUE DATE:
ISSUED BY:
PAGE:
May, 1998
J. Benson
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PRODUCT(S) AFFECTED:
ALL WITH GE (grounded emitter) / FLYING RAIL
POWER AMPLIFIERS (such as Stage 112SE guitar
amp, and some SR6300/8300 box-top mixers)
GENERAL THEORY OF OPERATION DISCUSSION
CONDITION:
Confusion sometimes occurs when, in servicing a flying rail amplifier, the technician
refers to the schematic diagram and notices that where he normally sees the speaker
connected, these amplifiers connect to ground. Then after finding the speaker on the
diagram, he sees it connected to "C.T."
GENERAL EXPLANATION:
In most complimentary solid-state amps, the output could be defined as the common
point between the emitter resistors. In a "normal" amp, the DC potential of this point is
near-ground - separated only by the speaker (or output) load. If this point were to be
grounded by removing the speaker and connecting the point to ground through a load
resistor, it would electrically operate the same way, but no sound would be heard.
In the "flying rail" amplifier this is exactly the case; and explains why it is sometimes
referred to as a "grounded emitter" amplifier. The "flying rail" name comes from the fact
that with the emitters grounded and the amp driven, the rails will swing in concert with
the driving signal.
To take advantage of this characteristic, a power supply exclusive to the power amp is
used. The positive and negative rails are derived from each end, respectively, of a
transformer secondary winding which has a center tap. All current drawn by the supply
TECH NOTE # TN98-4
PAGE:
2 OF 2
(for the positive
and
negative rails) flows through the center tap. If we now remove the
load resistor, insert the speaker in series with the center tap (positive lead connected to
the center tap, and negative lead connected to ground), we have accomplished two
things:
1. The load (speaker) has been moved from the power amp to the power
supply, and will now produce sound (driven by the current the power amp is
drawing.)
2. The power supply center tap is electrically (DC) near-ground.
Note: This is the reason that the speaker (or an appropriate load) must be
connected when servicing these amps.
The schematics of "grounded-emitter/flying rail" amps refer to "C.T." This stands for
"Center
Tap".
This is the same point as the positive terminal of the speaker.
When servicing this type of amplifier, it is important to find out whether the speaker
jack(s) is(are) isolated from the chassis, and ensure the unit being serviced is correct in
that area. At the time of this writing (May, 1998), all Fender amplifiers of this design
have isolated speaker jacks.
Typical hardware stackup of the jack is (from inside to outside) as follows:
1.
2.
3.
4.
5.
6.
7.
Jack
Toothed washer
Fiber washer
Chassis
Shoulder washer
Finish washer
Nut

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