Gibson ga15rv schematic

GIBSON ‘GOLD TONE’ GA-15RV

CIRCUIT DISCRIPTION

Please refer to circuit diagram for DC voltages and other information

INPUT SECTION AND PREAMP

SK1 and SK3 are the LO and HI sensitivity inputs respectively. When the LO socket is used
R5, R39 and R3 act as a potential divider to reduce the input level to the preamp.

C3 has two purposes, firstly to block any DC from the input that may unintentionally be
present, this would otherwise change the bias point of the first valve stage, secondly the value
of the capacitor has been chosen so that there is a slight roll off of lower frequencies, this
prevents the sound from getting too muddy.

V1a is the first gain stage and is configured as a cathode bias, common cathode, voltage
amplifier with bypassed cathode resistor for increased gain.

R6 and C6 give a slight presence lift and the frequency of the Bright effect is set by C7, which,
when switched in, is across pins 2 and 3 of RV1 (Volume). Obviously connected like this the
amount of brightness added will decrease as RV1 is turned up.

V1b is the second gain stage configured similar to before, C9 is added across the anode
resistor R8 to smooth out the top end.

The Tone network is passive and controlled by RV2. This is a dual ganged potentiometer, one
part of which effectively controls the mids (RV2B) while the other part inversely controls the
treble (RV2A).

R28, R34 and R35 act as a potential divider to lower the signal sent to the reverb circuitry.

REVERB SECTION

The signal from the preamp is fed into IC1A which is configured as a boot strapped voltage
follower. Z1 and Z2 have been added to prevent any nasty spikes from damaging the opamp,
this should in practise never happen.

C4 and R44 roll off a lot of the lower frequencies before the signal gets sent to IC1B and
IC2A. These opamps are configured for current gain, the actual gain being dependant on the
impedance/frequency curve of the reverb tray. Because of this it is crucial to the correct
operation of the reverb that the right reverb tray is used. This should be an Accutronics
8CA3B1B.

The output of the reverb tray goes into IC2B. This is configured as a differential amplifier as a
way of reducing any hum that may be picked up on the sensitive reverb return leads.

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RV3 controls the level of the reverb. Across pins 1 and 2 is TR6, a J175 FET. When SK4 is
shorted out, by a footswitch, TR6 is also effectively shorted which results in the reverb being
turned off.

The reverb signal is then mixed with the dry signal, via R58, before going into the phase
splitter.

POWER STAGE

The phase splitter (V2A and V2B) is a differential input splitter which produces the two anti
phase signals necessary to drive the push pull output stage.

V3 and V4 are the two EL84 output valves connected as a push pull, cathode biased class A
power amplifier.

The quiescent current is set by R24, which is bypassed by C19 for extra gain.
SW1A and SW1B is an electrically robust slider switch used to switch the power valves from

Pentode to Triode operation. For Pentode operation the screen grid (grid 2, pin 9) is
connected, via the screen grid current limiting resistors R25 and R26, to the highly smoothed
screen grid supply, G2 - positive pin of C23. For Triode operation the screen grid of each
valve is connected to the corresponding anode.
Triode operation basically reduces the power output to a bit less than half and also reduces
the high frequencies in the overall sound.

D5 and D10 have been added to give protection to the output transformer should a fault arise.

OUTPUT TRANSFORMER AND SPEAKER CONNECTIONS

The output transformer has secondary taps for 16Ω, 8Ω and 5.3 Ω. The 16Ω tap is used to
drive the External Speaker Output, SK2. When a jack plug is inserted into SK2 the internal
speaker is disconnected.

The three LINK positions are provided on the PCB so that different impedance internal
speakers can be used in production. Depending on whether the internal speaker is 16Ω, 8Ω
or 5.3 Ω the correct LINK should be fitted. This has been done purely so that different
impedance speakers can be used if there are any problems with supply.

POWER SUPPLIES

All three supplies, HT, ac heater, and DC supplies have secondary fusing on the PCB. This is
to protect the mains transformer and for approvals.

The HT supply is a very simple bridge rectifier diode network, with 4n7 1KV capacitors
across each diode for EMC reasons, which is then smoothed by C22, to supply the centre tap
of the output transformer. This is then further smoothed by R32/C23, R30/C25 and R31/C26
to supply the screen grids, phase splitter and preamp respectively.
R29 is added to discharge the high voltage capacitors when then unit is turned off.

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