Gibson ga30rvs schematic

GIBSON ‘GOLD TONE’ GA-30RVS

CIRCUIT DISCRIPTION

Please refer to circuit diagram for DC voltages and other information

INPUT SECTION AND PREAMP

SK1 and SK2 are the HI and LO sensitivity inputs respectively. When the LO socket is used
R51, R52 and R83 act as a potential divider to reduce the input level to the preamp.

C45 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.

R50 and C27 give a slight presence lift and the frequency of the Bright effect is set by C3,
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.

RL1 switches the extra gain stage in or out of the signal path. The relay itself is switched on or
off by the push/pull switch on RV1. With RV1 pulled out the relay can also be switched on or
off by the tip connection of SK3, Footswitch 1.

V1b is the extra gain stage configured similar to V1a but without the cathode bypass
capacitor. C62 is added across the anode resistor R87 to smooth out the top end, and R53,
R88 and C28 set the amount of signal sent to the next stage as well as giving a slight
presence lift.

V2a is the next gain stage configured similar to V1a but with C64 across the anode resistor
R91 again to smooth out the top end. The signal is then fed into V2b which is configured as a
cathode follower. This reduces the impedance to drive the tone network.

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).

R10, R11 and R12 act as a potential divider to lower the signal sent to the reverb and effects
loop circuitry. The J175 FET, TR1, is used to effectively short out R12 when the extra gain
stage is not in use. This sets the volume level difference between the two preamp
configurations.

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REVERB AND EFFECTS LOOP SECTIONS

The signal from the preamp is fed into IC3A 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.

The output of IC3a is fed to the reverb and effects loop sections.
C12 and R22 roll off a lot of the lower frequencies before the signal gets sent to IC3B and

IC1A. 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 IC1B. 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.

RV3 controls the level of the reverb. Across pins 1 and 2 is TR4, a J175 FET. When the ring
connection on SK3 is shorted out, i.e. by a footswitch, TR4 is also effectively shorted which
results in the reverb being turned off.

The reverb signal is then split and sent to the two mixer stages in IC4, however one side goes
through a unity gain inverting transistor stage, TR6, just before the mixer stage. This obviously
puts one of the reverb returns out off phase which produces the ‘surround sound’ reverb effect.

The signal from the output of IC3a is split two other ways for the effects loop. One goes to the
series/parallel switch, SW1A, the other goes to the SEND socket. R34 and R1 are a potential
divider to reduce the nominal send level to about -20dBu.

SK6 and SK7 are the LEFT/MONO and RIGHT RETURN sockets respectively. Each of these
are fed into the two sections of IC2 which are both configured as non-inverting gain stages.
The outputs of these then go into RL2. This relay basically turns the effects loop on or off.
Without a jack is inserted into SK6, LEFT/MONO RETURN, the ‘LOOP_MUTE’ line is held
down at 0V, this keeps TR5 turned off, which turns on TR3 and effectively shorts R70 to 0V.
This turns RL2 which means the effects loop is bypassed and cannot be switched on. When a
jack is inserted into SK6 the reverse happens, TR5 is turned on, TR3 is turned off and the
effects loop is activated. In this mode, shorting out SK4, Footswitch 2, will turn the relay on
and again bypass the effects loop. TR5 and TR3 were needed to isolate the ‘LOOP_MUTE’
line from the relay circuit to reduce switching noise.

MIXER SECTIONS

The two halves of IC4 are used as standard virtual earth mixer sections. Here the dry, effects
loop and reverb signals are mixed together depending on the settings of the series/parallel
switch and RL2. The outputs from each half of IC4 then provide the left and right signals sent
to the two power stages.

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POWER STAGES

The two phase splitters (V3 and V4) are differential input splitters which each produce the anti
phase signals necessary to drive the push pull output stages.

The outputs from V3 drive the EL84 power valves V5 and V6 for the left side power stage. V4
drives V7 and V8 for the right side power stage. The two pairs of EL84 output valves are
configured as push pull, cathode biased, class A power amplifiers.

The quiescent current of each power stage is set by R71 and R73, which are bypassed by
C41 and C43 for extra gain.

D3, D4, D5 and D6 have been added to give protection to the output transformers should a
fault arise.

OUTPUT TRANSFORMERS AND SPEAKER CONNECTIONS

The output transformers have secondary taps for 16Ω and 8Ω.
The four 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 or
8Ω 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.

SW3 selects the output impedance of the External Speaker Outputs, SK8 and SK9. These
can be set to either 16Ω or 8Ω. When a jack plug is inserted into either of these sockets the
corresponding internal speaker is disconnected.

POWER SUPPLIES

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

The HT supply is a simple bridge rectifier diode network, with 4n7 1KV capacitors across
each diode for EMC reasons, which is then smoothed by C82, to supply the centre taps of the
output transformers. R126, C81, R127 and C80 have been added to isolate the two centre
taps at high frequencies. The main supply is then further smoothed to supply the screen grids,
phase splitters and preamp.
R128 is added to discharge the high voltage capacitors when then unit is turned off.

The ac heater supply is simply connected via two twisted pair connecting leads to V5, V6, V7
and V8, after first going through the secondary fuses.

The 25.2V DC supply is a highly regulated supply using a BD647 (TR8) as the main
regulating device. (Please ensure that the heat sink is attached to TR8 and heat sink
compound has been used)

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