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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 16 , 8
and 5.3
. The 16 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 16 , 8
or 5.3
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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