3
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 16
Ω
and 8
Ω
.
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 16
Ω
or
8
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.
SW3 selects the output impedance of the External Speaker Outputs, SK8 and SK9. These
can be set to either 16
Ω
or 8
Ω.
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)
