http://www.ozvalveamps.org/coronet.htm | Created: 24/10/09 | Last update:
Source: David G
This is a very typical circuit for a small solid-state combo of the era and should be a useful guide to many similar amps.
The amp uses Japanese transistors with the numbering scheme “2Sx nnn”. These are silicon devices and the second letter gives the gender, 2SC = NPN, 2SA = PNP. Note that the first input device doesn't have the emitter arrow and is a 2SK-type, meaning it is a n-channel FET (similar to MPF102/2N3819). Tremolo is applied to the input stage from a typical phase-shift oscillator (lower right).
The power amplifier is a NPN quasi-comp, capacitor coupled to the speaker, using a single-rail supply.
Reverb tank drive and recovery are shown lower left.
For service in Australia the power transformer whould naturally have a 240V primary, the chassis would be grounded, and the 0.005uF/500V cap between chassis and the fused mains would be omitted.
The most common faults in amps of this sort is the failure of one or both of the output transistors, and generally these are subtitutable with BD139's or similar.
The most likely items needing renewal in an old amp will be the 1000uF/50V main power supply cap, the 220uF/35V output coupling cap, and possibly other electrolytics.
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