1962 Chapman Model 305 Audio Amplifier For Restoration
On offer today we have a vintage 1962 Chapman Model 305 Audio Amplifier For Restoration. Cosmetics are generally good, with the black grille cover slightly blemished with scuffs and corrosion, and a few scratches on the sides of the amp.
Manufactured by the well regarded British manufacturer Chapman Reproducers Ltd. / Derritron Ultrasonics, this amplifier is not a plug and play but a rare survivor of a compact footprint alternative to the Mullard 5-10.
Approximate Size (WHD):- 30 x 14 x 7.2cm
We are selling this amplifier as a Restoration project only. This is not for immediate connection to a mains power supply. The amplifier will be supplied with the input socket covered with a "Failed' PAT test label stating "For Restoration Only.'
It is offered in 'restoration condition' as it has been in storage for many years without use and, therefore, has not been powered. Accordingly, the passive components will probably have aged with the resistors having 'gone high,' and the electrolytic capacitors probably showing leakage. On removing the baseplate, we note that numerous capacitors have been replaced but the signal input and mains input have not yet been installed.
The valve line up is as follows:-
- Rectifier:- a yellow print Mullard EZ81,
- Input stage:- two Mullard Old Shield ECC83
- Output stages:- four Mullard EL84.
All seven valves have been tested and give good emission.
This example dates from approximately 1962 when Chapman Reproducers Ltd. changed their trading name to Derritron Ultrasonics after being acquired by the Derritron Group in 1960. It carries a makers plaque with this name under the top grille on the RHS of the top deck.
The circuit topology presents as dual mono ultra-linear output stage, comprising two EL84 valves in push-pull, feeding a grain concentrated Partridge transformer type 8913. This is the type of output stage normally used in high quality monaural amplifiers, providing no compromise in performance and reliability, and ensuring long life and long term stability.
From research, the specification was found to be:-
- Input sensitivity 350m/V RMS for 8 watts output.
- Frequency response within 0.2dB 30-20,000Uz range
- Valves 2 x ECC83 ; 4 x EL84 and EZ81.
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