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Copland CTA504

Stereo Valve Power Amplifier (1992)

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Copland CTA504

Specifications

Power output: 50 watts per channel into 8Ω (stereo)

Frequency response: 5Hz to 65kHz

Total harmonic distortion: 1%

Input sensitivity: 1.5V

Signal to noise ratio: 100dB

Speaker load impedance: 4Ω (minimum)

Valve complement: 5 x E83CC, 4 x 6550WA

Dimensions: 430 x 175 x 375mm

Weight: 25kg

Year: 1992

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CTA504 bias adjustment

The CTA504 bias adjustment procedure to 45mA per 6550 output valve can be directly derived from the CTA 505 service manual (available from the hifiengine database) recognizing one important difference:

The shunt resistors in the 504 are 10 Ohms hence the correct voltage adjustment is 0.45V or 450mV for each 6550 output valve.

The shunt resistors in the 505 are 1 Ohm hence 45mA bias corresponds to 0.045V or 45mV for each 6550 output valve.

This can be directly derived from Ohm's formula: U = R x I

The 45mA operate the 35W 6550 with its 460V grid voltage at slightly below 70% of its specified maximum - in other words: in long-life territory.

There are plenty of discussions and opinions as to what the optimum bias for any given valve would be. I am not getting involved in that. What is said above solely relates to Copland's bias specification for the 6550 valves in the CTA 505.

This website is not affiliated with or sponsored by Copland. To purchase CTA504 spares or accessories, please contact the company via their website or visit an authorised retailer.