Copland CTA-506
Copland CTA-506
Copland CTA-506

Copland CTA-506: Power amplifier based on KT120 tubes

The Copland CTA 506 amplifier is based on KT120 tubes, which are an updated version of the legendary 6550 / KT88 tubes. These classic devices have been the basis of numerous high-quality amplifiers for over half a century, and will undoubtedly continue to be an excellent choice for high-power tube amp builders.

Be that as it may, the KT120 is the most powerful audio amplification tube produced today. Compared to the KT88, these amazing tubes can deliver 30% more power. With a dissipation power of 60 watts each, a pair of KT120 tubes in a push-pull design can achieve an audio output power of 200 watts. When used in designs with more modest power output figures, the KT120 can provide incredible dynamics without distortion.


When output power begins to exceed 50 W, the input and driver circuits experience a significant increase in the current supplied to the inputs of the amplifier’s power stages. This situation does not depend on the choice of element base, be it bipolar transistors, field-effect transistors or vacuum tubes, as well as on whether the circuit has one amplifying element or several less powerful ones connected in parallel.

Even though FETs and tubes are theoretically voltage-controlled, they also require current to be supplied from driver circuits (with relatively low output impedance), which is partly due to the increase in input capacitances and partly to the often neglected fact that current flows in the lamp and through the signal input too.


To prevent excessive load on the amplifier stages, the Copland CTA 506 uses efficient buffer circuitry. Buffers are low-distortion current amplifiers that do not amplify the voltage signal, but make it stronger in current. Another positive side of using buffers is the resistance matching of driver and power tubes.

The Copland CTA 506 push-pull output stages are based on a pair of KT120 tubes in an ultra-linear design, delivering the low distortion of triodes combined with the power of tetrode tubes. A separate power supply for the amplifier’s input stages allows massive storage capacitors with a capacity of more than 3000 microfarads to operate independently on the output stage tubes.

To isolate fluctuations in the output stage power supply from the circuits of the input and driver stages, the Copland CTA 506 uses stabilization based on two powerful 6550 tubes, one of which is dedicated exclusively to the driver stages, and the second to the output stages.


The huge, powerful tubes used in this design are very attractive to look at, so we decided to place them behind the horizontal perforations of the front panel to ensure the stunning design matches the outstanding sound of the amplifier.

Characteristics of Copland CTA 506

Lamps: KT120 (4 pcs.)
6550 (2 pcs.)
12BH7 (4 pcs.)
ECC81 (2 pcs.)
Output voltage: Nom. 2V, max. 40V
Output power: 90 W per channel (20Hz-20kHz)
THD: < 1%
Frequency response: 15Hz-100kHz (-3 dB)
Input sensitivity: 1.5 V (at rated power)
Input impedance: 100 kOhm
Feedback depth : 17 dB
Load impedance: 8 Ohm, 4 Ohm
Signal-to-noise ratio: > 100 dB
Output polarity: Non-inverted
Power consumption: 600 W
Supply voltage: 120V, 230V or 240V.
Factory set according to country of destination
Supply voltage tolerance: +/- 12%
Dimensions (mm): 430 (W), 190 (H), 390 (D)
Weight (packed): 26 kg