Dynaco Mk. II Monophonic Power Amplifier
I got this picture from eBay auction Item #287094442.
The Dynakit Mark II is a complete 50 watt power amplifier kit which offers the highest possible fidelity, at low cost and in a compact arrangement. It uses a circuit of outstanding performance characteristics, along with top quality parts, including the new Dynaco A-430 output transformer – the finest available. The measured specifications show that the Dynakit is an amplifier of unequalled performance, and its listening quality is also unrivalled by any regardless of price.
The design features of the Dynakit which contribute to its superior listening quality do not show up in the usual steady-state laboratory specifications. For example, the Dynakit does not exhibit bounce and flutter when pulsed because of its wide margin of stability. The Dynakit, unlike other amplifiers, has been designed to provide its specified performance on a loudspeaker load, not just a resistive load as in the laboratory. In particular, the connection of a loudspeaker does not distort the high frequency square wave performance. In addition, the Dynakit’s power handling capabilities are maintained over the entire audio band without the sharp rise in distortion which characterises most amplifiers at the low and high frequencies.
The Dynakit uses a unique circuit. A pentode voltage amplifier, with parameters adjusted for minimum distortion, is directly coupled to a cathodyne phase inverter. High frequency compensation is employed in a capacitive feedback loop, which corrects the inherent unbalance of this type of phase inverter. The inverter drives the output tubes which are operated with fixed bias. The connection of the output tubes includes a small percentage of screen loading which improves the regulation of the stage and makes it comparatively uncritical of load impedance. The impedance match and bias conditions utilized in the output stage provide minimum distortion operation over a very wide dynamic range. In addition, 20 dB of negative voltage feedback lowers the distortion to an unmeasurable proportion at normal listening levels and to less than l% IM at 50 watts output.
Phase compensation at both high and low frequencies is incorporated into the circuit arrangement to provide a wide margin of stability and to make construction uncritical. This feature of the design means that there is no tendency toward motorboating or oscillation under any conditions of use.
Power Output: 50 watts continuous, 100 watts peak.
Intermodulation Distortion: Less than 1% at 50 watts;, less than .5% below 35 watts. These figures apply for any conventional combination of test frequencies.
± .5 dB 6 cps to over 60 kc.
± .1 dB 20 cps to 20 kc at any level between 1 milliwatt and 50 watts.
Power Response: 20 cps to 20 kc within 1 dB of 50 watts without exceeding 1% harmonic distortion over this range.
Square Wave Response: Essentially undistorted 20 cps to 20 kc on loudspeaker load.
Sensitivity: 1.5 volts rms input for 50 watts output.
Hum and Noise: 80 dB below rated output.
Damping Factor: 15.
Output Impedances: 8 and 16 ohms (no restrapping required for changing output impedances) (choice of 4, 8, and 16 ohms also available at extra cost).
Tubes: 6CA7, EL-34 (2). (6550s can be substituted in output without wiring changes), 6AN8, 5U4GB.
If it is desired to use 6550 tubes for output tubes in place of the 6CA7/EL-34 tubes these can be inserted directly. The only adjustment which need be made is to set the bias to give a reading of 1.92 volts at the bias check point.
Size: 9" by 9" by 6-3/4" high.
Weight: 27 pounds.
Pre-Amp Provision: Any pre-amp can be used. For pre-amps without power a spare socket is included which permits drawing 200 to 450 volts dc at 20 ma and 6.3 volts ac at 1 amp.
David Hafler, Ed Laurent
The first piece of equipment offered by Dynaco, in 1955. 50 watts continuous power. Identical in physical size with the later Mk. III.
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