Dynaco Mk. IV Monophonic Power Amplifier
The Mark IV Amplifier uses a circuit arrangement based on that of the Dynakits Mark II and Mark III which have become world famous for superior quality while essentially simple and trouble free. The new 7199 tube is used as a pentode high gain voltage amplifier directly coupled to a cathodyne phase inverter. All parameters are adjusted for minimum distortion. This type of phase inverter has the unique advantage that its operation is independent of tube aging so that no adjustments are required in maintaining optimum performance.
An internal capacitive feedback loop balances the phase inverter at the highest frequencies, and the arrangement provides accurately balanced driving signals to the output tubes, which are EL-34s operated well below their maximum ratings. The output tubes use a fixed bias arrangement which is set through use of Dyna Biaset (patent pending) which provides optimum linearity of the tubes and minimizes the effects of unbalanced components. The connection of the output tubes includes a small percentage of screen loading which improves the regulation of the output stage and makes it comparatively uncritical of load impedance.
The impedance match and bias conditions utilized in the output stage provide minimum distortion 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 1% intermodulation distortion at 40 watts output.
The use of a minimum number of phase shifting stages along with careful transformer design makes it possible to have unconditional stability at both high and low frequencies. This feature of the design means that there is no tendency toward motorboating or oscillation under any condition of use.
The inherent linearity of the circuit, its absolute stability, and the prefabrication of all critical wiring on the printed circuit assembly make construction completely non-critical. The use of conservatively operated, highest quality components insures superior and dependable operation over a long period of time.
Power Output: 40 watts continuous, 90 watts peak
IM Distortion: Less than 1% at 40 watts, less than .05% at 1 watt
Frequency Response: ±.5 dB from 10 cps to 40 kc; ±.5 dB from 20 cps to 20 kc at 40 watts output
Power Response: 20 cps to 20 kc without exceeding 1% distortion within 1 dB of 40 watts
Sensitivity: 1.3 volts rms input for 40 watts output
Hum and Noise: Inaudible; 90 dB below 40 watts (choke filtering)
Damping Factor: 15.
Output Impedances: 4, 8, and 16 ohms
Tubes: EL-34 (2), 7199, GZ-34, selenium rectifier
Finish: bright nickel chassis, vinyl-coated charcoal brown cover
Special Features: Matched tubes, Dyna Biaset for non-critical adjustment, preamp power socket, fuse post, on-off switch
Power Consumption: 115 watts
Size: 5" by 14" by 6-1/2" high.
Weight: 20 pounds.
Essentially half of a Stereo 70, this 40-watt monophonic tube amp was introduced in 1960. Thanks to Ned Carlson from Triode Electronics for the scan of the classic review!
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Copyright © 2000,2001 Greg Dunn