Mark IV

About the Mark IV

The Dynaco Mark IV is a single-channel (mono) tube power amplifier introduced in 1960 by Dynaco (Dyna Company) of Philadelphia, USA. It is a push-pull EL34 design rated at 40 watts continuous power with up to 90 watts peak output, intended to be used singly for mono or in pairs for true stereo systems, one amplifier per channel. The standard circuit uses a 7199 tube configured as a pentode high-gain voltage amplifier directly coupled to a cathodyne (split-load) phase inverter, driving a push-pull pair of EL34/6CA7 output tubes. The output stage operates the EL34s below their maximum ratings with Dynaco's Biaset fixed-bias arrangement, which sets the correct operating point by measuring across a shared cathode resistor, simplifying setup and maintaining good linearity and low distortion as tubes age. The power supply uses a GZ34/5AR4 tube rectifier with choke-input C-L-C-R-C filtering; a selenium rectifier is used in the auxiliary bias/preamp supply. Frequency response is essentially flat from 20 Hz to 20 kHz at full rated power within ±1 dB, total harmonic distortion is under 1% at 40 watts, hum and noise are more than 90 dB below rated power, and damping factor is about 15. Output taps accommodate 4, 8, and 16-ohm speaker loads, and input sensitivity is approximately 1.3 V RMS for full rated output. Physically, the Mark IV is a compact mono chassis measuring roughly 14 x 6.5 x 5 inches and weighing about 20-21 pounds. When used as a pair, two Mark IVs offer stereo performance broadly similar to one channel of the Dynaco ST-70, but with each channel having its own power transformer, rectifier, filter choke, and main filter capacitor. Originally sold in kit (Dynakit) and factory-assembled forms during the 1960s and 1970s, the classic Mark IV went out of Dynaco production around the late 1970s.