1980 MusicMan HD-150

This hybrid Tube/Solid State amp was made for guitar or bass and came to me because of severe issues with white noise/hiss/hum. The hiss/white noise was caused by the two TL072 IC chips beginning to fail. When analog amplifier chips or transistors fail they can either do so suddenly or they can begin to make a whooshing or hissing sound that grows in amplitude over time. The hum was caused by failing electrolytic capacitors in the power supply.

A complete overhaul was done - all electrolytic caps were replaced (F+T, Sprague Atom, and CE brands were used), two TL072 IC chips in the preamp were also replaced, new 2 Watt Metal Oxide power resistors, and fixing a botched modification that was previously done by someone else was completed. The amp was run through the original factory calibration for bias - 25mvdc across each of the 3.9ohm ballast resistors with a B+ of 712VDC - this provided the matched quad of NOS Winged C (Svetlana) brand 6L6GC’s with the correct amount of current to put out the full rated wattage. The amp now sounds like new.

Although it doesn’t look unusual from the outside, this is one of the strangest circuit designs I have ever seen. It uses a bias scheme for the four 6L6GC’s that claims 150 watts by using constant current sources from BJT transistors in the phase inverter and power transistors on the cathodes of the tubes - the signal is injected through the cathodes of the power tubes, not the grids as is seen in 99% of amps. Also the solid state section is in the preamp and not the power amp, like most hybrid designs.

These amps are somewhat famous for puzzling techs and owners alike when it comes to repairs - but they are rock solid amps and a good lower price option for a vintage amp - these sound absolutely killer and deserve “classic” status when it comes to tone, unlike most hybrid amps I have heard.