Tuesday, January 31, 2017

Sound Valves M60i Monoblock Rebuild and Review

Sound Valves M60i  Mono Power Amplifier

Today were showcasing a set of M60i Monoblocks I've acquired for my personal setup, a third Black faced M60i came with the trade thats still in repairs.  The M60i came with a "Ultra-Vent" proprietary case to protect the tubes and from what I can ascertain  with two faceplates, black for early productions and a brushed silver.

From the literature I have found on Sound Valves;
"Sound Valves is the name taken after Dynaco stopped selling DIY kits in the 1990s and set up shop in Columbus Ohio. The M60 amplifier could be considered the next version of the Dynaco MKIII.

The M60 audio circuit topology is a four stage, single channel power amplifier. The front end consists of an all triode, Class A gain stage, phase splitter, and low impedance drivers. The output stage uses 6550 tubes and operate in triode or ultra-linear mode selected by a switch between the output tubes and transformers (NOTE: Manual states the switch can only be used when amp is off). In triode / ultra-linear mode the first 17W / 30W are pure Class A, switching to Class AB1 for a maximum RMS output of 30W / 60W. Below is the information from the manual."

Each of these units were exhibiting similar issues I related to an initial design defect with regard to electrolytic voltages and factory sub-par components grades. Early failure of the 12Vdc regulated IC was apparently imminent as well from research I suspect to the initial current limiting resistor values and its current handling rating. Another issue is the premature burn up of a 15V zener which is erroneously noted as a 10V on the schematic. 

The construction consists on a outboard choke, Bell type Power and Audio Output Transformers that are exceptionally heavy and I think overkill for their output rating...which is NEVER a problem with in comes to tubes! The main circuit is on a single heavy layer double soldered PCB assembly with PCB-Type Ceramic sockets. Quality components such as Bourns, Nichicon were used during the initial construction.

The Chassis is made of two pieces of bent Chromed Steel (U-Body) with smooth corners and a fine finish. The rear outputs are a 5-way binding connectors with 2,4 and 8 OHM taps. The Tiffany style Gold-Plated RCA input is of heavy construction and feels very solid.

In each units all of the electrolytic capacitors were replaced with Nichicon PW high temp  (105C) versions with the same diameter for the chassis cutout with an increase of operating voltages to 250V. The two 4.7MFD BiPolar were replaced with VP Nichicons as well. Original Af path films and coupling films were all replaced with high grade MKP Polypropylene and ECW PP's films which made a significant impact. Several of the current limiting resistors were increased to 5W handling power as well as all of the small-signal BJT's and MOSFET to modern On-Semi, MicroChip and Fairchild TO-92 devices. The 12V circuit is based off a LM7812 type IC which was replaced with Fairchild robust, high heat (180C+) TO-220 7812's devices.

All new Electro-Harmonix compliments were purchased; 12AT7/12AU7 and two 6550 EH's.

Audio Notizen (PER Serial#)



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