These Mk3’s have been modified by Bill Johnson (later to start Audio Research) then operating as Electronic Industries. In particular, the power supply has been improved by adding regulated voltages using OA2 gas discharge tubes. This unit is very closely related to an Audio Research D-51. The circuit schematic can be found at the Audio Research website at http://www.arcdb.ws/M60/M60.html
This sale is for a pair outfitted with new 6550A output GZ34 rectifier and new driver tubes, good original cosmetics, but no cage. A general discussion of this amp can be found at Audio Asylum http://www.audioasylum.com/cgi/t.mpl?f=vintage&m=142522
Both amps weigh about 20 lb each. Don’t hesitate to ask any question which I’ll answer to the best of my ability. I have been using these happily with Quad ESL57 speakers. Some particulars:
- Driver circuitry: The driver board is made by Electronic Industries and has a compliment of Mullard NOS 1x 12AX7, 2x 6FQ7 (differential phase inverter) along with 2x OA2 tubes on the chassis. There are 2 jacks to use as bias test points for installing new output tubes. An additional control pot allows for setting the AC balance.
PAS 3 preamp: The PAS3 is in excellent condition with major phono circuit modifications by Electronic Industries. The high level output section remains the same original Dynaco, the chassis is original in every respect. The phono section has a new Electronic Industries circuit board (4 tubes now rather than 2) - interestingly the rewired controls are already the same wiring that is found in the SP series of Audio Research preamps. The rumble filter wiring is disconnected (a common mod) but left in place and can be easily reconnected if so wished. This PAS3 phono section is therefore not unlike a early Audio Research SP-type series of preamps (you can view the PAS2 version (mine is a PAS3) at http://www.arcdb.ws/preamplifiers.html - The preamp power supply is upgraded to solid state and greatly improved by extra regulation. Because there is insufficient room in the PAS3 chassis, it is housed in a second external chassis that is connected to the preamp via the octal plug and umbilical cord. Will split up this Audio Research PAS3 and Mk3 Power, but it would be a shame to do so. Will be happy to consider your offer for these very unique excellent condition amps. Moreover, I have a new standard Dynaco PAS3 phono board (fibreglass) if someone wishes to return the preamp to its original condition.
Amplifier Specifications are : Power output 50 Watts RMS 20Hz/20kHz Frequency response ± 1 dB 20Hz/20kHz Input sensitivity 1.15V RMS for 50 W RMS output Input impedance 500K ohms Output taps 4-8-16 ohms
I have ‘soft-started’ these amps with a variac a while ago to make certain the electrolytic caps are properly reformed - just in case. They power up as they should.
Re: Upgraded Dynaco MkIII amps and PAS3 preamp
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