The M-60 is one of Electronic Industries (early Audio Research by William Johnson) tube amps using chassis and transformer of the DYNA Mk lll with their own circuit design. The unit is more closely related to an Audio Research D-51 rather than a Dyna Mk3. A circuit schematic can be found at the Audio Research website at http://www.arcdb.ws/M60/M60.html
This sale is for a pair of these hard to find early model DynacoMk3/M-60 mono amps outfitted with all new tubes including 6550A output tubes, 5AR4 rectifier as well as 12AX7 and 6FQ7 driver tubes, excellent original cosmetics, no cage. This particular model is even more unique in that it is an earlier version where the power supply has regulated voltages using OA2 gas discharge tubes for voltage stabilization rather than solid state circuitry used in later M60 models. A general discussion of this amp can be found at Audio Asylum http://www.audioasylum.com/cgi/t.mpl?f=vintage&m=142522
Some particulars : - Output section: The 4 ohm tap is grounded to chassis and the Common ‘0’ ground tap is lifted. The 8 ohm speaker connection is made in a balanced way between the common ‘0’ and ‘16’ ohm taps. Output 6550 cathodes now connect to the 16 ohm and former common tap on the speaker side of the output transformer. Consequently, the ‘0’ and ‘16’ ohm output taps are balanced with respect to ground. So, unlike most amps, the ‘4’ ohm tap is now the ‘common’ or grounded tap. The screen supplies appear to be fixed, rather than coming off the ulta-linear taps of the output transformer. See for example the schematic of early AR tube amps http://www.audioresearch.com/ContentsFiles/D51D75D75AD76_ManualSchemPL.pdf
- Driver circuitry: The new circuitry by Electronic Industries has a compliment of 1x 12AX7 and 2x 6FQ7. On the chassis there are 2 jacks to use as bias test points when installing new output tubes. An additional control pot allows for setting the AC balance. Voltage regulation is incorporated using 2x OA2 tubes that regulate each stage separately and operate by gas discharge. The 2x 6FQ7 tubes, it appears, are used as a differential type phase inverter.
PAS 3 preamp: The PAS3 is in excellent condition with a new phono circuit by Electronic Industries with 4 tubes now rather than Dyna's original 2. The high level output section remains the same original Dynaco, the chassis is excellent and original in every respect. Interestingly the rewired controls already use the same wiring found in the later 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 wished. This PAS3 phono section now has striking similarities to an early Audio Research SP-type series preamp. - The preamp power supply is substantially upgraded and greatly improved via external solid state regulation. Because there is insufficient room in the PAS3 chassis, it is housed in an external chassis that is connected to the preamp by an octal plug via an umbilical cord (you can view the PAS2 version (mine is a PAS3) at http://www.arcdb.ws/preamplifiers.html).
Will split up this Electronic Industries PAS3 and Mk3/M60 Power, but it would be a shame to do so. Will be happy to consider your offer for these very unique and excellent condition electronics.
Amplifier Specifications are very likely very close to one of the AR M60 variants: 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 Distortion Less than 1% 30Hz/20kHz @50 Watts Rise time Approx. 20 µs. Hum & noise 90dB below rated output Power requirements 117V 50/60 Hz, approx. 150 Watts at idle, approx. 200 Watts at rated power Other Load regulation: Approximately 1-172 dB/1kHz Dimensions 9" W x 10" D x 7" H; Weight 21.5 lbs. Net; 26 lbs.
I have ‘soft-started’ these amps with a variac recently to reform the electrolytic caps, just in case, since they were sitting for a year or so. They powered up as they should.