|Address||Waterloo, ON N2L1P6
The M-60 is one of Audio Research early tube amps using transformers of the DYNA Mk lll only with AR’s own designs and circuit. This unit is very much more closely related to an Audio Research D-51 rather than a Dyna Mk3. A photo of the circuit schematic is attached, the Audio Research website where it can be found is http://www.arcdb.ws/M60/M60.html
This sale is for a pair of these hard to find early model M-60 amps outfitted with brand new 6550A output and driver tubes, good cosmetics, but no cage. This particular model is even more unusual in that the power supply voltage regulation uses 2x OA2 gas discharge tubes for voltage stabilization rather than the solid state circuitry used in later M60 models. You can find a general discussion of this amp at Audio Asylum http://www.audioasylum.com/cgi/t.mpl?f=vintage&m=142522
I have ‘soft-started’ these amps with a variac to reform the electrolytic caps and all appears to be good. Nevertheless, as these are vintage tube amps, the sale is “AS IS” and final. Pictures were taken on Nov. 15, 2016. Both amps weigh about 25 lb each. Don’t hesitate to ask any question which I’ll answer to the best of my ability.
Some particulars from what I can gather:
Output section: The 4 ohm tap is grounded to chassis and the Common ‘0’ ground tap is lifted. A 8 ohm speaker connection is therefore made between the common ‘0’ and ‘16’ ohm taps.
Output 6550 cathodes now connect to the 16 ohm and former common tap. Yes, that's right, 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 rather than the more conventional ‘0’ 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 circuit board is made by Electronic Industries and has a compliment of 1x 12AX7, 2x 6FQ7 and 2x OA2 tubes. There are 2 jacks to use as bias test points for installing new output tubes. An extra control pot allows for setting of the AC balance.
2x OA2 tubes are voltage regulators that regulate each stage separately and operate by gas discharge. The OA2's look pretty cool when they start to glow (see below for more details from Wikipedia). The 2x 6FQ7 tubes, I think, are used as a differential type phase inverter.
PAS 3 preamp: The PAS3 is in excellent condition with major modifications by Electronic Industries. The output section remains the same original dynaco, but the phono section has a new Electronic Industries circuit board (4 tubes now rather than 2) and is completely rewired to the controls (interestingly the wiring harness is the same as in the later SP series of Audio Research preamps. This PAS3 is therefore not unlike an 'early version' of the Audio Research SP-type series of preamps (you can view the PAS2 version at http://www.arcdb.ws/preamplifiers.html). The power supply is now solid state and greatly improved. Because there is insufficient room in the PAS3 chassis, it is housed in a second external chassis and is connected to the preamp by an octal plug via an umbilical cord. As well, the rumble filter wiring is disconnected but left in place and could be reconnected if so wished. 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 amps.
The OA2 voltage regulator tube: It resembles a vacuum tubes, but there are two main differences:
• Their glass envelopes are filled with a gas mixture, and
• They have a cold cathode; the cathode is not heated with a filament to emit electrons. Electrically, they resemble Zener diodes
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