Audio Database

SQ65
Commentary

A high-output pre-main amplifier developed on the basis of technologies obtained with medium-output amplifiers.
It is designed not only for high power but also for high power and low distortion, and achieves an output of 28W.

A cathode NF circuit is used for the output circuit.
In addition to the commonly used NF circuit, the cathode NF circuit is a type of multiple NF circuit method that applies local feedback to the output tube. By this method, the internal resistance of the output tube is greatly reduced and NFB is stably applied over a wide band.
The output transformer uses a special winding designed for the cathode NF circuit.

As the output tube, Nova tube 7868 of 7591 is used.
The Nova tube is a large 9-pin MT tube with the capacity of the GT tube in addition to the shape of the MT tube. It eliminates problems caused by the lacerations of the stem base and poor soldering, which were considered as defects of the GT tube.
The 7581 (7868) is an output tube for Hi-Fi that was used in the McIntosh 225 model and established a reputation. It is a high-sensitivity type ball of 6L6GC and can take out a large output of over 40W (AB class P. P.).
Each pair of output tubes is fully AC-balanced by independent regulators, and the output tubes are operated in a fixed bias mode.

The driver circuit uses the so-called LEAK-type, which was used in the point-one type amplifier by LEAK in the UK.
In addition, DC ignition is used for the first stage tube of this main section, and the overall SN has been successfully improved.

The preamp section uses the all-NF type as the rack type, which is used in the SQ5B and SQ38. A 5-pole tube is provided to prevent the increase of distortion and only to increase the input sensitivity.
There are two types of equalizer circuits for tapes : one for 19 cm of NAB standard and the other for 9.5 cm at low speed.

A lux-type tone control is used.

Extra Bass circuit is mounted.
This circuit is a kind of MFB circuit for low frequency compensation, and can automatically shift fo to low frequency according to the characteristics of the speaker.
For example, on a system with a fo of 80 Hz, the frequency can be reduced to 40 Hz, which is half of that, and the playback capability can be extended substantially by one octave into the lower range.
However, this circuit can only be used in closed speaker systems.

It features two types of low and high filters, a tape monitor with a 3-head tape recorder, and a blend control that adjusts the spread of the stereo sound source.

Model Rating
Type Tube type pre-main amplifier
Output (50 Hz) 28W+28W
Distortion factor 0.5% or Less (28W)
Frequency characteristic 20 Hz to 20 kHz - Within 1.5 dB
Input Sensitivity (20W, 1 kHz) Tape(9.5cm):1.7mV
Tape(19cm):2mV
Phono:4mV
AUX1:250mV
AUX2:500mV
Signal-to-noise ratio Tape:54dB
Phono:63dB
Tone control ± 16 db, LUX System NF Type
Equalizer Tape : NARTB (9.5 cm, 19 cm)
Phono:RIAA
Filter Low cut : 100 Hz, 6dB/oct.
High cut : 5 kHz, 6dB/oct.
Main part NFB Cathode NF : 6 dB
Major NF : 15 dB
Total 21 dB
Used vacuum tubes, etc.
Vacuum Tubes (11) 7868 : 4 pieces
6DT8 : 3
12AX7 (T) : 2 pieces
For 6AU6Hi-Fi : 2
Diode (6 pcs) SE0. 5a : 2
SE0. 5b : 4
Power consumption No signal : 125W
Maximum output : 156W
External dimensions Width 363x Height 127x Depth 320 mm
Weight 14.2kg