Often regarded as the first great amplifier, Peter Walker’s original, the Quad II, is a fairly unusual find in the United States. This classic '50s tube amp from across the pond cemented Quad's reputation as a company of influence and integrity. These examples have some character, and one side has a bit of a dent near the terminals, but electronically, they're square in spec and fully tested.

RESTORATIONS

• Full 30-point service check
• Replaced power cords with custom switchable, braided cloth cable
• Upgraded speaker terminals with Cardas solid copper
• Replaced signal caps
• Installed new EF86 tubes
• Cleaned controls, switches, and chassis

SPECS

Type   Monophonic Valve Power Amplifier

Type   Monophonic Valve Power Amplifier

Power output   15 watts into 8Ω (mono)

Frequency response   20Hz to 20kHz

Total harmonic distortion   0.25%

Input sensitivity   1.4V

Signal to noise ratio   80dB

Speaker load impedance   7Ω to 15Ω

Valve complement   2x EF86, 2x KT66, 1x GZ32

Dimensions   13"W x 6.5"H 4.75"D

Weight   18.25 lbs.

LINER NOTES

A Quad II, original or Classic, is a class-A push-pull monophonic amp with only two active stages. The input stage contains two EF86 miniature pentode tubes, each of which is capacitor-coupled to its own output tetrode tube, but only one of which (call it input tube No.1) gets the original input signal. From there, in addition to driving its corresponding output tube, input tube No.1 drives input tube No.2 with a portion of its reverse-phase output, attenuated by some 6dB. Thus the output of the second EF86 is equal to but out of phase with the output of the first, and this balance is maintained through the output transformer's primary, where the full music wave is put back together. This design, called a paraphase circuit, used to be quite popular, arguably because it accomplished phase inversion and voltage gain in one stroke—and so allowed the designer to keep both distortion and parts cost to a minimum.