"Circuitry of the 402-C, termed the "quadramatic," employs unusually high amounts of feedback to reduce distortion and improve linearity of response within the audible range. Each channel has two voltage amplifier stages and each stage consists of both halves of dual-triode tubes (type 12AX7). Normally, such a tube is used as two stages, each half serving an independent function. In the 402-C, the halves of the dual-triode are connected in parallel, so that it performs as one ultra-big or "super" triode tube. In addition to permitting high gain, this type of hook-up provides automatic compensation, without the need for adjustments, to help maintain original levels of tube performance despite aging. It also serves as a fail-safe provision in case of deterioration of a tube element. A similar approach is found in the amplifier's power supply, where two full- wave rectifiers (type GZ34) are used instead of only one. Normally, the GZ34—with two anodes—is connected to serve as a full-wave rectifier; here, the two anodes are tied together to serve as one.
This amplifier clearly has been built to a very definite "design philosophy"—which was confirmed by tests conducted at United States Testing Company, Inc. In genera], the 410-C shapes up as a high-powered amplifier offering interesting, and in some respects superb, performane. Power output vis-à-vis distortion was excellent: the wattage-per-channel exceeded the manufacturer's own rating, and harmonic distortion at mid-frequencies was too low to be measured accurately. The IM characteristic was generally quite superior, except for a rise at the 20-watt output level; even so, IM was practically immeasurable up to 10-watts output and was comfortably below the 0.5 per cent level at the amplifier's full rated output of 40 watts. The 402-C's damping factor was a whopping 181, which is the highest yet measured on any amplifier, back in 1965". This tube amp used very advanced circuitry for it's day..which was at the end of the Golden era of tubes. It was designed to compete with the mighty Marantz 8b. It had superb bass response, and 96db signal to noise ratio. The 4 12AX7's self biased..as did the 4 El-34 output tubes. It is conservatively rated at 40w per channel. Used matching Model 112 preamp(8 triple triode tubes!) This is truly one of the almost unknown and unsung all time greats. Rare..Dizzy Gilisibe owned Quadramatic products...THEN, decided to advertise for them. I would not trade this for a Marantz or McIntosh. I use Telefunken smooth longplate 12AX7's for incredible sparkle. GE fat bottles for outputs. I drive mildly modded Martin Logan hybrids w Klipsch RW 12 front firing sub. Mids are to die for..now that have I have correct phase on sub and crossover about 100hz to the sub.
Great tube Amp. The sound is smooth, directed and clear. After extended play (20 minutes), the sound adopts a mellow, sweet, smooth overlay that contains and makes fluid the sound coming from the source. I have used it for 8track tapes, reel to reel tape players, CDs and cassettes and the tube amp delivers a pure blanket sound as if ignoring the input source technology. Quite impressive for tube technology.
The Amp's front display supports 4,8, and 16 ohms speakers. You can also tamper with speakers with different ohms running at the same time. The amp is well designed. It sits well and quietly generates a sound power flow that is 'giant' in appeal.
On the flip side, the amp generates a lot of external heat. I play it in the Winter. However, the longer it is on, the better (mellow, fluid, engaging) the sound. Also, the front plate shows the model as 402C and the back label shows it as 410C. The casing can be easily removed to show the simple yet genius design.
On a cold Winter's night, the 410C can deliver the perfect acoustic environment....suggesting an unforgetable, interesting, superb, performance. It puts the memory to rest and envelopes the listener, front and center in the music.