The great grandmother of the Ultravalve amplifier is the original Dynaco ST-70, no big surprise. We started taking a close engineering look at the ST-70 in the 1970s and were able to identify several design weaknesses that could be fixed at a rational cost. These included getting the bandwidths of the power supply, audio circuits, and input acceptance into the right set of ratios so that no input signal short of output clipping could drive the unit into gross distortion at any frequency. This made the feedback loop job much easier and the whole amp cleaned up a lot. We also added a solid reference to the screen grid resistor and achieved even better stability.
Later, we came up with some unique, successful power supply design ideas which we first implemented in other products. Learning more, we incorporated these concepts into a brand new design, the Ultravalve.
To offer you a unique high quality, rationally priced pure vacuum tube amplifier, we designed a user friendly polished stainless steel chassis and added a silver double anodized faceplate.
The Ultravalve has premium input jacks, 5-way binding post output jacks on 3/4" centers for 4 ohm, 8 ohm, and 16 Ohm taps, a line fuse, and an standard IEC power jack and AC power cord. The chassis top has small pin jacks for bias adjusting and the bias adjust controls.
The chassis is 13" wide, 10" deep, 7" high. Shipping weight 31 pounds. To protect the vacuum tubes, the Ultravalve is shipped in custom made cocoon packaging.
The audio circuits of the Ultravalve are pure tube, with no solid state devices in the audio path. The power supply starts with a custom made power transformer that has reserve capacity. The power supply uses a diode protected 5AR4 rectifier tube.
Unique to AVA vacuum tube designs, the Ultravalve features independent regulated high voltage power supplies for each plate of each small signal tube and an additional regulated power supply for the bias circuit.
Like all other AVA products, we select appropriate parts for each task, all operated well within their ratings, to insure that you will have years of trouble free operation.
The Ultravalve operates with standard 120 Volt power, has an IEC power jack, and includes a polarized 120V power cord. For international use, an optional 240 Volt (50-60 Hz.) transformer is available at additional cost.
The audio circuits are built on a ground plane shielded double sided circuit board. The power bandwidth is 3 dB down points are 5 Hz and 50,000 Hz and maintain our goal of no feedback related overload under any condition of use. Engineering math provided us with excellent solutions for the bandwidths of the feedback and coupling circuits.
The Ultravalve uses two 6GH8A small signal tubes and four power output tubes. We supply matched quad sets of EH 6CA7 output tubes, but EL34 or KT77 tubes can be used too. Power is 35 watts per channel, but you can use it where you would normally use a 100 watt solid state amplifier. It does a great job of driving our 85 dB efficient speakers full range to reasonable levels. This is one amazingly wide band, dynamic, transparent and utterly musical amplifier. It just does not have typical vacuum tube power amplifier limitations.
The whole thing, except for some active and passive electronic parts, is made in the U.S.A.
The Ultravalve amplifier will require, easy to do, periodic tube bias adjustment, and occasional tube replacement. AVA stocks matched tube sets for the Ultravalve under the Accessories page on the AVA Web site.