Tube Power Amp Reviews

Owning a powerful tube amplifier is like owning a classic automobile. Great pleasure may be had, but ownership involves a little more care and maintenance than usual.

Jadis, an audiophile company specializing in all-tube amplifiers and operating out of a small French town, has enjoyed a good reputation for some years, even if some of its models have suffered from the reliability problems that occasionally afflict the largest tube amps. Another problem area is that of power consumption and heat output. In common with class-A amplifiers and high-bias A/B types, including solid-state models, larger tube amps give off substantial heat. The Defy-7's 240W idling consumption may or may not be welcome, according to your location and the season.

Your $6490 (give or take some lengths of good audio cable) gets you a massive stereo chassis in polished chrome weighing a hefty 70 lbs. A protective perforated cover is provided for the relatively fragile tubes. (Some thought may be given to whether this cover should be removed in families with young children. The tubes run too hot to touch; if broken, they provide access to dangerously high voltages.)

Power enters via a detachable 3-pin IEC connector; the speaker terminals are doubled up to facilitate biwiring, while the inputs are unbalanced, gold-plated phonos. For proper ventilation and control of microphony, the Defy-7 cannot be placed directly on a carpet unless on tall insulating feet; or, ideally, on a masonry slab itself placed on anti-vibration feet.

Technology
Beneath the Defy-7's glossy, polished exterior lies an object of mystery and wonder. Though apparently a simple, traditional tube design, some oddly modern, solid-state details have been bolted on to the undercarriage of this ostensibly all-glass amplifier. There are no solid-state devices anywhere in the signal path, however.

The generously dimensioned output transformers are equipped with terminal blocks which allow the use of the whole of the secondary winding in a range of combinations, much like those in the early Radfords. These combinations are selected by moveable straps to adjust the load matching and thus achieve maximum loudness and quality with a range of speaker loads. A particular feature is the 1 ohm setting, while others include 4 (to 8), 8 (to 16), and 6+ ohms. The 4 ohm setting, defined for 4 to 8 ohms, was the setting supplied and used on test. (Access is below the bottom cover; the amp should be disconnected from the supply and allowed to cool off for 15 minutes before any work is begun.)

A "low" output setting such as this improves the available output current and reduces the source impedance, if at the expense of maximum output level voltage. The 8–16 ohm tap may be used to advantage where the load is kind. Use of the 4 ohm tap—rated at a nominal 100W into a matched impedance—with an 8 ohm speaker will in theory reduce the maximum level by 2–3dB, to 70–50W (see fig.1 later). The 1 ohm setting in theory will produce only 12.5W of "level" into an 8 ohm load, but has a current capability sufficient to dump the 100W of output power into a 1 ohm loudspeaker load, such as the original Apogee Scintilla.

The input and driver circuits are long-tail–pair differential stages, direct-coupled from the input to their output. While pairs of ECC83 triodes, strapped to improve their power bandwidth, are used for the driver stage, in practice the signal passes through only three tubes before arriving at the output transformer windings. This is a beneficially short signal path.

An ECC82 double triode is present at the input, one tube per channel. An unusually high input impedance of 470k is defined by the input resistors. This loads the preceding source very little.

Two triodes, their cathodes each decoupled by an electrolytic capacitor, are used for each phase of the next stage, which is cathode-biased to +72V to match the preceding stage. The driver plate loads are quite high at 330k ohms; the plates are coupled to the output-stage grids via the usual 0.47pF polypropylene capacitors, these made by EFD of France.

The Defy-7's high output-stage gain helps reduce the demands on the driver stage; this keeps the whole amplifier relatively short and simple. The output stage operates in class-A/B "ultralinear" mode using General Electric 6550 beam tetrodes, three pairs operated in parallel to generate the necessary output. They are not overrated in this application, since the high-voltage rail is set to a fairly modest 500V. In theory, with a 750 or 800V supply one pair of such tubes could deliver 120W flat out; the use here of three pairs can be seen to be relatively kind to the output tubes. This also allows for a higher peak current and consequently a better load tolerance.

The output tube cathodes are partially decoupled, generating a degree of auto bias, but are also directly coupled into a feedback winding of the output transformer. The schematic on the transformer can is not representative, but the operating point for the tubes is primarily controlled by an unregulated DC bias voltage applied to the output tube grids.

In the "Ultralinear" connection the screen grid is coupled via a moderate-value resistor, 1k ohm in this case, to a tap in the primary winding of the output transformer. This confers two benefits: to begin with, it allows the screen to deliver some useful power; second, the local negative feedback it introduces lowers the high anode output impedance of the "pentode" from 12k ohms or more to less than 5k ohms, allowing a lower-ratio output transformer with improved bandwidth and coupling.

The "Ultralinear" mode is a selected transition point between pentode and triode operation for this class of output tube. As the screen tap on the primary winding of the transformer moves nearer to the plate, the triode connection is approached (ie, with screen and anode joined). On the other hand, as the tap moves nearer to the high-tension rail, less and less feedback voltage is applied, the feedback is reduced, and the pentode condition is approached. Somewhere in between—a 40–60% tap is common—the triple benefits of low output impedance, low distortion, and good bandwidth of the triode mode are combined with the higher output power and usefully high stage gain of the pentode.

Negative feedback is held to a moderate level and is taken from a connection at the speaker terminals. Its actual value varies according to the gain ratio defined by the chosen selection on the output transformer secondary.

The main circuit is built conventionally on a large fiberglass printed circuit board. However, other aspects of the Defy's construction leave something to be desired. For example, the heater supplies are regulated by a heatsinked TO-3–can device mounted in a relatively unventilated part of the unit beneath the mains transformer. As a result of this regulator's position and of its own heat, this transformer runs hot. Its can becomes almost too hot to touch in warm weather. And several large electrolytic capacitors, including the massive 1000µF 500V output stage reservoirs, are bonded—yes, glued!—with RTV silicone to the underside of the chassis. While it is unlikely that they would fall off, this and some other points indicate an amateurish approach to certain aspects of construction, surprising considering the number of years Jadis has been in the business.