Усилители мощности моно — VTL MB-185 Series III Signature

The MB-185 Series III Signature is a mono tube power amplifier from VTL of the United States, made to order, with delivery taking roughly three months. It follows the MB-450 Series III into the Series III platform, with the whole signal path reworked, while keeping automatic tube biasing and fault sensing from the earlier generation. Owners of the Series II version can have it upgraded at the factory.

Key specs that matter

Rated output is 225 W in tetrode mode and 140 W in triode mode into a 5 ohm load setting, from 20 Hz to 20 kHz at below 3 percent THD, and the output load range is 4 to 8 ohm. Input sensitivity varies between 1 and 2 V according to the chosen damping factor, while the input presents 45 kilohm. Noise is specified at -90 dB, 120 Hz. Mains draw is 270 W at idle and 550 W at full power. The primary fuse is a 12 A slow-blow type for 100 or 120 V and 6 A for 220 or 240 V, and the B+ circuit uses a 1.5 A plate fuse plus a 0.75 A screen fuse. Each unit is 47 cm wide, 45.7 cm deep and 23 cm high and weighs 38.5 kg without packing.

Engineering and use context

The Series III revision brought a switch to output valves of the EL34 family, and each monoblock carries eight EL34 or 6CA7 tubes with one 12AT7 and one 12BH7. A fully balanced differential input section is coupled to a differential phase splitter, which in turn feeds a push-pull output stage of lowered impedance, so even a single-ended input signal leaves the amplifier balanced. No global negative feedback is applied, and a shorter local loop needs no capacitor compensation to stay stable. The VTL Signature output transformer was redesigned for these tubes, with more interleaving and coupling of its windings. A Damping Factor control with LOW, MED, HI and MAX positions changes the output impedance by varying the amount of feedback. Screen and bias supplies are precision regulated and adjustable, which keeps the output operating point fixed when the AC line varies, and the autobias system removes the need to match output tubes. Mundorf silver oil capacitors are fitted in the audio circuit, the supply capacitors carry film bypasses, and a Standby Mute mode lowers tube current to extend valve life. The extruded aluminium fascia comes in black or silver.

Pairing

Stereo playback calls for two units, and the balanced input can be fed from the balanced outputs of the VTL TL-6.5 Series II Signature. The damping control helps set the amplifier's grip for loudspeakers within its rated load range.

Alternatives

The VTL MB-125 is the smaller monoblock in the Performance range, delivering 100 W in tetrode and 50 W in triode from four EL34 valves per unit.