DETAILS

A magazine reviewer once wrote the following about the Triode-Tetrode switching version of Manley 250's: "In triode mode a pair of Manleys produces a remarkably three dimensional soundfield that is both convincing and seductive. When switched into tetrode mode, the Manleys have the brawn to deliver a full symphony orchestra directly into your living room." "When I listened through the same cut through the triode mode tube amps, I cried. Yup, tears ran down my cheeks while I bounced up and down, waving my arms, physically buffeted by the raw emotion of the music. Emotion- that essential component at the very core of great music. The Manleys pass through that emotional information, but the (solid-state amps) don't." Tried and true: The Manley 500 and 250 Watt Monoblocks are our most powerful push-pull production-models. Both the circuit design and the selected componentry have been refined over years and years of development. Reliability that you can depend upon combined with musical accuracy and emotional authority is what these amplifiers promise- and deliver. They are able to switch operation modes allowing the listener to choose between the sonically seductive qualities of triode and the more powerful Ultra-Linear configuration. We use big, beefy reservoir capacitors in the high voltage supplies giving plenty of instant energy for dynamic performance of transient peaks and bass impact and weight which often exceeds that of rival solid-state amplifiers. We run high voltages on the plates of the output tubes but operate them at lower current which will result in their longer life. Although each output tube has its own bias-adjust, We carefully fit each amplifier with computer-batched tubes for optimum performance. The bias adjustment pots and measuring points are conveniently located behind the oval front panel insert.

To avoid a heavy power drain when the cold amplifier is first switched on, we conceived a "Soft-Start" mode which also functions as an "Ever-Warm" position allowing the amplifiers to be always warmed up for pleasurable listening. Each amplifier only consumes 30 watts of power in Ever-Warm mode. A blinking green LED reminds the user that the amplifier is in warm-up mode because as all the power supplies are at half-voltage, the amplifier will still operate at half power when the Ever-Warm mode is engaged, so the user should use the MUTE switch when in this mode.

The front-panel mute switch and angled-back chassis make it easier to hook up the interconnects, speaker cables, and IEC power cord.

In our own in-house magnetics department, our R&D team completely redesigned all of our output transformers in 1998 with the goal to reclaim that luscious rich mid-range of our vintage designs. In 2013 we reworked the output transformer to operate the output stage in Ultra-Linear mode which is a great sounding balance between the detail of triode and the brawn of Tetrode. We also specifically set out to achieve a deeper bottom register at higher power with lower distortion. Hours and hours of listening tests, measurements, and fine-tuning brought an exciting and stunning result to our ears. We hope you will agree.

RESOURCES

How do I bias my amplifier?

All of the sudden my amplifier is producing no sound! I double checked my cables and sources. All the tubes are reading normal bias. I exchanged the input and driver tubes with the other channel and this amp is just dead. But why?

OK, you have no sound on one of the amps, but the tubes are biasing. That tells us the output tubes & output stage are working. First step is to make sure that the BALANCED/UNBALANCED switch is set to the appropriate position. Second step is to exchange the signal cables coming from the preamp from right to left to ensure the unit is actually getting signal. If it has a bad input or driver tube(s), it will be silent as well. Exchange the input and driver tubes one at a time with the other amp.

(Customer reported back that the R58 20kOhm 5W WW resistor feeding the input tube’s B+ keeps overheating and dying. But why?)

There really isn't too much to this circuit. That the resistor overheats and fails would mean 1) Shorted or leaky 33uf 450V capacitor at the input tube’s anode resistor; 2) Shorted 12AT7 tube- unlikely that it could handle this kind of current, but possible; 3) Shorted pc board trace or tube socket

Scenario #1 is most likely. Measure the DC voltage at the (+) end where the cap connects (it is probably lower than the 287V shown on the schematic). Turn off the power, bleed down this supply, and lift the (+) lead of the cap. If the voltage goes up, the cap is bad. If it stays low, pull out the tube, then look for shorted traces or burned pc board. I believe the diodes he's talking about are the 3 series zeners that are regulators for the input/drive stage. If one or more are shorted, all of the supply voltage to the input and driver will be low; if they are open it will be high. Neither condition will cause the 20K resistor itself to fail.