Integrated Amplifiers

Manley Stingray iTube Stereo Integrated Amplifier


The Design

The iTube is an integrated tube amplifier. The input stage (voltage gain) is handled by two 12AT7 dual triodes, the driver stage by two 6414 tubes which are similar to 12AT7's but can deliver more current to the output stage than the 12AT7, and the push-pull output stage consists of eight EL84 pentodes. The power supply has two sections: an unregulated tube supply, and a linear, regulated supply for the logic circuitry.

In a push-pull amplifier, the + and - halves of the waveform are handled by two separate sections of the amplifier circuit. In order to do this, the input signal has to be split and one of the split sections inverted. This is accomplished by a "phase-splitter", and in the case of the Stingray, this is done in the driver stage, which is configured as a "long-tail pair", which is a classic tube circuit. Shown below is a generic schematic of the long-tail pair phase-splitter using a dual triode. The "long-tail" refers to the resistor at the bottom of the schematic. The phase split signal is then fed to the output stage. (I am indebted to my friends at DIYAudio in coming up with a general schematic for illustrating the long-tail pair.)


The output transformer only has one tap, which has been optimized for a 5 ohm load, connected to the five-way speaker binding posts. Manley has found this to be optimum, rather than having several output taps. There is a headphone jack (1/4") on the right hand side of the chassis, and an S-Video output jack (for use with Video iPods) on the left hand side. A Rec-Out and Loop Return (for recording), as well as a Sub-Out (for use with a subwoofer) complete the rear panel layout.


Speaking of transformers, the iTube is switchable between "Triode" mode and "Ultralinear" mode, using a toggle on each side of the amplifier, one for each channel. The EL84 output tube is a power pentode, but it is wired here either as a triode, or in ultralinear mode. The suppressor grid is always connected to the cathode, and the connection is inside the tube, permanently. This, in fact, confuses the pentode designation. The signal and bias go to the control grid, and the screen grid is connected either to the anode for triode mode, or to a dedicated tap on the output transformer for ultralinear Mode. This produces linearity (lack of distortion) that is part pentode and part triode.

Some consumers feel there is a difference in the sound quality between the two modes. I did not hear any difference, and in fact, the ultralinear mode bench tested a bit better, so I did nearly all of my listening in the ultralinear mode. You can see one of the toggle switches just above the Speaker Out posts in the photo above. (You can switch between modes while the amplifier is on, to compare the sound, but don't switch back and forth many times in a row, since there is high voltage running across that connection.)

Shown below is a generic schematic of how ultralinear mode is wired to the output transformer (red arrows).


Powering on the iTube involves a rocker switch on the rear panel. Then you press the blue button (the standby button) in the center of the front panel. After a few seconds (and some flashing lights for fun), the blue light in the center turns off and the amplifier is ready for use. However, it should be allowed to warm up fully (about half an hour) before any serious listening. Here is a video showing the power-on procedure.


The Stingray comes with a voltmeter for the purpose of setting the bias voltage (voltage applied to the grid) on the eight output tubes about once each year, and definitely if you change tubes. The process involves placing the ground probe (black) of the voltmeter into the grounded jack on the front of the Stingray, and the hot probe (red) in each of the eight jacks along the front right and left sides of the Stingray, one at a time, and adjusting the bias voltage. The voltage is adjusted by inserting the included small wooden screwdriver into the respective hole at the top rear of the Stingray (there is a row of eight holes), and turning the screw until the meter reads 250 mv. Here is a video demonstrating the bias adjustment procedure.

You can set the input level of each input according to the output of the sources that are connected to them (such as when one source has more output voltage than another). This is accomplished by first selecting the input you want to adjust, pushing the input selector knob in for 2 seconds and releasing it, rotating the input knob through its three menus (DIM, Input Level Adjust, Remote Control Configuration) and selecting the Input Level Adjust, then adjusting the input level using the volume control, and finally, pushing the input selector knob in again.

Secondly, there is what's called the DIM control, which lets you turn down the volume to a preset level for such things as having to answer the telephone. To do this, follow the same instructions outlined above, except that you select the DIM menu instead of the Input Level Adjust. Balance between left and right channels is achieved by pushing the volume control in for 2 seconds, releasing it, and turning the volume control clockwise to reduce the level of the left channel, and counter clockwise to reduce the level of the right channel. In all cases, to exit the various modes for changing the input level, the DIM, and chanel balance, push the respective knob in and release it quickly. Finally, you can adjust the brightness or "twinkle" of the blue LEDs by pressing both the input selector and volume control in for 2 seconds, releasing them, then cycling through the input selector positions, and adjusting each feature using the volume control. All of these features are controlled by a microprocessor.

The included remote control is very unusual. It looks like a "walkie-talkie" from the 1980's. Certainly, you won't have any trouble locating the buttons. You can select the input, adjust the volume and balance (relative volume of left and right channels), and navigate the menus of an iPod (to select albums and tracks to be played) when it is being used. It operates by your choice of IR or RF, so you can control the Stingray from another room. To select IR or RF, set the input selector to input 4, push the input knob in for 2 seconds, release it, and then turn the input selector to the 2 o'clock position for RF control, or 3 o'clock for IR. If both the 2:00 and 3:00 LEDs are illuminated, then the RF and IR modes are both active.