Radial Engineering PowerTube

500-series Tube Microphone Preamp

By Phil O'Keefe

Radial Engineering is a company that is widely respected in the pro audio community for making solid, reliable and great sounding products that perform exceptionally well at the kinds of tasks engineers and audio pros need to tackle daily. They've also been heavily involved in the highly popular 500 series market, and provide a wide range of different 500 series racks and modules for a variety of purposes. Today we're taking a look at one such module, the PowerTube single channel tube microphone preamplifier.

What You Need To Know

• Built in Canada, the Radial Engineering PowerTube is a true 100% discrete, high-voltage class-A tube microphone preamp. It is not a "starved plate" design. It uses a 12AX7 tube that has been selected for low-noise, and hits it with a full 140 volts, courtesy of a charge pump that increases the lower voltage supplied by the 500 series rack standard to properly power the tube. Since 140V is relatively low compared to a 12AX7's 300V rated maximum, tube longevity should be excellent in this unit.

• A front panel balanced XLR mic input is always a handy feature, and the one on the PowerTube is wired in parallel with the one on the back of your 500 series rack. Radial sticks with the AES standard pin 1 ground, pin 2 hot wiring configuration.

• The PowerTube also features a ultra-low noise Jensen JT-115K-E input transformer, and this high-quality "iron" is no doubt a contributor to its smooth sound. An additional class-A gain stage post-tube stage brings the levels up to the expected +4dB professional line level output. I appreciated seeing a block diagram included in the manual, as it allows you to quickly grasp the signal flow through the unit.  

• The PowerTube is a single-width 500 series module designed to work with 500 series racks and power supplies. Radial Engineering also makes rack and power supply units, as do other companies. I tested the PowerTube with my trusty Radial Engineering SixPack.

• There are two main knobs. The Trim knob adjusts the input sensitivity feeding the tube and class-A gain stages. Higher settings on this knob result in more character and harmonics. The Gain knob controls how hard you're driving the output. Just over 60dB of gain is available, which is plenty for most close-miking applications, even when using ribbon microphones, although you may occasionally wish for a bit more when recording very soft sources at a distance with some low-output microphones. Under most circumstances, there's plenty of gain on tap, and it's easy to dial up the amount of character you want, from clean and clear to quite colorful.

• All the other standard mic preamp features that you'd expect are here, including a polarity invert button, 100Hz high-pass filter for removing room rumble and unwanted mud, and 48V phantom power. The phantom power switch is recessed, so you'll need to use a small tool to depress it, but the upside is that you won't accidentally engage or disengage it. A red LED illuminates when phantom power is active.

• The metering is pretty cool on this preamp. A single "floating" lit LED indicator moves around the ten segment tri-colored (yellow./orange/red) LED display. The meter covers the range from -20dB to +6dB, and is designed to simulate VU ballistics.

• An unexpected yet cool feature is the PowerTube's Air switch, which adds 2-3dB of boost to the mids and high frequencies. The front panel switch on the review unit is marked with a shelving filter graphic and says "5kHz"; the boost starts ramping up at around 1kHz, is up by about 2dB at 5kHz, peaks at about 8kHz at 3dB, and extends past 20kHz, having only lost a dB or so from the peak in the highest octave. The net effect is somewhat like the opposite of the high-pass filter; instead of removing lows, you're adding more air and presence to the highs when you engage this switch. This can be useful in some situations, and when it's not, you can turn it off with the press of a button.  

• When using the PowerTube with a Radial Engineering 500 series rack, the Omniport becomes available, and in the case of the PowerTube, the Omniport is wired to function as a 1/4" unbalanced instrument input. This allows you to use the PowerTube as a DI for recording instruments direct. Plugging into the Omniport automatically disconnects the XLR inputs. The PowerTube sounded excellent with my Ibanez SR1100 bass - full, fat and punchy, but with great definition too. There is no dedicated line input.

• The 12AX7 tube can be easily replaced if you're familiar with swapping tubes. Thoughtfully, Radial includes a second 12AX7 tube from one of their Tonebone distortion pedals "for fun", and it can give you a little different character and a bit more grit, but don't expect gobs of distortion like you'd get from a dirt pedal. Even when driving the meters into the red, the PowerTube stays relatively clean; driving it harder does give it more harmonics, overdrive and character, but not heavy saturation. Noise is somewhat dependent on whatever tube you have installed, as well as how hard you're driving everything, but I never felt it was unreasonable or obtrusive, particularly by tube preamp standards.

• Changing tubes will change the sound of this preamp, but as with many tube swaps, the results can be somewhat subtle. Still, high quality tubes do matter, and I was pleased with what I heard while playing around with some of my personal stash of old Mullard and Telefunken tubes. I would encourage owners to experiment!

Limitations

• It's a tube preamp, and as you'd expect of a tube preamp, by 500 series standards, it draws a lot of current - up to 235mA maximum. You'll need to make sure that the 500 series rack / power supply that you have hosting the PowerTube has sufficient current capacity to properly power it, along with whatever other modules you might be running. This is one of the things I love about the Radial SixPack that I purchased last year - with its 1,600mA power supply, I don't have to worry about not having enough current and hearing things sag, regardless of what combination of modules I have installed.

• Still, when running multiple PowerTube preamps in any 500 series rack, even one of Radial's, you should use one of the PowerTube's "hidden switches." On the back side the module are two switches, one on each end of the card edge connector. The one on the left is a "power assign switch" that allows you to select either the positive or negative power rail of your 500 series rack's 16V power supply. This lets you run multiple units and balance the power draw. When using a single PowerTube preamp, you don't have to worry about it.

• The second hidden switch is a ground lift switch. The limitation here is that lifting the ground will disable the 48V phantom power. I'm glad that Radial includes this feature, since it's easily possible to run into ground loops when connecting an instrument or feeding the PowerTube a pre-recorded track for tube processing. Under those circumstances you wouldn't need the phantom power anyway.

Conclusions

Radial Engineering continues to impress me with the quality of their products, and the PowerTube is definitely no exception. Like all of the Radial gear I've had the pleasure to test and use, it's solidly built. It has a buttery smooth - dare I say "vintage" - sound that works great with a variety of sound sources, and you can drive it hard enough to give them a very appealing warmth and the kind of overdriven harmonic character that good tube preamps are famous for. The Air switch allows you to add a little excitement to the highs and help them cut through when needed, and the DI sounds fantastic too. While you may run into situations where you might long for just a bit more gain if you record quiet sources at a distance with ribbon microphones, pairing this preamp with a good ribbon and confident source yields a rich, detailed sound that I found extremely appealing. It works equally well with a condenser microphones too, and I loved the sound of it when paired with my Soundelux ELUX 251 for vocals. This is a true high voltage tube preamp, and it works and sounds like it. If you'd like to add a little tube color to your 500 series rack, it is something you owe it to yourself to audition.

Resources

Musician's Friend's Radial Engineering PowerTube tube microphone preamp online catalog page ($800.00 MSRP, $699.99 "street")

Radial Engineering's PowerTube web page

PowerTube manual (PDF format)

Radial Engineering PowerTube Specifications

Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Senior Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.