Select 12AX7 Tube

Radial Workhorse WR8 with 8 PowerTube Modules

Jensen JT-115-KE Transformer

Powertube and the PowerTube prototype


PowerTube™ development

When we set out to develop the PowerTube, we asked ourselves many questions. These included:

  • What kind of a tube should we use?
  • Will there be power issues with 500 series power racks?
  • Should it have a transformer?
  • How big should it be?
  • Does it need to have a front panel XLR?
  • Do we need a VU meter?
  • Should we have any form of EQ on it?
  • Will engineers use it for tracking instruments like bass?
  • And most importantly... What should it sound like?

To describe the development, the easiest roadmap is to simply follow the questions that lead to our getting the product made.

Selecting the 12AX7 tube

When we first started looking at a tube preamp, we began by asking ourselves what kind of tube would be best. We looked into NOS (new old stock) tubes that could maybe do the job and quickly found out that no matter which direction we took, stock was limited. Being able to source tubes without hindrance is essential when considering putting a product into the market. Just imagine if two years from now, the tube is no longer available... then what do you do? We also thought long & hard about after sales service. What happens if you are on tour in Brazil and need to pick up a tube for your PowerTube? Where do you get it?

After speaking with the guys who build our Tonebone distortion pedal tubes, we discovered that you can actually order select 12AX7s. These tubes exhibit less noise and distortion than the conventional ones and are set apart and sold at a premium price. The 12AX7 is the single most popular preamp tube made today and will likely be the one tube that will be on the market forever. This set the direction for the all important tube drive circuit in the PowerTube.

Power considerations

One of the biggest challenges with the 500 series format is limited power. The original API spec called for an average of 130 milliamps per slot. Folks complained that this was too low. So when we developed the Workhorse, we increased it by about 20% to 150 milliamps (average power per slot) with 1200 milliamps of total available power. Considering most modules draw between 30 and 130 milliamps, this enables engineers to mix and max modules without concern.

But with tubes... the current draw is much higher In fact most tube processors require between 200 and 300 milliamps to work. With the PowerTube, we need about 225 milliamps whereby 125 milliamps is used to heat the 12AX7 heater (filament) and another 100 milliamps to power the audio circuit. This is no problem when creating a channel strip using a Radial Cube or Powerstrip as the preamp is normally used alongside an EQ and a compressor. But when you stack a bunch of PowerTubes inside a Workhorse, you can run out of juice!

To manage the power demands, our engineering team came up with an innovative scheme whereby the source power going to the heater can be pulled from either the +16V or -16V rail by simply moving a miniature slider switch into the up or down position. By alternating the switch, you can balance the draw from each side of the power supply and use more units. This allows you to safely use as many as six PowerTubes in a Workhorse or more if you do not have the mixer option inside.

Transformers are amazing

Most audio engineers proclaim their love for all things vintage. But what few realize is that the 'sound' of vintage gear often has very little to do with the tube alone. Most vintage audio devices employ transformers and these play a significant role with respect to the silky smooth top end and warm punchy bottom. We asked ourselves... how cool would it be to combine a truly amazing transformer with the sound of a tube?

As Jensen's largest customer it was not hard to make a choice on what kind of transformer we should use. Jensen wrote the book on transformers and today are considered to be the absolute premium producer world-wide. We chose to use a Jensen JT-115K-E microphone input transformer to handle the task. This nickel-alloy transformer features an unprecedented frequency response that actually spans from 2.5Hz to 90kHz (- 3dB at the end points) with less than 0.065% distortion at 20Hz. Considering how noisy tube circuits can be, we loved the fact that the JT-115K-E is equipped with a MuMETAL® outer can to protect against stray magnetic fields – a problem when tubes are brought nearby. And when connected to the outside world, with better than -110dB of common noise rejection, you can sleep comfortably knowing that your PowerTube will behave quietly.

Size does matter!

When the first prototype was brought to table, company president Peter Janis exclaimed: "No! This cannot be a double-wide module... it has to fit into a single 500 series space." The engineering team quickly retorted: "It cannot be done! There are simply too many parts!" Six weeks later, the first single space tube preamp was on the table.

Janis continues: "The 500 series is all about keeping things small and being able to maximise space. Although a double wide would have been easier for powering, and of course with larger space, one could me easily eliminate noise, I felt that investing more time to come up with a single space design was key. The engineering team did a remarkable job to achieve this goal. And best of all, they did it without reverting to SMT (surface mount technology) computer parts. For years, audiophiles have been saying that bigger parts sound bigger. I felt that with a tube preamp, this was not time to move to a more clinical design."

Like all other Radial products, the PowerTube is made in Canada and employs 100% discrete parts. Each electronic part is hand inserted into the PCB, then wave-soldered before large parts like the tube and transformer are added manually.

A functional layout

Another choice we made was to incorporate a front panel XLR input. Most manufacturers do not do this as it takes up valuable space on the small front panel. But we felt that with so many artists and engineers now producing music in the comfort of a home-based studio, making it easy to connect a mic for a quick overdub was essential.

We also chose to recess the 48V phantom power-on switch so that it cannot accidentally be turned on. The inconvenience of using a pen to turn on phantom power was a good trade off versus blowing your favourite vintage ribbon mic or damaging a tweeter. We also chose to add a 10 segment LED meter where we could have had a simple LED overload switch. But given that the PowerTube will not easily overload unless driven real hard, we thought it would be nice to know when the signal is truly clean.

Controlling the gain

There are several 'philosophies' when it comes to gain structure. Some manufacturers use a switched input attenuator and a 'fine tune' control... others may take a different approach such as the PowerPre which employs a custom dual potentiometer that sets the sensitivity and gain simultaneously. With a tube preamp, particularly one that employs a transformer coupled front-end, these options may not be suitable. We chose instead to give the engineer a fully variable trim control to set the input sensitivity and then a drive control to set the overall gain output. This follows a similar approach to the way a guitar amp is set up whereby you can increase distortion by increasing the input sensitivity and driving the preamp harder into the red. We encourage you to try it.

We then incorporated an 'air' switch to accentuate the top end to accentuate the breath of a voice or the shimmer of an acoustic guitar. Unlike an EQ that can be added after the fact, the air actually occurs inside the preamp. This changes the character, particularly when driving the circuit hard. A high-pass filter was then added to reduce low frequency resonance which can often clutter up a mix. These handy features can of course be bypassed when not needed.

Using the PowerTube with instruments

Folks love the sound of tubes. They impart a rich character that is simply unattainable with any other type of tool. We also know that tracking instruments such as bass in the studio is often done in the comfort of the control room. With the PowerTube, the Omniport jack has been designated as an instrument input. This has been optimized for magnetic pickups to make them sound warm and natural. The artist simply 'plugs in' and records using the nearfields as monitors.

Omniport – a feature we developed with the Workhorse – allows the module manufacturer to assign a ¼" TRS jack to do anything. We chose to optimize the input to work with an electric bass or guitar. You simply plug in the back of your Workhorse, Cube or Powerstrip, and the PoweTube will automatically turn off the XLR mic input so that you can hear the instrument. Simple – efficient.

So how does it sound?

Nice. (What can we say... we built it!)