Guest Post: How to Use Amps and Iso Cabs on Stage, Part 2

Kevin McPeakKevin McPeak is the Creative Director at EastLake Church in Chula Vista, California. He is a proponent of both tubes and digits.  You can follow Kevin on Twitter @kevinmcpeak or find him on Facebook.  Kevin has kindly agreed to lend us some of his expertise in technical issues and will be posting from time to time.

Part 1 is available here.

Q: I play electric guitar and love my amp sound, but we donʼt use amps on stage because of stage volume. Is there some way I can use my amp so it does not impact stage volume?

Well, first off, letʼs identify the problem, because that will help us find a solution. The reasons that our mix engineer wants us to keep stage volume under control are many:

  • stage volume blurs front of house (FOH) mix clarity
  • stage volume creates ever-increasing demand for volume from stage monitors
  • stage volume leads to “hot spots” and “cold spots” where instruments are either too loud or too soft
  • stage volume dramatically limits the FOH engineerʼs ability to create a thoughtfully and musically designed mix.

So letʼs be clear: amps arenʼt the problem in and of themselves; the problem is that amps tend to get treated like personal monitors and turned up too loud. Furthermore, many amps donʼt sound their best without running at a decent volume. How the heck do we solve the problem?

Letʼs say that youʼve already got a combo amp – in other words, a single unit that has both the amplifier section and the speaker – that you love and you donʼt want to purchase an external cabinet or haul around something big. What can you do then?

In our case, we have an all-tube Mesa Boogie combo amp that our electric guitar 2 volunteers generally use. Although the amp can power an external cabinet, the amp already contains a perfectly matched built in 12” speaker, so we asked one of our volunteers to construct an iso cabinet for the amp.

This is what he made:

Iso Cube front

Iso Cube back

As you can see, itʼs effectively a four-sided cube. (For those of you trying to figure this out, a cube is a six-sided object with equally-sized sides. In the case of the iso cabinet above, we have a cube with no bottom and one side missing.

Okay, itʼs really not a cube because it doesn’t have equal sides. But I slept in Geometry class the day we talked about six-sided objects with unequally sized sides, so you will just have to go with it being called a “cube.”  If any of you would like to educate me as to the correct term for a six-sided object comprised of unequally sized sides, Iʼm happy to listen. If I can stay awake.)

You can see that the idea is to point the business end of the amp into the box. The top and interior walls are lined with sound baffling material and the floor takes care of itself by being carpeted. (If your stage floor is a hard, flat surface, you may want to throw a small rug in there.) This all works together to create a tremendous reduction in the sound output from the amp.

However, there is a catch in this case, and itʼs a very important one: this combo amp has an open-back cabinet.

Front side:

Mesa Boogie Amp frontBack side:

Mesa Boogie Amp back

As you can see, the back of the amp is open. (Thus the name, right?) Many guitarists prefer the tone produced by open-back amps. Open-back cabinets create a challenge because the back of the cabinet emits quite a lot of sound; putting an isolation box on the front of the amp only solves part of the problem of lowering stage volume.

In our case, we solved this problem by placing the amp off-stage, behind a thick curtain, and against a Tectum wall.  You could also create a matching iso box for the back side of the amp to limit the sound coming off the open-back side.  Make sure, however, that you allow sufficient airflow in and out of the amp because the amp will overheat and fail without airflow.

Have another technical question you would like Kevin to answer?  Post your question in the comments.

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