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Is that the vacuum tube glowing? Or yellow LEDs behind it?

A microphone preamplifier features a vacuum tube. You would expect the tube to glow wouldn't you? But perhaps it isn't even connected...

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I've always been amazed at Behringer equipment. It mostly works pretty well, and the price is extraordinarily low. And Behringer gear normally comes with an internal power supply therefore no nasty 'wall wart'. Granted there has been some upset in the audio manufacturing industry over some products for other than technical reasons, but the bottom line is that I have always found Behringer to be good value for money.

However, I don't like it when anyone tries to pull the wool over my eyes. If someone is testing me personally, and I fall for a convincing prank, then that's one thing. But when everyone buying a particular piece of equipment is being duped, that is quite another.

Here's the story. An engineer who I often work alongside recently had cause to take the lid off a Behringer MIC 2200 Ultragain Pro two-channel microphone preamplifier. Its claim to fame is that it incorporates a 12AX7 dual triode vacuum tube to 'warm up' the sound.

It looks like this from the front...

Ahh... the lovely glow of a vacuum tube, warming up my beautiful music. How wonderful it is to switch on a piece of equipment and see the tube gradually come to life with its soft, unique, translucent light.

Except...

Let's lift the lid and see where this glow is coming from. This is where the glow comes from...

Lovely tube glow? Or not...?

Three yellow LEDs positioned directly behind the tube. And yes, they ramp up in brightness a couple of seconds after switch-on.

Is that, or is that not a con? Plain and simple. We are encouraged to believe that the tube is warming our sounds by a trio of LEDs - which by the way are semiconductors, just like transistors - not only strategically placed but timed to mimic a tube warming up.

The thought did cross my mind whether the tube was actually connected into the circuit. There are good reasons not to connect the tube but to incorporate it purely as a decorative item...

  • It won't wear out. And even if it did lose its vacuum or become inoperative in some way, it wouldn't affect the working of the preamp if it wasn't connected.

  • All good electronic designers 'design out' the characteristics of the active devices so that variability among components does not affect the parameters of the circuit. What better way to design out a component than not to connect it in the first place?

My feeling is however that probably the tube is connected but the parameters within which it is running are not sufficient to activate any of the several mechanisms that make a tube glow. And if it doesn't glow, what would have been the point of putting it on display? Hence the LEDs to compensate. Perhaps they were an afterthought.

But if the tube is being run conservatively, maybe it isn't producing all that much warmth?

Ultimately, the Behringer MIC 2200 Ultragain Pro is still good value for money. Behringer should lose the LEDs though.

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By David Mellor Thursday September 22, 2005
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