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Old June 30th, 2009, 09:40 AM   #15 (permalink)
Stewart Ward
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Join Date: Jun 2008
Location: Basingstoke (BAzingstoke), Hampshire, UK
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Quote:
Originally Posted by Canuckcaster View Post
I have the AVT 100 watt heritage version. I have yet to play it out, but I love the tones that I get out of it at home. It matches up well with my Les Paul of course, but it sounds great with my Tele and 335 too. It has a clean channel, and 2 levels of OD. Great tones on all 3 channels. I can't justify a 1/2 stack so it's all the Marshall I'll ever need....unless the 40 watt Haze combo changes my mind.

CC

One thing that is seriously against the AVT series (100 & 150W), is the fact that they use the TDA7293 power amp chip from Thomson. They are 100W mosfet chips rated with a clean sinewave signal passing through them. Fine for TVs and hi-fis where the signals are always clean. The MODE 4 amps employ four to get the 350W output! Stay clear of any of these amps on reliability ground if you're seriously gigging at moderate volumes!

As we techies should know very well, guitar signals are rarely sine wave. The square wave signals generated means, at high power output, the waveforms being amplified will result in the average power of the chip far exceeding its 100W RMS sinewave rating. DANGER!!!

The mounting 'tab' of the chip which sinks the heat inside the chip to a heatsink to keep it running cool is only about 18mm x 9mm. That's no where near big enough for a guitar amp!! It acts like an hour glass where the heat cannot excape to the heatsink quick enough, and the mosfet die inside just overheats and fries after a while.

Marshall, mistakenly, have put a fan there to help out... but the real problem is the mounting tab being too small for the job, as already stated. So, you can bury the chip in an iceburg and IT WILL STILL OVERHEAT.

A 100 watt SS power amp with poor heat sinking will be most likely to fail whilst running at around half power. 50% power is when they get the hottest! (You can't tell this from the volume settings thought!) With SS devices, it's important to keep them running cool for a good long life. It's well known by power amp designers that letting the power transistors run hot seriously shortens their life.

Looking at auto motive electronics, the voltage regulators in 'some' cheap alternators fitted to cars are allowed to run hot... but only hot enough so they fail just outside the warranty period. The makers of those voltage regulators provide temperature/life derating curves so the designers can 'calculate' their expiry date pretty accurately! It's just the same with SS power trannies.

So, invest in a high quality PA with good heatsinking, then you can expect a lifetime of reliable service. Well designed SS amps are NOT that much cheaper than a valve equivalent. The money should go into keeping them running cool.

BTW, look how it's just the chip legs supporting the weight of the PCB and the thick heavy wires hanging off it! There's a real risk of the legs sheering off under the strain and vibration.

The photo below is of the 100/150 watt AVT.
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Last edited by Stewart Ward; June 30th, 2009 at 11:42 AM.
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