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Any love for the Marshall AVT's?

Discussion in 'Amp Central Station' started by ProToneThinline, Jun 24, 2009.

  1. ProToneThinline

    ProToneThinline Tele-Holic

    Jun 27, 2005
    Pittsburgh, PA
    Anybody else using one? I picked up an AVT-20 about a year ago. GC was blowing them out pretty cheaply ($200 brand new). I played it for a while with my Epi Dot, and thought is was an okay amp, but nothing spectacular.

    Well, about 2 weeks ago, I dug it out and plugged in my Tele. I never thought a Tele and a Marshall were a good match, but what the heck. WOW - did I ever get my eyes/ears opened. The AVT sounded really good with the Tele.

    Since I'm generally a "tinkerer" anyway, I opened up the AVT, and swapped out the stock tube for an old Amperex 12AT7 I had in my stash. The amp now sounds incredible. The Amperex has a nice warm sound, but still has some bite. The solid state power section sounds very much like a pair of EL-84's.

    I've gigged with that set-up a couple of times now, and it's worked out really well. I put the amp on a stand set in front of me. I use the DI out on the amp to run the signal to the mains. I get the tone I want without blowing our singer off the stage. We do a lot of small clubs, and even my DRRI tended to be a little too much amp. The AVT seems "just right". It's not too loud, but loud enough that I can hear myself well. It' s got a really nice "edge of breakup" tone, and is voiced to sit in the mix really well.

    Who wudda thought :lol: Too bad Marshall discontinued these.
    MDZimmerman likes this.

  2. BluezyBruce

    BluezyBruce Tele-Holic

    Feb 16, 2009
    Ontario, OR.
    I have no idea why Marshall discontinued the amp. I have always thought the Valvestate, AVT's where a good sounding amp for a solid state amplifier.

    I was going to mention changing out the stock 12AX7 for a better quality tube.

    For an inexpensive amp they work good.

  3. Michael Bolt-on

    Michael Bolt-on TDPRI Member

    Sep 4, 2008
    I always thought of rockin' an AVT 20 for a practice/jam/recording amp. They are built quite solidly for what they are, and made in the UK unlike the MG all SS combos(meh).
    The AVT 20 that I tried really cranked out some serious volume! Pretty unreal in that regard. At the time of testing the amp I was in one of my metal phases and I wan't quite sure if the low-end in the amp was everything I was hoping for. Go figure because now I have a Gibson GA5 tube combo for my amp, which isn't a metal amp at all as my likes have changed a bit. Hmmm, those AVT 20's surely look the part, if I could only test one out again to get a better opinion.

  4. littlewing6283

    littlewing6283 Tele-Meister

    Jan 1, 2009
    Southern Cal
    finally ! these amps have a really bad rap but i love mine. its just too loud for the apartment so im using a vox pathfinder 15r right now. mine is about 8 years old

  5. ProToneThinline

    ProToneThinline Tele-Holic

    Jun 27, 2005
    Pittsburgh, PA
    Yes, in the O/D mode the amp can get REALLY loud for 20 watts and a single 10. I don't know how well it would do metal, but it sure has the classic rock thing down.

    I run mine clean, and get my O/D from my pedals. I play Soul/R&B/Blues, and was surprised at how good the clean sound is.

    Several years ago I had a VS65R, which was an okay amp. Good cleans, but just okay overdrive. The clean on the AVT20 sounds better to me than the VS65. Plus, I really like the speaker emulated DI on the front panel.

    It's just a surprising little amp that fits for what I'm doing. Interestingly enough, I played through a JCM800 a few weeks ago (set for clean). The AVT20 sounds very similar to that monster 800.

  6. Nash

    Nash Tele-Holic

    Mar 8, 2009
    Fullerton, CA
    I have the predecessor to the AVT series and it's a great sounding amp. I'm selling it now because I bought a classic 30 and it's too big and heavy(80w 2x12) for my uses. If I didn't need the money I'd be keeping it because I love the sounds I get out of it. Many people put 'em down because it's solid state, but if they played one they'd see how good the amps really are. I'm not sure how much of the sound comes from the tube because it isn't even used on the clean channel, but it's a good amp that's very well built.

  7. guitarzan13

    guitarzan13 Friend of Leo's

    Jul 25, 2006
    LaGrange, GA
    I gig the big shows with an AVT 50 stack.....I LOVE it!!! But, I only use the clean side... with pedals of course...

  8. fabiomayo

    fabiomayo Tele-Meister

    I had an AVT 20 for years in my bedroom... It was the only time I practiced every day. So simple! Plug it in, good clean tone, press a buttom, good drive tone, late at night? plug in your headphones. Everything worked, noise was minimal. I sold it for dirty cheap after I got my first tube amp.

  9. The reason this range sounds better, is because they all have current feedback in the SS power amps - FDD or Frequency Dependent Damping as Marshall call it. On some Marshall amps you can switch it off too, so you can actually sample the difference it makes!

    This effectively (and exactly) replicates the unique properties of an amplifier equipped with an output transformer (OPTX). Note I did not specify a valve amplifier with an OPTX, because the OPTX affects the sound (tone) of any amp beit valve or SS (tranny) in exactly the same way.

    However, most valve guitar power amps are 'naturally compromised' by a narrowing power bandwidth as the output power is increased. This happens in the power transfer of the OPTX to the speaker, so cannot be seen with an oscilloscope on the speaker terminals due to varying and complexed voltage/current phase angles at different frequencies inside the speaker. In every day terms, the higher frequency response of a valve amplifier reduces as power increases. A valve amp which can amplify say 18kHz at 1 watt might only be able to reproduce up to 10kHz at full power; or much less depending on the cheapo OPTXs used in guitar amps. But then you probably wouldn't like sound of a high performance OPTX, as the amp could sound close to an 'old fashioned' SS amp!

    Modern SS amps (in the last ten years - but still some not eploying FDD) with current feedback mimic closely the OPTX effects, except they can comfortably reproduce the high frequencies through the speaker! This is a problem! Because, guitar players don't expect to hear so much treble in the sound. Sadly, many of the SS amps out on the market have not 'yet' tackled this problem. However it is easily remedied by installing a 6dB/octave roll off in the SS power amp from 1.5kHz. Then it will be very hard to tell the two technologies apart when coupled with FDD.

    Line6 is the only maker to do this that I've seen so far. Modelling cannot replicate the OPTX effects as it's far too complicated and changes when you change the speaker or cabinet dimensions. So they HAVE to employ analogue FDD and HF roll off in the power amp to get the amps sounding 'tubey'. No other way. Sadly, their SS power amps lack muscle (naf $3 TV power amp chips), otherwise they'd have it nailed!

    I have designed many 100% SS amps using these techniques and they easily compete with the classic valve amps in tone and distortion character.... but with all the SS reliability advantages. NO typical clanky SS top end at all!

    Sorry chaps, I just LURVE tranny amps... well mine that is!! :)

    I don't make them commercially any more... only for me!

  10. Ivan

    Ivan Tele-Holic

    May 8, 2006
    have Marshall really discontinued the AVT?
    I thought they still made them with the tribute stylings.

    When I tried my friend's AVT I didn't expect much out of it (I had a 1987x at the time). But I was blown away by it. I thought it was very versatile as I got a decent sounds no matter how I set it.

    I know people bash the AVTs and MGs but they really don't deserve that. Both amps sound great for what they are.
    Besides... I'm a believer that a good workman never blames his tools!

  11. ProToneThinline

    ProToneThinline Tele-Holic

    Jun 27, 2005
    Pittsburgh, PA
    Stewart, thanks for you explanation, although I only understood about 10% of it. :oops: I'd love to hear one of your amps - they sound very cool. I remember back in the 70's Gallien-Kruger made SS guitar amps that had an output transformer, and was supposed to mimic the sound of a tube amp. I never played one, so I can say if they did or not, but they didn't stay around for too long. Too bad.

    Ivan, yes, Marshall still makes a few AVT's. I think they're called the AVTH (heritage) series, but they only have a couple of models. It's basically the same amp with basketweave grill cloth, but they moved production from England to China. The AVT20 does sound pretty good regardless of the settings. I think that's because the tone stack has a fairly narrow range, but I think that's typical of most Marshalls.

    I've had (probably) a half dozen Marshall amps in my lifetime. I got my first one back in '75. It was a 1987 50 watt head. Gigged it for a few years then traded it for a silver-face Super Reverb (dumb move). A few years later I got a Studio 15 (wish I still had that one, too). Then I got a MOSFET half stack that lost its charm rather quickly. Had a couple more, but don't remember the models. All in all, I think the Studio 15 and the AVT20 are my two favorites.

  12. Yes, that inclusion of the OPTX in a SS amp and its reputation seems to bare out what I said.

    You can do this to any SS amp though. Just buy one of those multi-tapped impedance converting transformers and hang it between the amp and speaker. And hey presto, valve OPTX tone... exept for the excessive treble!

    Us SS designers are working hard to improve the reputaion of SS amps, so I guess it's just a question of time before we kill off our Thermionic-Retrosaurus friends!! Either we do... or the legislators will! We'll see in time. :cool:
    Last edited: Jun 30, 2009

  13. Canuckcaster

    Canuckcaster Poster Extraordinaire

    Jul 12, 2006
    Calgary, Alberta
    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.


  14. Colo Springs E

    Colo Springs E Doctor of Teleocity Ad Free Member

    Mar 4, 2003
    Colorado Springs
    I never cared for this series, but it's great you're getting some tones you like--that's all that matters!

  15. 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.

    Attached Files:

    Last edited: Jun 30, 2009

  16. gtrman100

    gtrman100 TDPRI Member

    May 9, 2009
    Maybe I'm just lucky, but I've had an AVT150 combo with ext. cab and gigged it for the last 7 years with no real problems other than a bad connector on the DSP board. I'm not sure if I've been running it at 50% level, but I run it pretty loud, not totally cranked though.

    I really like the sound of the amp, but the design is obviously not bullet-proof because of the cost factors.

  17. It is perfectly possible that you could get a reliable performance depending on how you use them. For example, if you run SS power amps fully cranked, then they will actually cool down. Same if they're run very quiet. About 40 to 60% power is the real danger spread. And it depends on the amount of distorted signal and for how long.

    If you are playing clean dispersed with distortion now and then... fine. But if you're playing death metal for 8-10 minutes or longer (estimates) at reasonable levels, then you could expect danger.

    Other factors can make the TDA7293 chip vulnerable too. And the most common is if the chip tabs are over tightened when mounted on the heatsink. For this type of chip case where the mounting bolt goes through a hole at the top of the tab, there is a strong risk that the mounting plate (and tab) will bend away from the heatsink, thus causing poor thermal bond; this allows the silicon die (the tranny) inside to overheat and burn out. The bolt should be tightend using a torque wrench, but is often done by hand and can be a lottery. With the mounting plate being so small, the case to heatsink thermal resistance increases rapidly if not perfectly mounted.

    Ambient temperature is another. In USA, many more venues have effective air conditioning. That's not the case here in England, although we're getting more gradually. So I guess we'll see more failures due to hotter venues and the fact that the chip starts out much warmer!

    The TDA7293 is, IMHO, a most unsuitable power chip for this arduous application. To be fair, Marshall are not alone in using it. Laney do and some Carlsbro amps do too. They all suffer similar problems and we have seen many examples coming through our workshops in the last seven or eight years.

    As far as cost goes, they have wasted a lot of metal work money and employed a CPU fan to try and keep the chip cool, as the photo shows. That alone says "we have a problem mission control!" Why did they not just use a better designed power amp using conventional power trannies and dump the metalwork and fan costs? Well, it's my guess that once committed to using the chip early in the AVT design cycle they could not go back, so had to find a way of 'making it work.'

    Musicians are used to spending lots of money on their valve amps, so often this moderate repair cost level is just accepted as 'normal' or 'not so bad.' Fender and Peavey seem to have applied logic here, and resisted using these chips in their bigger amps... they do still use proper discreat tranny designs as far as we've seen so far and are very reliable as a result. Sadly, I have to admit, it's only the British makes who risk cheapening their names by using this kind of corner cutting to meet a price point.

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