AVT Thread

Discussion in 'Marshall Amps' started by mrpipster, Nov 14, 2015.

  1. Sidewinder

    Sidewinder Member

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    In my experience the fans don't stop because of a electrical related problem. The bushings wear out because the stock fans are not ball bearing. You'll hear the fan making a winding noise if this starts. If the winding noise stops, it my be not turning at all.

    Here is a link to a photo of the fans in place on a chassis. You can see the twisted red and black leads that go to the board.

    http://www.fearsomesound.com/images/New Folder (3)/Marshall AVT150H Chassis.JPG
     
  2. benoityip

    benoityip New Member

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    A useful and very long article explaning solid state and tube. Details of the valvestate 1 and 2 and avt are explained. It said AVT has more gain stages by feeding the signal back to another mosfet

    http://www.jeanpierrepoulin.com/PDF/transistor.pdf

    Extract as below on page 276

    Aside employing a conventional mixed-mode feedback configuration that provides a “tube-like” frequency response, Valvestate amplifiers contain no circuitry that would specifically emulate characteristics of a tube amplifier. However, excluding the lowpower models of the first two Valvestate lineups the amplifiers do employ an actual dual triode ECC83 in the preamplifier circuit. Yet that fact just makes them simple hybrids - and not even very novel as such. It can be argued whether mixed-mode feedback should be considered as tube emulation technique or not but based on the fact that some Valvestate models include a switch between “Valvestate” (current feedback path toggled) and “Linear” (current feedback path open) it is clear that Marshall has formed their opinion about the issue. Yet, current feedback is not even exclusive to Valvestate lineup: For example, it was already in use in the MG series where the FDD (Frequency Dependant Damping) feature simply disabled a low cut filter that attenuated the resonance peak. It should be noted that, excluding the tube stage, circuit designs of various Valvestate amplifiers are very different from each other. Basically, the more advanced (and expensive) the model is, the more sophisticated circuit it contains. Considering the triode circuit, first and second Valvestate lineups were virtually identical. In the AVT series (Advanced Valvestate Technology) the section is revised. A typical Valvestate preamplifier has nearly separate signal paths for clean- and overdrive channels. Common nominator is sharing the tube stage. Solid-state stages of the clean channel are basically configured to be virtually transparent within the realistic range of input signals, thus the channel will provide only tube overdrive. Overdrive channel, however, utilizes diode clipping and provides both solid-state and tube overdrive. As a rough generalization, the first and second lineup used a shunting diode clipping configuration while the circuits of AVT series utilize diodes in the OpAmp feedback loop, thus being less prone for driving OpAmps to clip into rails. The limiting stage is located before the tube stage but the placement of gain-, and in some cases tone, controls varies widely in different models. Typically tone controls follow the tube stage forming the cathode follower buffering arrangement that is similar to many Marshall circuits. The dual tube stage of series 1 and 2 Valvestate amplifiers consists of a common cathode stage, which is directly coupled to a cathode follower. This has been a typical circuit in Marshall amplifiers ever since JTM45 and it allows driving the tone stack from fairly low impedance source. The B+ voltage of Valvestate amplifiers is approximately 109 V so it is somewhat lower than the B+ of typical tube amplifiers. The moderately low value is pretty typical for all similar hybrid circuits. Although the voltage gain of a cathode follower is less than unity the whole tube section, counter intuitively, has fairly symmetric clipping characteristics: According to magnitude of drive signal the clipping turns from asymmetric (cathode follower is overdriven) into “semi-symmetric” (the common cathode stage clips as well). Both configurations also have different clipping characteristics: Cathode follower generally clips “harshly” while the common cathode has “softer” clipping. The input impedance of the following stage also has huge effect on the behavior: Purely resistive load creates very harsh clipping while the irregular loading impedance of a typical FMV tonestack circuit can actually soften it a bit. Aside from having lower gain the performance of the circuit is identical to similar circuit in JTM45, Fender Bassman and various other “classic” tube amplifiers.

    In the AVT series the tube stage is configured to provide more gain by wiring the second tube to common cathode configuration as well. The typical cathode follower concept is still maintained but the tube is replaced with a MOSFET that shares the high rail voltages. Not shown in the schematic is the zener diode protection that connects between the source and the gate and consists of two back-to-back 15 V zener diodes. It is claimed that a MOSFET is a very “transparent” replacement of a tube in a follower configuration. While being partially correct the claim is also a bit misleading: In the concerned circuit the cathode (or source) resistance is very high, which biases the follower so that it is driven into clipping. (This is exactly the same thing that happened in the earlier Valvestate circuit as well). If one would decrease the cathode resistance from the typical value of about 47 – 100 kilo-ohms to, say, 3.3 kilo-ohms the follower would operate in the linear region and would not clip the signal. However, knowing this doesn’t change the existing design to anything else than what it already is. In this case the follower will clip – and this is an essential ingredient of the circuit’s tone. However, I haven’t yet seen a MOSFET that in the source follower configuration would have similar clipping characteristics as a triode in cathode follower configuration. This is not saying that the circuit couldn’t clip “softly” or “tube-like” – just that a MOSFET does not behave exactly like a triode. MOSFET choice is an extremely important aspect of the design and the device choice dominates the clipping characteristics of the preceding tube stage as well.
     
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  3. Rokinroller

    Rokinroller Well-Known Member

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    Interesting AVT thread here .... sorry I had not seen it and I had started a "NAD" thread for my newly acquired AVT100 1x12 combo which I am really liking a lot. Having owned many all tube Marshall heads and being very pleased by all of them ... ( 800's 900's DSL 50 /100 ) , this AVT 100 is no less satisfying than any other Marshall I've played. I bought it on impulse and have no regrets at all . Very happy to own this fine amp.
     
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  4. benoityip

    benoityip New Member

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    and in guitar magazine, russian edition, it compares AVT20 to a TSL60, and it sounds good. I have to use google translate to translate from Russian to English to read

    http://www.guitarsmagazine.ru/read_002.php?marshall-avt20

    Also, I have made a mistake in my previous post for the power amp section compared to older valvestate
    AVT uses frequency dependent power amp damping and Marshall's proprietary, dynamic clip level technology (which emulates the HT supply of a valve power amp)

    From my ears, I can tell the difference they made in the power section of this amp, It sounds like "virtual 3D". Older valvestate sounds 2D, hence some people thinks AVT sounds loose compared to older valvestate, but this is my speculation

    and I guess the technology is similar to peavey transtube series (solid state)
    https://peavey.com/support/technotes/hartley/Chapter_3.pdf
     
  5. gearhead

    gearhead Member

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    One thing I found on the AVT150H, is that if you run a cable from the effects out to the effects in, and crank the effect mix wide open it really adds a lot to the sound! The effects loop isn't in series, it's in parallel, so it adds volume and more overdrive.
     
  6. Mauro

    Mauro New Member

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    I can confirm that!
    It adds a lot more punch to the overall sound... simple "mod" :hbang:
     
  7. Mauro

    Mauro New Member

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    I was looking across the internet for some deeper infromation about this amp, but found almost nothing!

    Short story: Went to a gig and used the backline cabs, which supposed to be 8 ohms... but they were not! so I fucking burned the amp...

    When I took it home and checked (not much of a pro electronic guy, but I can do some things), the fuse and both amplifiers were fried... so I got them for around $10 each and started the repair.

    Amplifiers are TDA7293, and when they get fried, they get short circuit, so to make sure there wasnt another component in failure, I disconnected both amp boards. Good... no other component failed and the voltage was right . So I started the surgery

    [​IMG]

    This 15 pin amplifiers are soldered directly to the board, and it was a pain in the ass to unsolder them, and as this is the component that often fail, would damage the board if solder and unsolder too many times, so, I looked for a base to adapt and make them more of a plug and play component.

    No base was found :(

    So i bought a regular microchip base and cut it in 2 parts , and solder it to the board. voila... looks and works great!

    [​IMG]

    After putting the amps on and testing every pin, I was ready to mount and power on

    [​IMG]

    Worked, and now I will make sure before plugging any cabinet ....

    Cheers!
     
  8. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    :welcome:to the forum...

    cool 1st posting (well 2nd, but, hey!)

    :cheers:
     
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  9. Mauro

    Mauro New Member

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    Thanks bro!

    1st marshall also, sent directly from england :uk:
     
  10. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Yea don't they suck removing? I cut the individual pins and pulled them seperately. After I did one I saw the whole output module for sale cheap, and I wished I would have went that route instead, and saved some time.
     
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  11. Mauro

    Mauro New Member

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    I did the same :D

    And the module was too expensive for the piece of crap it is, thats why I only replaced that :pops::pops::pops:
     
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  12. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member

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    Welcome to the forum Mauro
     
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  13. Mauro

    Mauro New Member

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    Great to be here bro
     
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  14. DervishMoose

    DervishMoose New Member

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  15. Zensee

    Zensee Member

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    I love when someone goes off the beaten path and finds the good in a forgotten amp. I hope you have a blast with it!
    :applause:
     
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