Amps with higher than their rated output

TTBZTTBZ Frets: 515
I'm getting a two notes captor for silent/direct recording with IRs and would like to use it with some 100w marshalls (2203, 2555, 1959 etc) - the Captor is rated at 100w handling.

Seem to recall someone on here (probably @ICBM ) mentioning that some 100w marshalls actually put out more than 100w. At what point in the master dial roughly do they tend to reach that 100w? I don't fancy frying my load box straight away so wondering how far up I can safely put the masters!
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Comments

  • jpfampsjpfamps Frets: 1282


    Depends on how the power is rated in your attenuator and your amp.

    Most amps are rated for a continuous sine wave at a specified level of distortion which more or less represents the onset of clipping (distortion rises rapidly at the onset of clipping).

    As you drive the amp further into clipping output power increases, theoretically to twice the rated output power, although in practice around 160 % of rated power seems the norm.

    Many early 70s Marshalls put out more than 100W at the onset of clipping. I've measured up to 140W clean and over 200W fully clipped.

    Thus ideally you should use an attenuator that can take twice the rated power of your amp to give a margin of safety, especially as you are very likely to be thrashing the amp.

    Now it could be that your attenuator is rated at 100W because you can use it with a 100W amp, but I'd want to be 100% sure.

    If you want my honest opinion, starting with a100W amp and attenuating for silent recording is not ideal.



     
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  • TTBZTTBZ Frets: 515
    edited February 14
    Hm yea you may have a point there!

    It doesn't necessarily have to be silent, I just like the idea of recording direct with a line out after the power section to make use of impulse responses for consistency. Fwiw most of the time it'll be used with my 2525C.
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  • ICBMICBM Frets: 30242
    edited February 14
    I would not normally use a 100W-rated attenuator with a fully cranked vintage 100W Marshall, regardless of whether the 100W is true electrical power input or 'cranked amp of that power' - as jpfamps says, many old Marshalls can exceed 200W when driven really hard, including the early JCM800s. Likewise, I've measured up to 225W into a resistive load on some. About the only exception is the Marshall Powerbrake, which Marshall designed specifically for the purpose and which I've used with 200W amps under test a few times without any apparent ill effects - although not much beyond the point of clipping usually.

    You cannot normally go by the control settings to know whether a non-MV 100W amp is exceeding 100W or not - it totally depends on the input signal, the gain structure and the pot tapers. Some early-70s Super Leads can reach clipping with the volume control at only about 2 with a high-output guitar, and just get progressively more distorted beyond that. MV amps are more predictable because the preamp clipping will act as a limiter and is reasonably consistent. 2203s typically reach power-stage overdrive with the master around 6 if the preamp is already overdriven.

    There is a simple way to make it safe - pull two power valves, preferably the inner pair with old Marshalls - because on some the inner pair do not have grid stopper resistors. Set the impedance of the amp to half that of the load box to keep the impedance correctly matched. That way the amp can't exceed 100W (or not by much) even wide open, and it's also less hard on the transformers... and you're only burning through your valves at half the rate overall. (You should swap the pairs every now and then to equalise the wear.)
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • TTBZTTBZ Frets: 515
    edited February 14
    Thanks for the in depth replies looks like I'll have to keep the Captor for smaller amps then !
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  • ecc83ecc83 Frets: 783

    You could build a  simple load box and parallel it with the Captor. However the Captor would need to be say 16 Ohms and then set the amp to 8 Ohms for a 16 Ohm load box. Same process for 8+8=4R.

    You need a 100W allyclad resistor, an ally box and a couple of jacks. Whole thing should come in well under a nifty. Maybe someone makes one? Not looked.

    Dave.

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  • TTBZTTBZ Frets: 515
    edited February 16
    According to Two Notes the load box is disconnected when using the thru output of the Captor so the 100w is irrelevant in that situation and they say it's safe to use with 200w-300w amps. So in theory when connected to a cab, I should be ok to use it as a direct box for recording a 100w amp at a healthy volume!
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