So why are 2x6L6 or 2xEL34 50 watts?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ari, Feb 22, 2006.

  1. ari

    ari Member

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    There must be something mechanical about those tubes that lend themselves to 50watt designs or something. The reason I'm asking is because personally I think 50 is still too much -- 30 or so is probably more manageable. But I like the sound of those tubes over smaller ones (EL84s, 6V6)

    There are some Fender-based designs with 6L6s for 40 watts or so, but there doesn't seem to be any for Marshall-based or other gainier amps.

    Your thoughts? Thanks!

    ari
     
  2. Reeek

    Reeek Member

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    I'll be a warm up for the smarter guys but the 50 watts probably means the maximum plate disspation for two tubes which equates into the maximum output if the circuit is designed for maximum output. An EL34 dissipates 25 watts each while a 6L6GC is 30 watts. Other 6L6 types like WGB and 5881 are in the 25 watt range.

    Class A/B allows for higher output from tubes while Class A is lower output.

    Reasons will have to come from the big boys.
     
  3. avwalker

    avwalker Member

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    it's really all about voltage and bias. Older amps were lower voltage and therefore had lower power. Also, some of the early fenders were cathode biased, which produced less output power cause it's a less efficient design. When you deal with a certain tube in a design, you have to find the sweet spot in its dissipation. That being said, 50 watts isn't much louder than 30 watts. I love 18-22 watters (Marshall 18 watt, Fender Deluxe Reverb). They can still get hella loud.
     
  4. Shea

    Shea Member

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    The plate dissipation rating doesn't define the max output power. You can get 100 watts out of a pair of EL34s, but the plate dissipation rating is still only 25 apiece.

    The max clean output power depends on plate voltage, screen voltage, and the plate-to-plate impedance of the output transformer, as well as the tube type. You can't assume that any circuit with two 6L6s or two EL34s puts out 50 watts. If you look up old spec sheets for EL34s or 6L6GCs on the internet, you can find design parameters for different circuits using a pair of power tubes in push-pull that can make anywhere from maybe as low as 15 watts (IIRC) for EL34s or 6L6s, or up to 60 watts for 6L6GCs or 100 watts for EL34s. 25 to 30 watts is pretty typical for the types of circuits they usually list on those spec sheets.

    Here's what I'm thinking of as the "typical" sort of class AB1 circuit you might see on a tube spec sheet, from page 4 of the Amperex 6CA7 / EL34 spec sheet: http://www.triodeel.com/6ca7ap4.gif

    See? With a B+ supply of 375 volts, a shared screen resistor of 470 ohms, a shared cathode resistor of 130 ohms, and a 3k4 primary output transformer, they predict you'll be making about 35 watts.

    But on the previous page, they list some other circuits that might strike the typical guitar player as a bit exotic -- Class B2 circuits squeezing up to 100 watts out of a pair of EL34s: http://www.triodeel.com/6ca7ap3.gif

    So, a pair of EL34s doesn't necessarily produce 50 watts.

    BTW, I've heard that either Matchless or Bad Cat makes a 30-watt amp using a pair of EL34s.

    Shea
     
  5. teefus

    teefus Silver Supporting Member

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    my hayseed teefus special runs 2xel34's, cathode biased for somewhere around 30-40 watts. cathode biased el34's rule.
     
  6. donnyjaguar

    donnyjaguar Member

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    I think you guys are all forgetting a couple of things. The output transformer plays a very big part in how powerful the amplifier will be. Some transformers couple much better than others and I think its pretty safe to say that musical instrument amplifiers don't have the creme de la creme of conversion efficiency. If you want an example of superiority here, have a look at the iron on a McIntosh MC240. This amplifier uses 6L6/5881 tubes in the output stage. If you can't get 55Watts out of each channel on this amplifier your tubes are definitely whipped. Guitar transformers don't need the same kind of bandwidth as hi-fi units so consequently the windings could very well be shorter and have lower inductace, use conventional round wire instead of flat thus increasing inter-winding capacitance and resulting in coupling less than optimum.

    The second thing to remember is that many amplifier manufacturers are full of baloney. Seldom, if ever, do they qualify how they "measure" the output power. Frequency and distortion level are seldom noted. I've even seen "max power" stated by taking the peak-to-peak waveform and calculating the wattage using V^2/R. This is totally bogus. To add to the confusion some manufacturers purposely de-rate their amplifiers. So their X amount of watts is louder than another's X wattage.

    One of the most absurd things I've seen is a speaker that had a power rating written on the back with something to the effect of: "20Watts USA, 16Watts UK". I guess those British watts are more powerful? ;)

    D-J
     
  7. Red Planet

    Red Planet Member

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    Marshall JCM 800 Artist Two EL34's 30 watts.
     
  8. Shea

    Shea Member

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    I think those factors have more to do with the bandwidth at the maximum power, than the maximum power itself. IOW, a guitar amp output tranny might have crappy response below 50 hz and above 30khz, but still be putting out the max power at 800 hz.

    Yup. Which leads me to suspect that some manufacturers might have just gotten lazy and decided to label any amp with two EL34s or two 6L6s as a 50-watt amp.

    Shea
     

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