RichP714 3,140 Posted April 2, 2009 Share Posted April 2, 2009 From wiredstate audio community: actually the "ode" came from the term electrode so a diode is a vacuum tube device (wala pa SS when they coined the term in the turn of the last century) with two electrodes or di-electrode or simply diode same with tri-electrode for triode, tetra-electrode (4) for tetrodes and penta-electrodes for pentodes. Huge difference between the two, triodes and other multigrid amplifying device (barest amplifying device is a triode, a diode do not amplify) like tetrodes and pentodes have non linear characteristics but can handle more gain, very efficient and more power the triode on the other hand triode is inefficient, small gain but highly linear. Linearity dictates amount of distortion, a pentode without feedback will generate 15% distortion (because of it's non-linear response), with feedback will go as low as 8% distortion pero panget tunog even with low distortion (due to phase shift, which is an imaging killer). Triode naman usually have 5% distortion but only 10 to 15% efficient so a 30 Watt triode tube can only spew 2 to 8 watts the rest is converted to heat. But can reproduce near exact tones and thousandfold better than pentodes. and Q: What is "ultralinear" operation? A: In ultralinear operation, the screen grid is connected to a tap on the output transformer primary such that the screen voltage varies in proportion to the plate signal voltage. The constant of proportion- ality typically ranges from 30-50%, although other ratios will also work. The resulting tube characteristic has properties intermediate between those of the triode and the pentode. In essence, the ultra- linear connection forms a local negative feedback loop around the output stage. This may be advantageous depending on circuit topology and gain distribution. Advocates of ultralinear operation claim this connection combines the best features of triode and pentode mode, while detractors claim it lack the virtues of either. Q: Compare the general properties of triode, pentode, and ultralinear power amplifiers. A: It is probably misleading to characterize the various types of output stage connections in terms of sound quality. Many factors contribute to the sound of an amplifier, and a good designer will blend these elements in order to achieve a particular goal. On the other hand, certain sonic qualities are associated with each type of output stage frequently enough that they deserve repeating here. In addition, there are objective differences that are worth mentioning. The triode amplifier is characterized by low efficiency and low power output. This is because a smaller voltage swing is available from the triode for a given DC plate voltage. Consequently, the triode amplifier burns up more power at idle relative to its peak output. The sound of the triode amplifier is often described as "rich" or "sweet", conveying in a natural and realistic way the harmonic structure of musical instruments and voices. The pentode amplifier is often described as having a more analytical sound than comparable triode units. Others may accuse it of sounding harsh. Objectively, the pentode output stage tends to produce more high-order distortion products than a comparable triode. In addition, the pentode is more sensitive to load impedance variations and may clip more sharply than the triode. The ultralinear amplifier combines the benefits (or flaws, depending on your point of view) of the triode and pentode connections. The ultralinear characteristic curves resemble those of the triode in some ways, those of the pentode in others, and have unique characteristics as well (regretably, they are hard to render in ASCII). While the general concensus favors triode mode above all, there seems to be no strong trend supporting ultralinear over pentode mode, or vice-versa. Perusing the high-end magazines, one can find examples of well-regarded amplifiers ueing either type of output connection. The one certainty is that the ultralinear connection is the cheapest way to get good performance and high power out of a pentode. Whereas a quality pentode design requires a stiffly regulated screen grid supply, all that is needed to implement an ultralinear output stage is a pair of transformer primary taps. Perhaps the economic argument leads the sound quality argument in this case. Q: Can I convert my amplifier back and forth between pentode, triode and ultralinear modes in order to hear the difference for myself? A: In general, the answer to this question is "No." Under some circumstances, it may be possible to perform such experiments, but subject to limitations. If the amplifier is an ultralinear design, it is possible to convert it to pentode operation by connecting the screens to a fixed voltage source. The correct screen voltage depends on the type of output tube, the B+ supply voltage, and the output transformer primary impedance. For audiophile performance, a regulated screen supply may be required. This makes the pentode conversion a major modification. The most common conversion is to modify an ultralinear or pentode mode amplifier for triode operation. In many cases, this modification can be made successfuly and with little effort, but some caveats apply. One would like to be sure that the maximum triode-connected plate potential is not exceeded. For many EL-34/6L6/KT-66/5881 amplifiers running B+ supplies on the order of 400V, there is no problem converting to triode operation. On the other hand, a 6550 amplifier with 550V on the plates is probably not a candidate for triode conversion without a reduction in B+ voltage. When the conversion is made, a 100 Ohm non-inductive resistor is usually specified, connected directly between the screen and plate pins on the tube socket, to suppress RF instability. Changing the output stage connection from pentode to triode mode typically lowers the open-loop gain of the amplifier. As a result, the closed-loop global feedback factor also goes down. The output impedance of the amplifier, its sensitivity, the total harmonic distortion and the distortion spectrum will all change. Overload behavior and stability will likely be improved. Typical comments are that the triode-connected amplifier sounds "more relaxed", "warmer", and "sweeter" after the conversion. Whether this is due to an inherent quality of triode-strapped pentodes, or is a consequence of modifying a topology that was not designed with triode output in mind, is open for debate. Q: What about "pure-triode" amplifiers? A: The vintage triode power tubes, such as the 845, 2A3, and 300B, are classic devices from the earlier days of vacuum tube technology. They are still available in limited supply and at high cost (although there are now Chinese copies on the market that offer a reasonable, lower- price alternative). A significant structural difference between these tubes and more modern units is the use of a directly-heated cathode. In this design, the cathode heater also serves as the emissive element. In contrast, newer tubes employ a separate heater that is electrically and mechanically isolated from the cathode. These tubes are "pure triodes", meaning that there is no screen grid to be strapped to the plate in order to achieve triode operation. The classic triodes have very low plate resistance and low voltage gain. Many require significantly higher plate supply voltages than ordinary pentodes. In exchange for these limitations, these tubes offer very linear characteristic curves, making possible the design of low- distortion amplifiers that use little or no local or global feedback. The sound of a pure-triode amplifier is reputed to be exceedingly musical, with a natural harmonic structure, very low grain or noise, and a realistic, inviting nature. Triode adherents claim that the pure-triode output stage is sonically superior to one constructed with strapped screen grid pentodes. Other listeners will find the pure-triode amplifier to be colored, restricted in bandwidth, inefficient, and overpriced. Single-ended triode amplifiers have been very popular in Japan for some time, and are making a limited comeback in North America. Q: What is the difference between a single-ended and push-pull amplifier? A: A push-pull output stage uses one or more pairs of output devices connected in a symmetrical arrangement such that output current flows to the load first through one half of the circuit and then through the other half. The advantages of the push-pull topology are higher efficiency, higher power output, much lower even-order distortions, immunity from power supply ripple, and zero DC current in the output transformer primary. In contrast, the single-ended output stage employs only one set of output devices which conduct continuously throughout the output current cycle. This forces the stage to be operated in class A mode, limiting the available power output and greatly lowering efficiency. Total harmonic distortion is higher because there is no cancellation of even-order harmonics. Power supply ripple is not rejected by the single-ended output. The most significant difficulty of the single-ended output stage is that the output transformer is required to carry a large DC current in its primary. Due to magnetic saturation and nonlinearity effects, a very special output transformer design is required. Such a transformer is large, heavy, expensive, and has a low power rating. The resulting amplifier is restricted in bandwidth at both extremes of the audio spectrum and produces a great deal of distortion. To minimize distortion (and to add to the single-ended mystique), it has become fashionable to design single-ended amplifiers with pure-triode output stages. While no one claims the pure-triode, single-ended amplifier is "neutral" or "accurate", devotees of the genre describe in almost mystical terms the sonic attributes of these amplifiers. The word "magic" is often used. Listeners will have to judge for themselves. Link to comment Share on other sites More sharing options...
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