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Silver Member Username: Joe_cOakwood, Ga Post Number: 840 Registered: Mar-05 | Can someone explain this to me in regards to a tube amp? |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4797 Registered: May-04 | Can I assume you know what a sine wave looks like and that one complete sine wave is from 0° through 180° positive back to 0° through 180° negative and then back to 0°? |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4798 Registered: May-04 | http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm 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. Q: What are the meanings of Class A, B, and C? A: Because virtually all active devices pass current in only one direction, it is necessary to go to some trouble in order to amplify audio signals, which are alternating currents. There are basically two strategies for making one-way components amplify two-way signals. The first is to use a pair (or pairs) of devices arranged so that one half of the circuit conducts current exclusively during positive swings of the signal, and the other half conducts exclusively during negative swings of the signal. This arrangement is called "push-pull" operation. The other strategy is to superimpose a direct current on the AC signal of such a magnitude that the combined current remains net positive at the negative signal peaks. The result is that the amplifying device is never required to reverse the direction of its current flow. The superimposed direct current, which is known as a bias current, may be filtered out of the amplified signal using a transformer or blocking capacitor. In its simplest form, this type of circuit is known as "single-ended". If we consider purely sinusoidal signals, it is clear that the output device in the single-ended circuit conducts current during 100% of the audio signal cycle. In contrast, each device in the push pull circuit conducts during exactly 50% of the signal cycle. It is also possible to construct amplifiers in which the output devices conduct for less than 50% of the signal cycle, although these circuits are generally not employed in audio amplifiers. The Class A, B, and C designations refer, respectively, to these three modes of operation. Returning to the push-pull, Class B amplifier, it can be seen that during zero crossings of the output signal, the positive half of the circuit switches off just as the negative half begins to conduct, and vice versa. At the precise instant that the output current is zero, no current flows in either half of the circuit, i.e., there is no bias current. In practice, amplifying devices are quite nonlinear near their low-current cutoff points. A type of nonlinearity called "crossover distortion" can be eliminated if the Class B circuit is modified so that a modest bias current flows while the amplifier is idling. The small overlap in conduction between the two halves of the circuit smooths over the transition that occurs during zero crossings. Because the bias currents in the positive and negative halves of the circuit are are equal and opposite, they cancel one another automatically. This is why the net DC current in a push-pull output transformer primary is zero, and why solid-state push-pull amplifiers require no output DC blocking capacitors. As the push-pull bias current increases, the conduction angle of each half of the circuit increases from the minimum value of 50%. If the bias current is set to a value equal to one half of the maximum output signal current, the conduction angle will equal 100% and the amplifier will be operating in Class A mode. The flow of bias current in the absence of signal current dissipates energy in the form of heat. Thus, the efficiency of the amplifier is reduced as the bias current increases from zero to full Class A operation. An amplifier that is biased part-way between Class B and Class A operation is said to operate in Class AB mode. In vacuum tube amplifiers, an additional distinction between Class AB1 and Class AB2 is made. In Class AB1, the driver stage has a high output impedance and clips at the onset of output tube grid current flow. In Class AB2, the driver stage has a low output impedance, allowing it to drive the output grids linearly into the positive grid current region. This allows greater output power with a given bias current. Despite the efficiency penalty, many listeners believe that Class A amplifiers drive difficult loads with more authority and a smoother sound than comparable class AB units. http://www.ken-gilbert.com/techstuff/vtf.html |
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Silver Member Username: Joe_cOakwood, Ga Post Number: 844 Registered: Mar-05 | Thanks Jan. Is there a price difference between push pull and single ended tube amps? |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4802 Registered: May-04 | No, not really, there are single ended designs that are more expensive than most P-P designs. That has mostly to do with the transformers (most two channel tube amps will have three transformers; one power trans. and one output trans. for each channel) with single ended designs requiring a more elaborate and therefore more expensive set of transformers. There are a few OTL (output transformerless) tube designs on the market, but they represent a very rare breed of tube amp. Because of the cost of winding and then shipping transformers of sufficient quality, tube amps tend to not be cheap. You can find some very good deals in the used equipment market, but new tube amps begin around $1,000. If you choose a S.E.T. tube amp, you will be looking at pretty low wattage; typically under 12 watts per channel. This most often necessitates a reasonably high efficiency speaker to get sufficient volume in an average size room. If you're contemplating a S.E.T., you may ultimately need to consider the cost of new speakers also. Even more efficient speakers on paper may not be a good match for a S.E.T. if the X-over is fairly complex. Also most S.E.T.'s are affected by the impedance of the speaker, so you'll require a speaker system with a fairly benign impedance curve. There are P-P triode designs beginning to show up. This seems, to me, to negate the purpose of using a triode as the tube of choice. But, the end results are a matter of circuit design. It is certainly much less difficult to begin experimenting with tubes using a P-P design. You can get about 40 watts for around $1,000 and you will not probably require different speakers. |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4803 Registered: May-04 | P-P designs typically use tubes deignated as tetrodes and pentodes, with the pentode being the most common. The more elements a tube contains (pentodes have more elements [5] than tetrodes [4]) There are some tubes called beam power tubes (overall similar to a tetrode), but not many companies design with beam power tubes for some reason. The B.P. tubes have very low distortion and clean sound. The McIntosh amps I have use 6L6GC tubes which are beam power tubes. Most tube equipment manufacturers use pentodes such as the EL34 or the 6550/KT88. Tetrodes, beam power and pentodes can be wired to operate in triode (3 elements) fashion; though, once again, I don't see the point. How tubes are wired to take their plate and grid voltages off the power transformer and couple to the output transformers will vary from manufacturer to manufacturer. Most utilize UltaLinear transformer windings. A few use the McIntosh Unity Coupled designs. And, there are a few odd balls out there, but they are probably best avoided. |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4804 Registered: May-04 | Sorry, I accidentally deleted the end of the sentence: "The more elements a tube contains (pentodes have more elements [5] than tetrodes [4])" ... the more power it can produce. 6550/KT88's are usually found only in amps capable of more than 50 watts per channel. |
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Silver Member Username: Joe_cOakwood, Ga Post Number: 851 Registered: Mar-05 | Took the plunge Jan, I got a Jolida 502b for $600. I hate and love going out of business sales. Let you know later how it sounds. |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 4855 Registered: May-04 | I see from your other post you're having problems. |