There are many guitar tube amplifiers schematics that don't use negative feedback.
I was wondering how the negative feedback can help in the life cycle of the tubes? (If can help)
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\$\begingroup\$ Help how? Extend their serviceable life? \$\endgroup\$– Phil FrostJun 22, 2013 at 23:05
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\$\begingroup\$ @PhilFrost: By my understanding, it's common for some of the characteristics of vacuum tubes to change as they age. In the absence of negative feedback, circuits may be rendered unusable by even small changes to tube characteristics which negative feedback would be able to accommodate. With regard to guitar amplifiers, I would expect that changing tube characteristics may affect the relative behavior of the "gain" and "volume" knobs, but that an operator adjusting those knobs may provide a form of negative feedback not shown on the schematic. \$\endgroup\$– supercatJun 23, 2013 at 1:31
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\$\begingroup\$ @PhilFrost: If one wanted to feed a vacuum tube output stage with an op-ap that took feedback from the tube's output (plus some high-pass-filtered feedback from itself to prevent ringing) one could probably design a circuit that could work well with a wide variety of tube characteristics. Of course, doing that would likely negate the purpose of using tubes in the first place. \$\endgroup\$– supercatJun 23, 2013 at 1:35
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\$\begingroup\$ especially with guitar amps, where vacuum tubes are prized for their behavior when pushed into their nonlinear regions. be a shame to iron that out with a pile of transistors. \$\endgroup\$– JustJeffJun 23, 2013 at 2:18
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\$\begingroup\$ @JustJeff On the other hand, pushing them non-linear with transistors (e.g. op-amp based overdrive pedal in front of a tube amp) is widely accepted. Some of the nonlinearity even comes from a "soft clipping" pair of diodes. \$\endgroup\$– KazAug 11, 2016 at 18:33
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2 Answers
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Voltage negative feedback with valve amplifier is here mostly to enhance the bandwidth, reduce distorsion and the output impedance. If this is desirable in the hifi world, it has less important in a guitar amplifier where distorsion is wanted and 50Hz-15kHz is common bandwidth. Furthermore, if on the paper negative feedback has only advantages exchanging gain for stability, in the reality, it kills spontaneousness this is why it is often presented as an option in a guitar amplifier.
So no, negative feedback has nothing to do with valve serviceable life. Heater voltage and respect of the DS values are by far more important.
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\$\begingroup\$ I was going to upvote this until I saw "kills spontaneousness". This is a engineering site. We deal in measurable and defined quantities. If you think there really is something quantifiable called "spontaneousness", then you need to define it, else its just a bunch of superstitious audiophool nonsense. \$\endgroup\$ Jun 25, 2013 at 13:40
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2\$\begingroup\$ Negative feedback doesn't have to reduce output impedance. It does that if it it is based on the output voltage. Negative feedback based on output current increases output impedance (while doing those other things, like reducing gain and improving bandwidth). \$\endgroup\$– KazAug 14, 2013 at 0:46
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\$\begingroup\$ @Kaz I took me time to realize a current «negative» feedback does tend to reduce current as it increases thus it increases the output impedance. Thank you, I am going to edit my answer. \$\endgroup\$– gregAug 20, 2013 at 9:01
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No, whether a vacuum tube is used in a used in a circuit with negative feedback around it or not has no direct bearing on the tube's service life. As long as the tube isn't abused, it should be fine. It may be easier to design a circuit with feedback to guarantee it runs the tubes within some parameters, but that is just one more thing to take into account when designing a circuit without feedback.
The primary cause of tube failure, barring outright physical damage, is cathode wear. The cathode has a special coating on it to reduce the work function. This allows for reasonable cathode currents at lower temperatures than otherwise. However, this coating is degraded over time, especially as electrons leave the surface of the cathode. Very roughly, each cathode coating is good for some number of total coulombs, although there is a lot more to it than that. Feeback or no feedback doesn't by itself indicate higher or lower average cathode current. Usually these tubes are biased at recommended operating points, so both types of circuits would probably have about the same average cathode current.
answered Jun 25, 2013 at 13:49
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\$\begingroup\$ While the extent to which a tube wears may be a function primarily of the number of coulombs that are pushed through it, I would expect that the amount of wear a tube can sustain before it becomes unsuitable for use in a particular circuit would be a function of the circuit's tolerance to changes in tube parameters. If a circuit attenuates a signal by a factor of four and then feeds it to an open-loop amplifier with a gain of 20, a 25% change in gain would affect the output by 25%. If, however, the circuit instead used negative feedback to reduce gain, the output would be affected far less. \$\endgroup\$– supercatAug 20, 2013 at 15:45