Oscillation in tube amps. There have been many questions asked about oscillation in tube amps. These have been scattered across so many different threads that the search engine can't find them all.

I am starting this thread to collect the thoughts in one place. I am doing this because I was searching for something else last week and I found an old thread where someone had asked me how I check for oscillation.

I realized that I never saw that question so I posted the answer, but the original poster may have not seen it since the question was buried in a non related thread. Now I am working on some new circuits and I captured an oscillitory gremlin hiding in my amp. It is the evil hard to catch kind.

Unstable Amplifier Examples

I have not found the cause yet, and with my current work schedule, it might take a couple software developer jobs in germany with visa sponsorship weeks. I am copying the original questions and answers in the next post. I have been working on my "universal driver board" on and off since late last year.

Before my recent trip I had been testing the first prototype with various output tubes in screen drive and conventional grid drive. No anomalies have been seen but all testing has been on the bench with test equipment and a load resistor. No listening tests were done since I only had about one week to work with the board before I had to make a long road trip.

Hard clipping is near 40 watts. Tonight I turned the power on for the first time in 3 weeks and the amp still worked. The scope traces were clean even when pushed well into clipping.

OK now it's time to listen. Righe now there is only one channel, and it is operated by 4 seperate bench power supplies. There are clip leads for connection to the output tubes and power supplies.VTA amp goes into low freq oscillations the thermionic watercooler. The cone excursion was really scary. I immediately turned it off. I saw this once before, but power cycling the amp fixed it then. It happened again today after many days. Normally, this amp works great.

This time, even after powercycling, a couple of times, it went into oscillations. So the input AC voltage is obviously fluctuating. Bias seemed to be fine at 40ma per tube.

To be on the safe side, I readjusted the bias. Looking forward to suggestions to fix this instability. It is really scary to see those cones nearly pop out! I had this same thing happen on a project that I did, and I found that I did not decouple enough the driver tube from the power tube on the psu. If my boss is a bit more stupid, I have to water him twice a week! The filter is CLC with uf, 1. I noticed that the oscillation seemed to happen around 30 s after startup.

The oscillation issue is gone, but I probably need some kind of mosfet based slow start nowas my chassis cannot accomodate a 5AR4 now. Looking at the driver excursion, I guess the frequency is quite low - about 10Hz or so. I now have no slowstart in the amp. The NFB values were as recommended by Roy, for the 8 ohm tap - 5. Could flaky 12AT7 tubes on the driver board cause oscillation?

I have some old, untested Telefunken 12AT7s which I want to try soon. Any help is most welcome and appreciated It appears one channel has more oscillation both amplitude and frequency than the other, though it is hard to say for sure, as the speakers are 10 feet apart. Wish I had a scope. Make sure it is sound, not loose or dodgy.Parasitic oscillation is an undesirable electronic oscillation cyclic variation in output voltage or current in an electronic or digital device.

It is often caused by feedback in an amplifying device. The problem occurs notably in RF[1] audioand other electronic amplifiers [2] as well as in digital signal processing. Parasitic oscillation is undesirable for several reasons. The oscillations may be coupled into other circuits or radiate as radio wavescausing electromagnetic interference EMI to other devices.

In audio systems, parasitic oscillations can sometimes be heard as annoying sounds in the speakers or earphones. The oscillations waste power and may cause undesirable heating. For example, an audio power amplifier that goes into parasitic oscillation may generate enough power to damage connected speakers. A circuit that is oscillating will not amplify linearly, so desired signals passing through the stage will be distorted. In digital circuits, parasitic oscillations may only occur on particular logic transitions and may result in erratic operation of subsequent stages; for example, a counter stage may see many spurious pulses and count erratically.

Parasitic oscillation

Parasitic oscillation in an amplifier stage occurs when part of the output energy is coupled into the input, with the correct phase and amplitude to provide positive feedback at some frequency. The coupling can occur directly between input and output wiring with stray capacitance or mutual inductance between input and output. In some solid-state or vacuum electron devices there is sufficient internal capacitance to provide a feedback path.

Since the ground is common to both input and output, output current flowing through the impedance of the ground connection can also couple signals back to the input. Similarly, impedance in the power supply can couple input to output and cause oscillation. When a common power supply is used for several stages of amplification, the supply voltage may vary with the changing current in the output stage.

The power supply voltage changes will appear in the input stage as positive feedback. An example is a transistor radio which plays well with a fresh battery, but squeals or " motorboats " when the battery is old. In audio systems, if a microphone is placed close to a loudspeaker, parasitic oscillations may occur. This is caused by positive feedback, from amplifier's output to loudspeaker to sound waves, and back via the microphone to the amplifier input. See Audio feedback. Feedback control theory developed to address the problem of parasitic oscillation in servo control systems β€” the system oscillated rather than performing their intended function, for example velocity control in engines.

In practice, feedback may occur over a range of frequencies for example the operating range of an amplifier ; at various frequencies, the phase of the amplifier may be different. If there is one frequency where the feedback is positive and the amplitude condition is also fulfilled β€” the system will oscillate at that frequency. These conditions can be expressed in mathematical terms using the Nyquist plot. Another method used in control loop theory uses Bode plots of gain and phase vs.

Using Bode plots, a design engineer checks whether there is a frequency where both conditions for oscillations are met: the phase is zero positive feedback and the loop gain is 1 or greater. When parasitic oscillations occur, the designer can use the various tools of control loop engineering to correct the situation β€” to reduce the gain or to change the phase at problematic frequencies.

Several measures are used to prevent parasitic oscillation. Amplifier circuits are laid out so that input and output wiring are not adjacent, preventing capacitive or inductive coupling.Oscillations can take the form of sustained audible signals, very low frequency 'motorboating', or supersonic 'thud-hiss distortion '. These are frequently sensitive to control settings, such as motorboating to the bass setting, and supersonic to the treble. If you have oscillation the problem is how to get the feedback around the loop to a low level when it is in phase; conversely how to keep it sufficiently out of phase negative or opposing when the loop gain is high.

In older amps this is almost always due to large value electrolytic capacitors 'drying out' or basically losing capacitance with age. Sometimes these can be 're-formed' but replacement is often a better option. Other causes can be due to where, exactly, the various filter capacitors are 'grounded'.

In one case intractable low frequency hum and instability was traced to a filter cap being grounded to the signal shields. Single point earthing at the main filter capacitor isn't a new idea but it still works.

Many guitar amps ignore this with many grounds to chassis all over the place, inputs, output, power supply, and others. The older the resistor, the more suspect it must be. Very old 'dumbell' resistors see Amplivox chassis pic with wire connections wrapped around the end, using body-end-dot coding, must be checked.

These are now so old that, like waxed paper caps, you should consider replacing them on sight. Higher values, 1M and up are always suspect as these are particularly prone to drifting high.

tube amp low frequency oscillation

Daryl's experience shows this is not always the case, a low value going really low. This is a fragment of a Goldentone power supply showing the decouping networks, so-called because they are intended to provide DC power while de-coupling the stages from each other for AC signals. There are a couple of different ways you can look at decoupling networks depending on what you're doing.

The simplest way is that the shunt cap has to have a low reactance to signal frequencies or the stages will have a load in common to impress signals across. Another way is as a set of low-pass CR networks which are tuned to a very low frequency well below the amplifier passband so that when the frequency is low enough to pass the decoupling network, the amplifier gain has already dropped to a low value below its bass cut-off frequency.

But it may produce other odd effects if the unwanted coupling is of opposing phase see below. A similar supply decoupling network is frequently used for the screen of pentodes in low-level stages rare in guitar amps except possibly as the reverb driver stage and similar considerations apply. It is not unknown for screen by-pass caps to go low or open and cause problems. Below this setting everything may appear to be fine.

Amps with this kind of fault, particularly solid-state ones, should not be run in this condition for more than a few seconds. You may not be able to hear it but assume that it's close to full available output power and flogging the guts out of your amp - you may even see the pilot light dim. It is particularly deadly for piezo tweeters. This may come in the form of a solid oscillation, or more insidiously as a parasitic oscillation. This is a burst of oscillation that only occurs when the amplifier is driven, over only part of each input cycle, and possibly only at high levels.

The only real way to investigate this is with an oscilloscope where the oscillations may appear as fuzz on signal peaks. The workshop AM radio tuned off a station can sometimes give an indication of RF-oscillation in an amp. Most valve guitar amp output stages are fitted with small RF-stopper resistors right on the grid pin, VHF-style, typically 1k5. The object is to kill the Q of the hidden VHF oscillator lurking in the black heart of each output valve.There are many ways to tweak the tone or "voice" of a guitar tube amplifier.

Amp designers and modders can alter the clean and overdrive tone by changing component values, adding components, removing components, adding gain stages or even adding complete circuits like negative feedback.

This page also discusses how your amp's controls work which will help you get the most out of your guitar and amp. Knowing what a resonance control or cut control actually do to the guitar signal can make you a better player. You can tweak the voice early in the amp circuit for the greatest overall effect, especially on the overdrive tone, or you can tweak it late for minimal side effect.

It's ideal to have early and late tone controls in an amplifier circuit to offer the most versatility. This is especially true in high gain amps with lots of preamp distortion. Why can't you just turn down the tone or treble control to remove overdrive fizz and ice pick highs?

Because they are typically caused by harmonic and intermodulation distortion added by the overdriven stages in the amp--after the tone controls.

Using an early-in-the-circuit tone control to remove fizz and ice pick can smother the tone because you not only remove high frequencies but also all the high frequency harmonics that would have been generated as the guitar signal cascaded through the gain stages. A late-in-the-circuit tone control can usually remove the fizz and ice pick without killing the lush and layered overdrive harmonics and sparkle. I'm a fan of adding a VOX style Cut control which trims high frequencies very late in the amp circuit--from the power tube input.

It allows you to do a final tweak of the overdriven signal to darken it to taste without affecting the substance of the overdrive tone. When working with a new amp pay close attention to what knob tweaks do to the overdrive tone because a minor tweak early in the amp circuit can have a major impact. An added bonus of many of the tone tweaks on this page is they can help suppress oscillation and boost amplifier stability.

The higher the gain of an amplifier the more critical its voicing becomes. The Fender 5E3 Deluxe is beautifully voiced with rich overtones and a full bodied if not lose bottom end. Its clean tone is exceptional and when lightly overdriven sounds wonderful but push the amp too hard and all those low frequencies running through the amp wreck havoc on the amplifier circuitry.

The 5E3's big coupling and cathode bypass caps and lack of grid stopper resistors all work together to cause extreme blocking distortion known as "farting out. Reducing the size of the coupling and bypass caps and the addition of some grid stopper resistors can allow you to play Van Halen covers with the best of them. It just comes down to tweaking the voice. Why do most high gain channels sound thin and anemic when played clean?

Because to get the best overdrive tone you have to filter the guitar signal and remove a lot of the bass to prevent a muddy overdrive tone and prevent blocking distortion. But drive the high gain channel into overdrive and harmonic and intermodulation distortion fill in both the bottom and top end for a nice fat overdrive tone.

Keep that in mind next time you evaluate an amp with a high gain channel. The opposite is generally true for amps designed for their clean tone. Coupling and cathode bypass caps are usually large to pass low frequencies for a lush, full bodied tone. It's difficult to design an amp to do both clean and hard core overdrive well and that's why many amps that attempt it have one channel that's "not so great.Welcome, Guest.

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tube amp low frequency oscillation

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Everything works well and the amp sounds good to my taste. The issue is that I have a low frequency oscillation which can be heard at idle. I think this sound would be considered "motorboating" It appears with all gain controls set at zero. I seems to be associated with channel one. THe amp is built very close to this schematic.

I have moved all the wires around to no effect. I tried installing a low wattage tube in V1 to no effect. I have re-flowed many solder connections. The cap changes did make the amp sound better Everything is built per the schematic except the cathode caps are 25uf and not 22uf. There is a error on the schematic with the value of the resistors in the power supply which I changed to 10K to provide correct plate voltages.

I have pretty much done everything I know how to do to try to resolve the issue. At this point I really need some guidance on how to proceed. Thanks, Billy. Hi Do you have an oscilloscope or other test gear? Best bet is that it actually is a high frequency oscillation that impacts the bias.

Amplifier Compensation.

That gets it down to the point you can hear it. Probe through the circuit with a scope that has at least a 10 MHz bandwidth would be better and you likely will find the problem stage. Then it's a matter of fixing the bypassing and grounding to make it go away. I also used a probe connected to another amp to hear the sound it was making.

The sound does not appear on the plates of V1 and V2. I am not very skilled at using my scope. When looking at the signal at the output transformer there is a voltage both positive and negative that appears with the sound. Perhaps you could give me some guidance on how to set up the scope and just what to look for.

Here is a photo if that helps.It was only evident at certain frequencies β€” prevalent when I strummed low chords or struck low notes. At first the rattle was faint and only occurred occasionally. But over a number of weeks the rattle grew in prominence and occurred almost every time I played a low chord or note. I spent a few hours sitting at the back of the amplifier, strumming chords trying to identify where the rattle was coming from.

It seemed to centre around the power valves also known as tubes that hang behind the steel cage at the back of the amp. Valves are mercurial beasts. They can last a couple of weeks or a couple of years. Power valves are more prone to failure than those in the pre-amp stage. The more you play and the louder you play, the greater likelihood your valves will wear out sooner rather than later. Combo amps, like my Hot Rod, are particularly cruel to valves. Unlike a head and cabinet partnership, the valves in a combo live in a highly violent, reverberating environment directly behind the speaker.

Unless they stop working altogether, power valves begin to fail in a number of ways.

tube amp low frequency oscillation

Tell tale signs are:. If valves do rattle, the rattle will normally only be heard at low volumes. A cranked amp will drown the sound of filament rattle.

I used a quick test for tube rattle. I let the amp cool down and unplugged it. I unscrewed and removed the back panel to expose the power tubes. I gave the one on the right a few gentle taps with my fingernail, nothing. Then I tapped the left power valve. Ahh…it rattled like a broken light bulb. I was a bit lucky with this test method. Tube internals expand when they are hot and contract when they are cold.

We are really only concerned if the tube rattles when the amp is on and the tubes are hot. To test for rattle when the amp is on you could very gently tap the power valves with something non-conductive, like a tooth pick.