Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Thisismyname on March 30, 2025, 02:59:05 pm
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Hello, I have an AB763 based amp that I'm often tweaking suddenly exhibit a super high frequency oscillation. I've gone back on the mods I did to no avail.
The oscillation is so high I can barely hear it. It makes my ears ring, even when I turn the amp off. So I need to be able to measure the oscillation so I can visually see it disappear when I fix it.
I have a digital oscilloscope, but limited experience. The manual briefly touches on relevant points. I experimented myself with the FFT function in particular, but only found octaves of 60 cycle hum going up to include 15kHz, which may be what I'm hearing :dontknow:
The amp has quiet background noise too.
How can I confidently search for oscillation in my amp with my scope?
What settings or methods do people use and what might this oscillation look like?
I'm aware of bad layout designs and fixes, so I'm sure I can troubleshoot the problem once I can verify it's there. I haven't completely ruled out that it's just my tinnitus playing tricks on me.
Thank you for any and all help.
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even when I turn the amp off.
going up to include 15kHz
my tinnitus playing tricks on me.
I always start hunting with a known good signal that I inject into the amp
with all knobs set at "5" and all "effects" OFF (verb, Trem) I expect to see the same basic waveform at the speaker + - that I see at the input
If you don't see what you hear than something is most likely off in your setup.
FFT is a great way to "see" unwanted frequencies but tube amps are great harmonic makers, so you have to understand what the fundamental frequency looks like and know what are the harmonics of that fundamental.
so, start with a signal source directly into your scope, adjust for about 80mVrms that's "bassline"
stick it into the input, set knobs, measure the output at the speaker, post screenshots of baseline and speaker.
1 how long after the amp is off do you hear the tone?
2 Pets and wives are great for verifying whether it's tinnitus or real :icon_biggrin:
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I was able to input a 1kHz sine wave from my phone, into a keyboard (that doesn't produce sine waves) and out through the output into my amp. Probably not the cleanest signal, but it's all I can get right now.
With master at 10, 3-band eq at 5, the amp starts to oscillate around 3-4 on the volume. I can actually hear the 1K sine wave being produced somewhere in the power amp/phase inverter circuitry! I tried other pitches with the signal generator and got a corresponding pitch change from within the circuitry.
Is it normal to hear oscillations from within the circuitry when the amp has parasitic oscillation?
When the oscillation occurs, it pulls down voltage in the amp, which I've read is common.
The yellow trace is the 1K signal at the input jack. The blue trace is the signal at the speaker jack. The amp volume was only at 3-4 for the blue trace to look like that.
At least I know for sure it's there now lol
It's odd I can hear it inside the circuitry. Does that narrow it down at all?
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" I have an AB763 based amp that I'm often tweaking suddenly exhibit a super high frequency oscillation. "
1-Has this amp ever run fine, without oscillation?
If so, what happened to make it oscillate now? Mods ?
2-Was this amp built by you or a manufacturer?
Oscillations are very often the result of poor assembly, and it's only by revising the assembly that you can find out what's causing the problem. More than if you use an oscilloscope.
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" I have an AB763 based amp that I'm often tweaking suddenly exhibit a super high frequency oscillation. "
1-Has this amp ever run fine, without oscillation?
If so, what happened to make it oscillate now? Mods ?
2-Was this amp built by you or a manufacturer?
Oscillations are very often the result of poor assembly, and it's only by revising the assembly that you can find out what's causing the problem. More than if you use an oscilloscope.
The amp was fine for years, even when it was full of a mods and switches during my early years as an amp builder. I rebuilt it a year ago to be a single channel, no fx Deluxe Reverb, since that's what the iron is for.
The amp is a lot simpler now, with a much better layout. I added a reverb circuit, noticed the oscillation, then removed the reverb circuit from signal and power. It's still oscillating. I keep bringing the amp back closer to the stock DR. The oscillation is still there. I can't escape the feeling that some part is slowly failing.
I'll work on it more this evening.
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IMO chance are a wires dress issue more than defective parts.
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First off, hi-res photos (gut shots) and a schematic will help the forum help you. There are as many ways to implement an AB763 as there are amp builders.
Personally, I can't work while listening to a high pitched sound like you're describing. I'd suggest using a dummy load across your output jack rather than a speaker. It won't exactly match the impedance, but close enough and you'll be able to trace in comfort. I use a 50W 8ohm resistor.
I do suggest getting a signal generator. A cheap sig-gen makes it easy to inject signal without fuss, and without the risk of damaging your other expensive equipment. No need for an expensive fancy unit -- an option might be something like FNIRSI DSO510 which is a combo pocket scope and sig-gen, fairly inexpensive on ammozonia, and a pretty capable unit for what it costs.
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First off, hi-res photos (gut shots) and a schematic will help the forum help you. There are as many ways to implement an AB763 as there are amp builders.
Personally, I can't work while listening to a high pitched sound like you're describing. I'd suggest using a dummy load across your output jack rather than a speaker. It won't exactly match the impedance, but close enough and you'll be able to trace in comfort. I use a 50W 8ohm resistor.
I do suggest getting a signal generator. A cheap sig-gen makes it easy to inject signal without fuss, and without the risk of damaging your other expensive equipment. No need for an expensive fancy unit -- an option might be something like FNIRSI DSO510 which is a combo pocket scope and sig-gen, fairly inexpensive on ammozonia, and a pretty capable unit for what it costs.
Very good advices !
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First off, hi-res photos (gut shots) and a schematic will help the forum help you. There are as many ways to implement an AB763 as there are amp builders.
Personally, I can't work while listening to a high pitched sound like you're describing. I'd suggest using a dummy load across your output jack rather than a speaker. It won't exactly match the impedance, but close enough and you'll be able to trace in comfort. I use a 50W 8ohm resistor.
I do suggest getting a signal generator. A cheap sig-gen makes it easy to inject signal without fuss, and without the risk of damaging your other expensive equipment. No need for an expensive fancy unit -- an option might be something like FNIRSI DSO510 which is a combo pocket scope and sig-gen, fairly inexpensive on ammozonia, and a pretty capable unit for what it costs.
I am using a dummy load. The pitch I'm hearing is coming from somewhere in the power amp/phase inverter area. It's like a component is vibrating at whatever frequency I inject into the amp.
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First off, hi-res photos (gut shots) and a schematic will help the forum help you. There are as many ways to implement an AB763 as there are amp builders.
Personally, I can't work while listening to a high pitched sound like you're describing. I'd suggest using a dummy load across your output jack rather than a speaker. It won't exactly match the impedance, but close enough and you'll be able to trace in comfort. I use a 50W 8ohm resistor.
I do suggest getting a signal generator. A cheap sig-gen makes it easy to inject signal without fuss, and without the risk of damaging your other expensive equipment. No need for an expensive fancy unit -- an option might be something like FNIRSI DSO510 which is a combo pocket scope and sig-gen, fairly inexpensive on ammozonia, and a pretty capable unit for what it costs.
Thanks for the advice 👍 I'll look into that scope/sig generator.
I haven't run out of ideas of my own for troubleshooting this, so I won't upload pics and a schematic just yet. I don't want to waste anyone's time before I've exhausted my knowledge/experience. I do appreciate anything people post though 👍
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"hearing" the injected signal may indicate your bias is way off, I've had amps that were biased TOO HOT make some interesting sounds
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"hearing" the injected signal may indicate your bias is way off, I've had amps that were biased TOO HOT make some interesting sounds
I will double check bias 👍 I usually set it at 70% max dissipation in my fixed bias amps.