Can I do this with an oscilloscope?

[Planobilly]

Hi,

I am frequently concerned with audio "noise" coming from a guitar amp. I can inject a 500hz signal for example and obviously see that signal at the output transformer. There are other frequencies also being produced by the amp at the same time which are typically hard to see or not displayed at all. For example low amplitude 120Hz.

Is it possible to build some sort of filter that would filter out all frequencies greater the say 150Hz? The signal generator would be driving the amp at 500hz but I don't want to see that signal on the scope.

I am wanting to look for 60Hz, 120Hz signals and measure their amplitude. I also want to look for frequencies greater than than 20KHz.

Thanks,

Billy

 

[jim hardy]

Is it possible to build some sort of filter that would filter out all frequencies greater the say 150Hz? The signal generator would be driving the amp at 500hz but I don't want to see that signal on the scope.

That'd be a real sharp cutoff low pass filter.

These will do a fine job
http://www.ebay.com/itm/Rockland-852-Dual-Hi-Pass-Lo-Pass-Cutoff-Filter-0-to-10MHz-/252661654268?hash=item3ad3cee2fc:g:P9sAAOSw-0xYP1SG

but if you're always after say 500 hz, i'd say you can build a
"Biquad Filter"
with Q 50 or so, put it in a plastic box with BNC's and a 9v batery, call it your o'scope accessory.
I used the one in fig 52 here for a very narrow bandpass , worked quite well . Cascading them gives sharper cutoff.

http://www.ti.com/lit/ds/symlink/lm359.pdf

 

[Averagesupernova]

You could play with some of the available software that shows a spectral display on a PC. Just feed something into the mic input. Be careful here, you want to make sure to keep high voltage out of your PC.
-
If you want to troubleshoot noise generated by the amplifier you would just short circuit the input and go from there. I see no reason for a filter if you can utilize a spectrum analyzer.

 

[Planobilly]

LOL Jim...you are going to pull me into transistor electronics yet!

Seriously though, I assume the concept at least is logical. Build a device of this nature would be fun. The one on ebay is not too far out of my budget.

Thanks,

Billy

 

[Planobilly]

Hi Averagesupernova,

I actually have a recording program ( Sonar Professional) on my main computer system up stairs that is chock full of spectrum analyzers. Plus many types of filters. I had assumed that it might prove useful. I have not tried it yet because of the issues of moving the amps from the workbench down stairs to the computer system upstairs which is pretty large and complex and can not be easily moved.

I also had some concern about introducing ambient noise into the microphone. I don't really think that will be an issue that I can not overcome as I have a lot of ways to control the microphone.

I had in mind to run a microphone cable down stairs as I can remotely control the computer from this computer on my work bench. Too many projects..too little time...lol

Thanks,

Billy

 

[tech99]

Hi,

I am frequently concerned with audio "noise" coming from a guitar amp. I can inject a 500hz signal for example and obviously see that signal at the output transformer. There are other frequencies also being produced by the amp at the same time which are typically hard to see or not displayed at all. For example low amplitude 120Hz.

Is it possible to build some sort of filter that would filter out all frequencies greater the say 150Hz? The signal generator would be driving the amp at 500hz but I don't want to see that signal on the scope.

I am wanting to look for 60Hz, 120Hz signals and measure their amplitude. I also want to look for frequencies greater than than 20KHz.

Thanks,

Billy

Why do you not set the reference level using 500Hz tone and then switch it off and measure the noise?

 

[Planobilly]

Hi tech99,

If I understand what I think I know, there is the noise that is being generated at quiescent conditions and other noise being generated as a function of amplification. The injection of some frequency like 500 Hz which is arbitrary to begin with, 1000Hz being a more common reference frequency is used to drive the amp. As the amplitude of the reference frequency is also adjustable other conditions that may exist at quiescent but are not noticeable can only be seen or heard with signal input.

The whole idea is to identify frequencies that should not be there and define where they are coming from. Listening to a amp, one may be able to hear an unwanted low frequency of 120 Hz but it may not be being produced by the usual places it normally comes from such as the power supply. Faulty tubes, caps, and many other conditions produce low frequency noise. These faults may or may not be apparent at quiescent conditions.

I guess the simple answer to your question is that when you turn off the 500hz signal you are no longer causing the tubes to amplify.

Cheers,

Billy

 

[tech99]

Hi tech99,

If I understand what I think I know, there is the noise that is being generated at quiescent conditions and other noise being generated as a function of amplification. The injection of some frequency like 500 Hz which is arbitrary to begin with, 1000Hz being a more common reference frequency is used to drive the amp. As the amplitude of the reference frequency is also adjustable other conditions that may exist at quiescent but are not noticeable can only be seen or heard with signal input.

The whole idea is to identify frequencies that should not be there and define where they are coming from. Listening to a amp, one may be able to hear an unwanted low frequency of 120 Hz but it may not be being produced by the usual places it normally comes from such as the power supply. Faulty tubes, caps, and many other conditions produce low frequency noise. These faults may or may not be apparent at quiescent conditions.

I guess the simple answer to your question is that when you turn off the 500hz signal you are no longer causing the tubes to amplify.

Cheers,

Billy

Hi Billy
The tubes are amplifying even with no drive, so most noise sources will be visible. It is possible however, though unusual, for something to arise only during drive conditions, for instance a Class B amplifier might become unstable. The other source of "noise" is if the amplifier generates harmonics of the 500Hz tone. This will be a consequence of non linearity and you should be able to see it by looking at the shape of the 500Hz at the output, where it should be a perfect sine wave. Any instability of the amplifier should also be visible on the 'scope.
A technique for measuring total noise and harmonic distortion utilises a notch filter tuned to 500Hz at the output. This notch filter is traditionally a Wien CR bridge. You can check the measurement system by using the filter directly on the signal generator without the amplifier in circuit.

 

[Planobilly]

Yes I understand what you said. The conditions I am speaking about are sort of unusual.

Well, I have been setting around for the last two hours playing the new amp and every now and then it has started to make a small almost inaudible static sort of sound only on certain notes I play which it does not make when it has no signal. If I am lucky, I will get it to reproduce this fault hooked up to the scope. This will most likely be easy to see because it will look like a voltage spike of some sort. Finding where it is coming from may or may not be easy. This situation could be caused by several things actually. The guitar cable or guitar. May also be a cap going bad somewhere.

Also I have FFT on my scope to look a harmonics. It is not a super good feature on the Owan scope I have but it is what I have to live with at the moment.

Thanks,

Billy

 

[Svein]

Well, I have been setting around for the last two hours playing the new amp and every now and then it has started to make a small almost inaudible static sort of sound only on certain notes I play which it does not make when it has no signal. If I am lucky, I will get it to reproduce this fault hooked up to the scope. This will most likely be easy to see because it will look like a voltage spike of some sort. Finding where it is coming from may or may not be easy. This situation could be caused by several things actually. The guitar cable or guitar. May also be a cap going bad somewhere.

Sounds more like an instability in the amplifier - possibly a periodic high frequency oscillation.

 

[dlgoff]

... every now and then it has started to make a small almost inaudible static sort of sound only on certain notes ...

My thought is that those notes are being clipped somewhere. From Wikipedia Clipping (signal processing):

Hard clipping results in many high frequency harmonics; ...

 

[Planobilly]

First thing I did was re-seat the tubes. That seem to fix the issue or at least it has not returned yet. I doubt anyone but me would have even noticed the sound. It happened at random intervals.

There could be high frequency oscillations going on, that would not be unusual with these old amp circuit designs. There are some ways to make them more stable but generally at the expense of good tone. This could also be environmental EMI. I do have some noise from a modem I am aware of.

Not much to do until it comes back, if it does.

Cheers,

Billy

 

[UsableThought]

A few comments about the comments so far - FYI, my perspective by the way isn't a physics/EE perspective, nor even a "really good amp tech" perspective. However, as a hobbyist, in the past 2 years I've done a lot of work chasing down & eliminating various sorts of noise in tube guitar amps (and a bit in SS), and I've done a huge amp of reading on this as well. I also frequent forums elsewhere where the perspective is exclusively to do with building, repairing, or modifying guitar amps - both SS and tube, but most of the time tube. I've previously mentioned a couple of these forums to @Planobilly & recommended visiting them; they can be helpful if you are able to separate out the 95 percent witless comments from the 5 percent really good EE/tech comments.

Anyway - starting with the question of whether a scope or something else is best used for identifying & diagnosing problems with more or less continuous noise (e.g. 120Hz ripple, 60Hz hum, etc.):

I am frequently concerned with audio "noise" coming from a guitar amp . . . For example low amplitude 120Hz.

You could play with some of the available software that shows a spectral display on a PC. Just feed something into the mic input. Be careful here, you want to make sure to keep high voltage out of your PC.

I actually have a recording program ( Sonar Professional) on my main computer system up stairs that is chock full of spectrum analyzers. Plus many types of filters.

I agree with @Averagesupernova that separating out different frequencies is a lot easier using an FFT app that can draw a spectrum. A scope is far more difficult for this but can be used. You can avoid frying the computer soundcard by 1) plugging the app into a portable USB sound card with a line level input (better than mic for this purpose), and 2) building a variable voltage attenuator (with a diode clamp too) to place after whatever probe you use & before the USB sound card; I can point to a good recipe if anyone is interested. OR, you can just be really careful; but the attenuator is more useful & safer.

Also I say "probe" because you would want to use this device in conjunction w/ a probe you build w/ a DC blocking cap & ground clip, similar to a scope probe but much simpler; this allows you to directly check nodes in the amp. This typically excludes the power amp due to the very large voltage swings on the plates there (although you can build yourself a 100-to-1 attenuator if you really wish for that); but it includes the preamp signal path right up through the phase inverter or driver. Also you can carefully look for AC (ripple and/or audible noise resulting from diode ringing) on the rectifier output. Thus you can often find exactly where a noise is entering the signal path. It's a great way to speed up diagnosis & supplement the old "divide and conquer" routines. The late & wonderfully generous Bill Machrone, former editor in chief of PC Magazine, for many years had a small business selling mod kits for a specific kind of Fender amp, the Blues Junior; his site has a neat page showing how to build such a probe: http://billmaudio.com/wp/?page_id=1254

Also Billy, no need to try & move your big computer; just get a laptop, tablet, or other small device; they make the appropriate apps for these devices at between $10 and $20 or so.

However . . .

Well, I have been setting around for the last two hours playing the new amp and every now and then it has started to make a small almost inaudible static sort of sound only on certain notes I play which it does not make when it has no signal . . . First thing I did was re-seat the tubes. That seem to fix the issue or at least it has not returned yet.

Sounds more like an instability in the amplifier - possibly a periodic high frequency oscillation.

. . . I wouldn't chase after the noise w/ an FFT setup quite yet, because I agree w/ @Svein that since the noise is associated w/ playing a note, it is not your typical mains hum/rectified mains buzz/diode ringing sort of situation, but most likely some kind of instability. There are many sorts of short-lived oscillations that can occur that will cause what are called "ghost notes." Billy, since you seated the tubes & the noise seems to have gone away, you've tried the #1 thing that typically gets recommended on the guitar forums just to start out with. Microphonic tubes; poorly seated tubes; tube sockets that need tightening and/or cleaning; these can all sometimes cause a small instability.

BTW, @dlgoff mentioned clipping; this noise that Billy was hearing was almost certainly not clipping; clipping is very different & is not experienced as a sound like "static", nor as separate from the originally played note. Also the high frequencies associated with clipping vary depending on the type of clipping and how hard it is, but generally speaking clipping is experienced as fairly musical in nature. Anyway that's a whole other topic.

There could be high frequency oscillations going on, that would not be unusual with these old amp circuit designs. There are some ways to make them more stable but generally at the expense of good tone. This could also be environmental EMI. I do have some noise from a modem I am aware of.

Actually, although tracking down oscillation can be difficult - the causes are potentially quite numerous - getting rid of it need not be at expense of tone. Some tube guitar amps by design may live near the edge of instability, but they don't actually have to be unstable to sound good. Instability generally occurs w/ older amps and is introduced by either aging components, or mistakes made with either component values or layout during repair or modification. Poor layout or lead dress can lead to unintentional positive feedback; a filter cap going bad can cause problems due to increased leakage; apparently coupling caps can cause issues; etc. Once identified such issues can be corrected w/ no alteration to the amp's originally intended behavior. FYI, here's a nice page someone put together on sources of instability: http://www.ozvalveamps.org/stability.htm

Also regarding EMI, low-frequency EMI would be constant & not just when notes are played; however if it's a really old amp, it may not have grid stopper resistors on the input triode(s) and thus might be admitting RF frequencies. If I remember computer modems do have some RF, but the operation that causes interference (switching of some sort?) is apparently intermittent & unlikely to cause instability. Generally people seem to diagnose modem interference in their homes as an intermittent clicking sound down in the audio spectrum.

BTW if you ever do want to add grid stoppers, so as to avoid noise from cell phone calls, AM radio stations, and other sources of RF, these can be added quite easily (put them right up next to the input pin on the socket) & again won't hurt tone a bit. In theory a grid stopper can add a small amount of resistor hiss for when the amp is cranked up, but today's metal foil resistors are very quiet; plus if it's a really high gain preamp & you still are concerned about hiss, you can take the further step of reducing the value of the grid stopper down to say 10K by adding a very small cap, say 300pF, after the grid stopper & running its other end to ground; this improves filtering even w/ the smaller 10K resistor such that once again you are cutting off all frequencies above say 20KHz (obviously you can do out the math if need be).

Anyway glad the issue seems to have stopped w/ the reseating of tubes. Hopefully it won't come back any time soon. From that page I linked to above about causes of instability:

An amplifier is only an oscillator waiting for the chance.

 

[Planobilly]

Hi Usable Thoughts,

Thank you for taking the time to post all your comments. I will take the time to study all you said in detail.
There are some functions on my scope I don't know how to use. I started to make some photos but there is so many screens I need to make a video so anyone could make sense of it. I will post it when I get it made.

I turned the amp on this morning and the intermittent crackling noise came back. It occurs on the low E string and A string. It is the typical sort of noise I have heard many times associated with issues with tube sockets. The sort of noise that is caused by arcing. As this is a new amp built with the best components I could find so some of the typical issues with old amps do not apply. Also the AB763 Fender circuit has been used in thousands of amps in one form or another and the Deluxe Reverb is not known for instability.

After I finished the amp there was a problem with the reverb recovery circuit causing hum and unusual noises. I measured the value of the components and found nothing wrong. I finally got tired of dealing with the issue and replaced the plate resistor, cathode resistor, cathode cap and the .0033 coupling cap. That solved the issue and the amp started working without issue. I don't know which of those components had actually failed or why they failed. Bad component? Overheated them when I soldered them in? Who knows. This problem is part of what prompted me to ask questions about troubleshooting noise.

It is entirely possible that I got a bad batch of caps. They were all best quality brands, Sprague Atom, Nichicon, Cornell Dubilier 715/PS Film capacitors.

So...the issue is more or less constant now and I hope I can reproduce the fault with signal injection which is a lot easier to deal with than playing the guitar and looking at the scope. I don't think this is circuit design instability. If I am lucky it will be a failing component. Worst case is a bad solder joint or something unusual like a bad stand by switch that is arcing. If the fault is in fact arcing it can be hard to find.

If I can find what stage or tube is associated with the issue the problem can be fixed even if I have to take the shotgun approach and just replace the components associated with that stage.

I am going to take the chassis out now and start looking. I will post the results. I really want to get this issue fixed so I can get back to my original questions about using scopes and other methods for tracking down noise in general.

Best wishes to everyone for the coming New Year,

Billy

 

[dlgoff]

BTW, @dlgoff mentioned clipping; this noise that Billy was hearing was almost certainly not clipping; clipping is very different & is not experienced as a sound like "static", nor as separate from the originally played note. Also the high frequencies associated with clipping vary depending on the type of clipping and how hard it is, but generally speaking clipping is experienced as fairly musical in nature.

You are correct. I just checked out http://www.sfu.ca/sonic-studio/handbook/Peak_Clipping.html where there's an http://www.sfu.ca/sonic-studio/handbook/Sound/Peak_Clipping.aiff.

 

[Planobilly]

Here is a video of the issue.



I have made a lot of changes and tried a lot of things with no results. I really have no idea at this point what could be causing this issue. I guess it is possible the amp had done this from the beginning. It is hard to imagine I would not have noticed it after I resolved the hum issue in the reverb recovery circuit. At that point the amp sounded great as I tried the two new different type of speakers I was testing.

Perplexing to say the least.

Billy

 

[jim hardy]

Bill this is speculation

try "divide and conquer". Amp or speaker ?

i'm not sure whether i heard the buzz on my cheap headphones
but i see a small speaker trying to play bass notes at high volume

as you know
distance= rate X time

to make a 100 hz bass note a speaker cone must move forward for 1/200 second and backward for 1/200 second.
How far does it move during that 5 milliseconds ? Depends on how loud it's asked to be.

Small speakers have limits to travel.
Outward limit is where the voice coil starts to exit the magnet area, or the surround gets stretched so tight it stops motion.
Inward limit is where the voice coil former , usually a thin plastic or aluminum cylinder, hits the bottom of the magnet structure. That makes a loud 'clack' and a series of 'clacks' is a 'buzz'.
I don't know what hitting the "out" limit would sound like, i'd think it might not even be noticed.. But the "In" limit is a physical collision.

To isolate if that's what you are hearing , try a bigger speaker, or close the back of that amp making it a sealed enclosure that'll limit cone travel naturally.

Good stereos have a LF rolloff button that prevents subsonic frequencies from reaching the speaker, that's in case somebody is using vented enclosure with a high powered amp that's capable of driving the speaker beyond its limits. It'll also keep your speaker cone from trying to follow a warped LP record..
Sealed encosures limit cone travel because they compress the air behind the cone.
Vented ones don't so you have to be aware of your amp's capability .
Guitar amps I've seen have open backs. So there's no inherent limiting of cone travel. If you "souped up" that amp it might need a speaker with 'longer throw' .

What's low end response of that little amplifier? Do you think it's capable of overdriving your speaker? Gently place a finger on the cone while playing that note and see how much motion you feel...

one step at a time.

old jim

 

[Averagesupernova]

It'll also keep your speaker cone from trying to follow a warped LP record..

Interesting. I never thought of that. A former coworker and I used to discuss whether the subsonic filter did anything at all. In that case it is likely it could.

 

[Planobilly]

Hi Jim,

Sorry the video did not "show" much. It was meant just to let you hear the sound.

I have played the amp though a quad Marshall cabinet with four 12" Celestion with the same results. The little Orange amp is a small transistor amp I use to hear the results of a probe placed at different points in the circuit. The speaker you hear in the video is a 12" Weber DT12 30 watt guitar speaker.

The issue is definitely in the amp. The greater question is about what the amp should normally sound like. Is this buzzing normal for this design? I don't think so but I have not found any defective component as of yet.

I also plugged the amp into a another Weber 12" 65 watt speaker designed with a Neodymium magnet. The speaker is more efficient and has a different bass response. That speaker reduced the buzz by a small amount.

The buzzing noise happens on both channels. It happens when channel one is disconnected and the reverb stage and tremolo stage are disconnected.

That only leaves V2A and B, V4B, and the phase inverter and of course the power stage. I went back and checked all the voltages and ripple values plus the bias on the output stage tubes. I replaced coupling caps and cathode caps and resistors on V2 A and B. I measured the resistor values on the rest of the amp. All this without finding any issues. No change.

If I have too I guess I can start changing coupling cap values to change frequency response and reduce the capacitance of the cathode cap. All that is a bit over my head to calculate but I can figure it out if need be.

Jeezs...I just want to build amps, not worry about being in the corner with any frequency ...lol I know, I know, silly joke

Cheers,

Billy

EDIT: The amp is around 20 watts, and yes it would be capable of overdriving a small speaker. The whole video is with the amp volume turned up less the 50%.

 

[jim hardy]

Okay, use your scope's FFT spectrum analyzer to find the frequency of that string.

Look for difference in the harmonics in amp output at low and high volume. Distortion should appear as bigger harmonic content

Use your signal generator to feed that frequency into the amp continuous so you can look with scope for distortion stage by stage, examining both the raw signal and its spectral content via FFT .

Sometimes a triangle wave will make distortion more obvious to the eye.

Remember when Svein explained what is "Transient Intermodulation Distortion", overdriving intermediate stages within a closed loop ? I'd learned that phenomenon by another name, so was mistaken about what it meant.
Could be that's going on, is there a schematic of this little guy someplace ?

 

[Planobilly]

Hi Jim,

The frequency of the low E string on a guitar is 82.41 Hz when properly tuned. The A string is at 110.00Hz which is two octaves down from standard 440HZ tuning on the piano and is the second string down from the top. The amp buzzes when either of these strings are played very hard.

There is a big difference in the waveform of the E string from the guitar injected into the amp and the signal generator injecting a 82.41Hz. Even playing/striking only one string produces a bit of a complex waveform with a least three harmonics and no real way to accurately control the amplitude, well, at least by my ability to play guitar...lol

I will try what you said to do. I never thought about using a triangle wave

Here is a schematic. There are a few changes in the amp I built relating to the tremolo circuit. I have disconnected the tremolo circuit which did not change anything with the fault.

I have the reverb driver and reverb recovery disconnected, also what they are calling the vibrato circuit disconnected to make looking for the fault less of a problem.

Cheers,

Billy
EDIT: here is a link to the same schematic http://schematicheaven.net/fenderamps/deluxe_reverb_ab763_schem.pdf

 

[UsableThought]

I turned the amp on this morning and the intermittent crackling noise came back. It occurs on the low E string and A string. It is the typical sort of noise I have heard many times associated with issues with tube sockets. The sort of noise that is caused by arcing.

Suggestions:

1) The video is pretty good for showing the sound; but it left me wondering, have you tried other guitars into this amp? You seem confident the guitar is not involved, but it never hurts to rule things out.

2) Presumably you've swapped out all your tubes, one by one, with replacements that you know work properly (e.g. tested in another amp & don't have problems)? This is always the first recommended thing to do w/ any sort of problem like this.

3) If swapping tubes didn't locate a problem tube, a good next step is to check for a loose socket. You can do this by gently wiggling each tube in turn while the amp is live - you don't need to be playing nor have the amp volume turned up particularly high; you just want to see if you get a burst of static for any particular tube. You would probably want to wear leather work gloves or otherwise protect your fingers when touching the power tubes! A very small amount of static is unlikely to be a problem, but a big burst of static might indicate that a pin or pins on a socket needs tightening.

4) If the tube sockets don't seem particularly noisy, you might still go ahead & clean the pin holders on the sockets if they are anything but brand new; this is to make sure no oxidation is getting in the way and creating an intermittent contact. DeoxIT type D is commonly used for this purpose. You can Google for what the best tool to use for the job might be; you definitely don't want to loosen the pin holders while doing this, so something as simple as roughing up the end of a big paper clip with a file might give you enough of a tool to work with. Of course if the sockets are brand new & looked in good shape when you installed them (no visible oxidation), you could probably skip this step.

If tubes & tube sockets have been ruled out, then the issue is more complex. It is unlikely to be 'arcing' in the sense normally meant in troubleshooting a guitar amp; arcing typically produces more of a popping sound, and is not usually confined to notes of a particular frequency: see this page on the Geofex web site. BTW if you're not already familiar with Geofex, now is a good time to get acquainted; the site is maintained by R.G. Keen, who is something of a demi-god in the DIY guitar tube amp world. Here's the home page & here's the tube amp troubleshooting index page.

Ghost notes or buzzing notes can have a wide variety of causes, e.g. a leaky filter cap, speaker rub, sympathetic resonance of something in the cab or even in the nearby environment, etc. There's also the possibility (as already mentioned) that it's an instability somewhere in the amplification chain that is leading to positive feedback & thus unwanted oscillation (see Parasitic oscillation in Wikipedia). For a troubleshooting case study of instability of this sort that was solved, see this thread over on The Gear Page (one of the forums I previously mentioned) in which someone experienced a problem with a "fuzzy ghost note" on the low E string: SOLVED - '74 Bassman 100 Renovation - Fuzzy Ghost Notes on Low Strings? The thread wanders about quite a bit, with considerable speculation by the OP (original poster) about whether the output transformer might somehow be the problem. However, if you go to the OP's final comment in the thread, here, and scroll down past the pictures, you'll see that he wraps up the thread by reporting that one of those commenting (someone with the handle "pdf64") was correct & the problem had been caused by an incorrectly sized shunt resistor in the negative global feedback loop, leading to instability that caused the "ghost notes." Replacing that resistor w/ a more appropriate value fixed the problem.

Now, an incorrect resistor value in the feedback loop circuit seems unlikely to be the problem in your amp; I get the impression you built this either as a kit or just as sourced parts, but either way, strictly according to the Deluxe Reverb schematic? So presumably you're using the stock resistor values everywhere. HOWEVER - even so it's very easy to create a similar problem (instability in the negative feedback loop) without realizing it. For example poor lead dress can cause this - see comments #21 and #25 in that "Ghost Notes" thread for more. So one thing you could do is review your lead dress to see if you've violated any guidelines, e.g. routed wires from different stages too close together as well as too close to being parallel.

I'd also suggest you get more narrowly targeted expert help with this problem than is likely going to be possible on a forum devoted to physics; and so I would again urge you to post on a forum such as The Guitar Page which is specific to tube guitar amps. The "Ghost Notes" thread above appeared on The Gear Page forum titled Amps/Cabs Tech Corner: Amplifier, Cab & Speakers

You'd get a fair number of mediocre comments over there; but if you got lucky (and it happens pretty often) you'd also get comments from folks there like pdf64, who was so helpful to the OP in the "Ghost Notes" thread. pdf6 happens to be a retired EE who has been modifying & repairing tube guitar amps for several decades, so he not only has an outstanding theoretical background, but also a wealth of guitar-amp specific practical experience that you are somewhat less likely to find here on PhysicsForums (no offense intended to anyone here). There are several EEs over on that forum w/ just as much experience as he has; plus there are several extremely experienced guitar techs, including a number who specialize in troubleshooting & repairing Fenders in particular.

So those are the sorts of people over on that forum who could give you the help you really need - whether or not the problem is in the feedback loop, or elsewhere. The base level membership is free, just as here; and they use the same forum software (Xenforo); so the hassle in starting a new thread there would be minimal. If you do go ahead & post there, I'd suggest starting with a description of the problem, including the video of what it sounds like; a description of the amp build (when, what, whether it's a kit or not, etc.) plus schematic, as you have done here; and then, if you have time, adding a second post with "gutshot" pics of the layout, showing lead dress.

Another good forum is "Music Electronics Forum" - specifically the sub-forum on solving amp problems; they too have some great members who are either guitar-happy EEs, extremely experienced guitar amp repair techs, or both: Maintenance, Troubleshooting & Repair

 

[UsableThought]

1) The video is pretty good for showing the sound; but it left me wondering, have you tried other guitars into this amp? You seem confident the guitar is not involved, but it never hurts to rule things out.

Forgot to mention, it might seem silly, but to rule out speaker rub or sympathetic resonance with something in the cab, you can try playing the amp through an external speaker of the correct impedance. Use a long speaker cable if you like to get extra distance. Resonance occurs more often than you might expect, and usually manifests only for particular notes.

 

[jim hardy]

Usable, you've been around tubes, haven't you ?

Well Bill
when i get puzzled because nothing will show itself wrong, i start making things prove themselves right. By challenging parts that way i also am challenging my understanding of the circuit.

First thing i think i'd do is squirt some kind of cleaner solvent down those tbe sockets. I like Miller Stephenson MS185 because it has a teeny bit of acetone to remove traces of flux.

Next thing i'd do is lift the feedback resistor.

It only wraps the phase splitter and output stages and splitter gain is only about 1, so i'll be surprised if that's causing your trouble, bur i might be missing something...
If it's really negative feedback then lifting it should make the amp louder. If it lowers volume instead then it was positive and the output transformer leads are rolled.
If lifting it changes things in some curious way i'd patch a 10K pot in its place and experiment with value of feedback resistor .

Above should tell you something about feedback.

Observe Svein's TIM link says the fix is to have feedback wrap individual stages. That comes natural to us old control guys , we spread out gain of a loop and take precautions to not drive the middle against a limit for that destroys your linearity.
Your feedback only wraps two stages, not bad. Just make sure it's really negative.

My two cents.

Probably be absent a couple days , going to visit my sister .

Keep poking at it. You have a powerful 'scope there . Put input and output on two channels, compare them. Surely it has storage? See if you can get it to trigger from the buzz.

old jim

 

[UsableThought]

Next thing i'd do is lift the feedback resistor.

This is a good suggestion, more efficient perhaps than my suggestion about cleaning up lead dress - disconnecting the feedback loop should immediately tell you whether it's involved. If not, not, and you can move on to other candidates.

 

[jim hardy]

Resonance occurs more often than you might expect, and usually manifests only for particular notes.

I'd guess his speaker in that open back cabinet is basically in free air. They publish free air resonance frequency for speakers, and he could sweep a signal generator...

ps that 47 ohm completes the feedback voltage divider . 'Scope it with and without feedback resistor. I'd expect nothing there with fb resistor lifted. But as i said, this poking around challenges my preconceived notions about how the circuit works.

old jim

 

[UsableThought]

'd guess his speaker in that open back cabinet is basically in free air. They publish free air resonance frequency for speakers, and he could sweep a signal generator...

It's not so much the speaker, but the cab itself, I think. With the gain & volume up high, I get a lot of resonance for the low A fretted on the low E string (not the low A string itself, have no idea why) in the combo cab of my little 10W Laney Cub; and when I go to the external speaker this goes away. Combo cabs are famous for being noisy this way. The head is bolted in pretty tight, so maybe it's the frame of the cab, I don't know. I don't play that loud often enough to worry about it; if it was a real problem for me, I'd just go with the external speaker cab rather than try & chase down the exact cause.

But you can also get resonance from objects nearby - sometimes this is very bizarre, i.e. people report discovering that the buzz for certain notes when they play at home comes from the ceiling fan fixture overhead, or an aluminum window frame, etc. Admittedly this is not the usual cause of buzz but it happens.

 

[Planobilly]

Hi Guys,

Lots of info you guys have posted for me to think about.

It is also not easy for anyone to troubleshoot over the internet not being there to look at things.

The amp is new and built from new parts over the last couple of months. I built everything myself including the chassis. It follows the AB763 schematic pretty closely except for the following.

1. Different style of tremolo circuit. I disconnected this circuit and the fault still exist.
2. The chassis is aluminum and built in 5 panels screwed together. The original is one piece bent steel.
3. I installed a Mid control. The original has no mid control and has a 6800 ohm resistor going to ground off the bass control. I installed a 10K pot for the Mid which provides the 6800 ohms at apx 70% rotation of the pot. Note: It is hooked up as a variable resistor not a pot.
4. I used a 33uf filter cap in the B voltage to reduce the ripple from 116mv to 65mv. The original uses 16uf on B. Note: I installed a 16uf and the fault remains.
5. I installed a master volume control after the phase inverter. Note: I removed it to test and the fault remains.
6. The power transformer produces slightly more voltage than the original. The B+ voltage is around 430 VDC. All the pre amp voltages are within 5 volt or less of the original.
7. I used all 600V rated 5% Cornell Dubilier 715P polypropylene orange drop style caps. Who knows what was in a original...the cheapest thing Fender could buy I suspect.
8. I used made in England Cliff jacks to provide isolation from the chassis.
9. I used ground busses in my grounding scheme. Note: The amp is very quiet with a very tiny amount of hum at max gain.

Here I will try to reply to some of your comments. This is what I think is true and why.

1. This is not caused by speakers. The fault exist when plugged into four separate and totally different speaker systems.
2. This is not caused by bad tubes. I have rocked the tubes around and cleaned the tube sockets with De-Ox-Id and changed all the tubes with three or four new sets of tubes from a very reliable vendor. I keep over 100 new tubes in stock.
3. This is not caused by the value of the negative feedback resistor. I had installed a switch which provided three conditions of NFB. 820 ohms, 470 ohms, and no NFB. The fault appears at all three conditions.
4. The issue occurs when plugged into either the normal channel or the vibrato channel.
5. The issue occurs when everything is disconnected to the point that the signal is only flowing through V2 A and B and V4B and through the inverter and out to the power tubes then through the output transformer.
6. The power tubes are biased at 21 ma and the bias circuit is working correctly. The tubes are matched within 1ma.

The next step for me is to check and change out components in the inverter as I have replaced just about everything else in the signal path before that stage.

Another question I have is do you think it is electronically possible for the output transformer to cause this sort of fault based on the fact that the amp sounds very good except for the buzz?

Whatever the cause turns out to be, it has certainly not been easy to discover.

Cheers,

Billy

 

[UsableThought]

That's a lot of work you've done.

Whatever the cause turns out to be, it has certainly not been easy to discover.

Discovery is aided by tools such as a scope & signal tracer, along w/ the knowledge of how to use these. Noise problems can seem very mysterious & complicated, but turn out to be very simple.

In this case I still recommend using a signal tracer with FFT app as I've described, to check nodes in the signal path for where the noise actually enters; it takes away the guesswork. There are ways to set up the probe (for example temporarily tack-soldering) so that you can watch hands-free as you play a guitar into the amp, if that seems more effective at evoking the problem. If you need further info about FFT I can provide links; but it sounds like you have ideas of your own you'd rather pursue. In any case, good luck.

 

[Planobilly]

Final analysis.

After digging through every connection in the amp and removing even the smallest fault I did the following.

I ran the output into a 8 ohm resistive load and disconnected everything connected before and after V2. At that point there is only the input jack, the grid resistor and plate resistor and the cathode resistor connected to V2A. There was no noise on the plate of V2A. The low frequency "noise" is present on V2B plate. It is not present on V2 grid. I replaced the V2 tube socket and tube and replaced the cathode resistors and cap. At this point I was 99.9% sure all the components were working correctly per the schematic. I had previously replaced components in the tone stack.

I listen to each connection of V2 with a probe connected to another amp while playing my guitar.

The cause of the low frequency noise is a function of the design of the circuit.

After removing every little bit of crackling in the amp due to very small connection issue like imperfect tube socket fit, I was left with only the low frequency distortion/buzzing being created by the amount the V2 tube was being driven past it's optimum curve.

This is evidently inherent in the original Fender Deluxe Reverb AB763 circuit design.

With all other issues that compounded the problem eliminated, the amp actually sounds fairly normal.

As the configuration of both channels are the same, that is the tone stacks are the same, I will redesign one of the channels to produce less preamp drive and therefore less distortion and less bass response. The general idea is to have one channel be the drive channel and the other be the less driven channel. This will also require the installation of a channel switching circuit which I am sure will be a bit of a challenge to design and install. Also a foot switch that then will have three switches. One for the reverb, one for the tremolo, and one for channel switching.

Nothing is simple if you want what you want...lol Huge learning experience so far.

Thanks to everyone for all the feedback and education you have given me. My band mate is on his way over so now I am going to play music instead of work on stuff!

Cheers,

Billy