How useful is an Oscilloscope for working on Guitar Amps?

[random_name]

....I'm talking about using it for faultfinding as well as tweaking/modding and building them.

Hi there, new member, 1st post and all that...I've been working on and modding amps for quite a while now, I even built a couple recently (albeit from kits) and up until now I've only been using a DMM to test/faultfind etc, but one of the amps I built appears to have developed a problem with parasitic oscillation and I was wondering if it would be worth investing in an oscilloscope.

I'm pretty sure that a good scope would help with that particular problem, but I'd also like to know if there are any other common Guitar amp/FX pedal problems that are much easier to diagnose if you have a scope.

One other possible use that occured to me was checking for ripple/noise on the output of an amp's rectifier, but I gather that you are supposed to use a differential probe for this...

[MWJB]

The scope would largely confirm, what your ears are telling you...you still have to work out why you have the PO and cure it.

A scope is a good thing to have for determining if an amp is making the specified output, for identifying unusual/unwanted artefacts at various points in the circuit, for ensuring that your PI is balanced etc. But if you have an issue, you have probably come to that conclusion from what your ears are telling you in the first place, if there is a malfunction there will be signs somewhere to support this.

It's not a silver bullet. A good thing to have if you know how to use it, not essential for 99% of situations.

Perhaps start a thread regarding your build's PO issue, furnished with dc voltages, description of the grounding & pics of the build.

[random_name]

A scope is a good thing to have for determining if an amp is making the specified output, for identifying unusual/unwanted artefacts at various points in the circuit, for ensuring that your PI is balanced etc.

...Thanks, that's good info and cheers for the quick reply!...I was thinking that I might be able to look at the output of the amp, see the frequency of the PO on the screen and then use the scope to find the part of the circuit that was creating that frequency.

Pricewise, the best scope I'd seen so far was the Rigol DS1052E DS1052E::RIGOL but I was wondering if it's Max input voltage would be suitable for working on amps (I know you can get upwards of 500V on the HT part of the circuit.

..I will also think of making a thread about the PO problem if I can't sort it myself, ta for the suggestion.

[Enzo]

A scope is a tool, like anything else. Imagine doing without a tuner. You can do it just fine. You can play a CD in a known key and tune the guitar to it. You can tune to a keyboard. You can just tune the guitar to itself or to some other guitar. Then one day you get a tuner. Now you have a specific tool that is very efficient at doing that job. You could do without, but it makes that aspect of guitar so much more easy.

A scope is another way to look at something you may already see. Then again, there are things like crossover distortion that you can see starting before your ears can hear it. Ripple? Is just AC riding on DC. No differential probe needed, just flip the scope coupling switch to AC mode.

MAybe a signal is clipping, and you don;t want that. Your ears probably won't tell you if it is positive or negative peaks that are clipping. And that might just be the clue to why it is happeneing. And while you can learn what different sorts of distortion sound like, to me, the appearance of a distortion on the screen tells me a lot about what is causing it.

[random_name]

Cheers Enzo, more good info....I guess it doesn't matter how high the DC voltage is if you set it to AC coupling, hadn't thought of that. From what you're saying about the distortion, it sounds like it would come in handy for tweaking circuits to get the tone I want too.

[Steve Conner]

That's true, but you can't connect it straight to a tube circuit with some alligator clips. You'll need X10 probes, ideally X100 too if you can get them cheaply.

[exclamationmark]

I don't really find my o-scope usfeful for tone chasing ventures, but it's invaluable for troubleshooting. And by troubleshooting I mean checking for oscillation/ripple or weird noises that shouldn't be there. I actually built my first scope from a PIC32 microcontroller and a $5 ADC... only had a sampling rate of 5msps but it worked pretty well for audio and helped me out a few times. I have a big boy scope now - much fancier

Also - with guitar amps it would be wise to get x10 or x100 passive probes; you can then set the oscope to scale the x-axis by 10 or 100.

[GregS]

I've found the scope invaluable for designing in particular. If your building amps from proven circuits it's less essential I would say. If you get into designing though it's particularly useful for things such as:
- getting the gain structure through the amp exactly as you want it (theoretical calculations only get you in the ballpark and you can also make mistakes doing them too).
- determining the circuit is functioning as you planned (i.e. the stages you intend to distort are the ones that are)
- troubleshooting tone problems. For example if the sound lacks bass you can find where in the circuit you might have an unintended high pass filter. Otherwise you'd just be guessing and making changes at random.
- determining which stages are distorting first as you turn the gain up by using the dual trace (assuming the scope has it).

I'm sure it's useful for many other things as well besides what I listed.

[riz]

Get it. You will enjoy it!

[Wombaticus]

...Thanks, that's good info and cheers for the quick reply!...I was thinking that I might be able to look at the output of the amp, see the frequency of the PO on the screen and then use the scope to find the part of the circuit that was creating that frequency.

Pricewise, the best scope I'd seen so far was the Rigol DS1052E DS1052E::RIGOL but I was wondering if it's Max input voltage would be suitable for working on amps (I know you can get upwards of 500V on the HT part of the circuit.

..I will also think of making a thread about the PO problem if I can't sort it myself, ta for the suggestion.

I built quite a few amps with just a multimeter, so you *can* definitely do fine without. But once you have a signal generator and scope in your lineup, you'll quickly wonder how you ever did without, especially if you're working on new designs.

Incidentally, when my old quad channel Tektronix died, I decided to go the new-but-on-a-budget route and purchased the Rigol DS1052E that you reference. Overall, the unit has been a pleasant surprise, and I have no regrets over the purchase. It's more than adequate for audio work. Someday I'll get around to resurrecting the old Tek scope, but in the meantime, I'm happy.

[Chuck H]

I built quite a few amps with just a multimeter, so you *can* definitely do fine without. But once you have a signal generator and scope in your lineup, you'll quickly wonder how you ever did without, especially if you're working on new designs.

+1

[riz]

I've got an old loaner scope, and just use Audacity on the laptop as a signal generator, through the headphone out, with a little volume knob i rigged up to accurately control the voltage. It does sine, square, and I think triangle waves(?), at whatever frequency I want. It's primitive, but effective when I need it. Would I LIKE to have something fancier? YES! Do I NEED something "better"...honestly, no, and probably won't get it any time soon. But the insight this rig provides has been fascinating. I am a "visual person", so actually SEEING the waveforms, and how they behave/distort etc., on the screen is awesome for me. Not to mention the troubleshooting/repair power it affords...

[g1]

From a repair perspective, a scope is the most indispensable tool for troubleshooting. The amount of time it saves is often overlooked. If you are charging for repairs and don't have a scope, this is not fair to your customers. You will not become authorized for repairs for any company without a scope.
Yes you can get by without one. But if you are used to using one and you get stuck somewhere doing field service without it, you realize what a time saver it is.
You can do all your voltage readings with a scope, pretty much anything aside from current and resistance. For most tube amp service, you don't need high bandwidth and a single trace will often suffice.
If I had to choose between a great multimeter, or a cheap multimeter and a cheap old scope, I'd take the scope option every time. I can't imagine trying to measure output power without a scope, or signal tracing or analyzing power supply ripple.
There are online tutorials with scope demonstrations etc. Once you get used to using a scope you will never say "I don't really need this".

[random_name]

That's true, but you can't connect it straight to a tube circuit with some alligator clips. You'll need X10 probes, ideally X100 too if you can get them cheaply.

Don't worry, I'm not that much of a noob. The scope that I'm looking at comes with an adjustable probe (1:1 or 10:1) they do make a 100:1 probe, but it costs another $240 I'm just wondering what sort of measurements I'd be able to make with just the stock (10:1) probe...

Or maybe I should look for a budget scope that comes with a 100:1 probe.

[random_name]

I built quite a few amps with just a multimeter, so you *can* definitely do fine without. But once you have a signal generator and scope in your lineup, you'll quickly wonder how you ever did without, especially if you're working on new designs.

Incidentally, when my old quad channel Tektronix died, I decided to go the new-but-on-a-budget route and purchased the Rigol DS1052E that you reference. Overall, the unit has been a pleasant surprise, and I have no regrets over the purchase. It's more than adequate for audio work. Someday I'll get around to resurrecting the old Tek scope, but in the meantime, I'm happy.

Cheers, that's encouraging, I was thinking of using some sort of software/soundcard thing for the signal generator or maybe building it myself.