How useful is an Oscilloscope for working on Guitar Amps?

[random_name]

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...

Well I've got Audacity, but I didn't realise you could use it as a signal generator, so I might give that a go...I've also got a looping pedal, so I suppose I could set it looping with a guitar part and watch the trace...not sure how useful that would be though.

[GregS]

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

I haven't found that necessary myself. I think most scopes can handle at least 300V and with a 10:1 probe the scope only sees 1/10th of what you're measuring. I think the issue is more if the probe can handle the high voltage. You can get a signal generator for around $75 or less. Check out Test Equipment Depot.

[random_name]

I haven't found that necessary myself. I think most scopes can handle at least 300V and with a 10:1 probe the scope only sees 1/10th of what you're measuring. I think the issue is more if the probe can handle the high voltage. You can get a signal generator for around $75 or less. Check out Test Equipment Depot.

This is from the manual:

Probe× 2 (1.5m), (1:1 or 10:1 adjustable) Passive Probes

The passive probes have a 6MHz bandwidth with a rating of 150V CAT II when
the switch is in the 1X position, and a Full oscilloscope bandwidth with a rating of
300 V CAT II when the switch is in the 10X position.

...maybe a bit on the low side?

[GregS]

Maybe, but it's possible they're conservatively rated. I'm not sure though. I got a couple of new 10:1 probes with a $100 used scope I bought and could not find any voltage rating on them so I doubt they're any special high voltage capable probes - just common ones. I scoped places with a little over 300V DC without a problem though.

However there are others on this board with a lot more experience than me that may be able to give more information on this.

[riz]

Well I've got Audacity, but I didn't realise you could use it as a signal generator, so I might give that a go...I've also got a looping pedal, so I suppose I could set it looping with a guitar part and watch the trace...not sure how useful that would be though.

You can loop it right in Audacity. Look under Generate>Tones. Just make a 30-second sample, hold the shift key while hitting the space bar, and it loops itself until you stop it.

[random_name]

You can loop it right in Audacity. Look under Generate>Tones. Just make a 30-second sample, hold the shift key while hitting the space bar, and it loops itself until you stop it.

Yup, I just wondered if inputting an actual guitar signal (rather than a sine wave) might sometimes be useful.

It looks like the scopes that are in my budget range tend to come with only 1:1 and 10:1 probes, so I am probably going to have to buy a probe with a higher voltage rating separately...I've also just seen an Instek scope that has a built-in function generator.

...I've just found a guide to making your own 100X probe...he says you shouldl use plenty of insulation!

http://how-to.wikia.com/wiki/How_to_...lloscope_probe

[J M Fahey]

The expensive 10:1 or 100:1 probes are so because they *guarantee* several MHz bandwidth, will pass a high frequency squarewave intact, etc. but you don´t need that much in audio work, much less for *guitar*, so you can build your own attenuator probe with just suitable resistors.
I used 4 x 470K resistors in series for the high side (so as to have better arc-over protection) into a 18K resistor, across whose ends I clip the 1:1 probe. Nice 105:1 attenuation.(call it 100:1).
The whole shebang was put into a plastic tube and potted for safety.

[exclamationmark]

Yes, probes guaranteed for bandwidth are terribly expensive, because of the various types of RC compensation they use (and the guarantee to not blow up your expensive scope :P). I just used a plain old voltage divider from a couple of resistors like JM does and scope it from there. It probably doesn't really matter until you get into the several hundred thousand kHz or low MHz range... Or intend to scope square pulses at high frequencies. There are some cheapie 100x probes that I probably wouldn't mind using as long as the insulation is up to spec.

[Kazooman]

I recently purchased the Rigol DS1052E and I am very happy with it's performance. I got it from this company:

Rigol DS1052E Digital Oscilloscope 50MHz Rigol DS1052E DS 1052E DS-1052E

Good price and fast service. The scope arrived two days after I placed my order. The day after I ordered they ran a sale and dropped the price by $20.00. I sent them a note and they refunded the difference. Nice business practice!

I found a good signal generator software package here:

Realtime Signal Generator for windows

You can get any frequency you want with sliders or entering the value manually. The freeware version gives a nice sine wave and can also output square and triangle waves. The output level is controlled by selecting various dB attenuation settings. This software works really well. The total package is pretty inexpensive and is even more versatile. You can adjust the balance between the stereo channels so you can open two versions of the software and have each totally on the left or right channel. Set up the scope for X - Y and make some Lissajous patterns to impress your customers

[random_name]

The expensive 10:1 or 100:1 probes are so because they *guarantee* several MHz bandwidth, will pass a high frequency squarewave intact, etc. but you don´t need that much in audio work, much less for *guitar*, so you can build your own attenuator probe with just suitable resistors.
I used 4 x 470K resistors in series for the high side (so as to have better arc-over protection) into a 18K resistor, across whose ends I clip the 1:1 probe. Nice 105:1 attenuation.(call it 100:1).
The whole shebang was put into a plastic tube and potted for safety.

That's what I was thinking - I'm not going to need massive amounts of bandwidth or accuracy when I'm taking those measurements, so I might as well just build my own or find a cheapo one that's compatible with whatever scope I get (which is probably going to be the Rigol one as I've just noticed that the one with the Function generator is an Analog scope)

[kurtdaniel]

Im into how to spam activities right now, as well as being a new technician.. So, what brand of scope i should buy for my guitar tube power amp? Thanks

[J M Fahey]

I think the "Affordable" brand is very popular.
Any scope is good for guitar work, thanks God.

[Gingertube]

I use exactly that RIGOL oscilloscope you mentioned, I bought a pair of X100 probes when I bought the CRO. I really only use the X100 when probing anodes of output tubes etc. otherwise the X10 probes will do (for up to 500V max).
There are good X100 probes with 100MHz Bandwidth being supplied out of China at the moment - around US$30 a pair, for that money it's is not worth mucking about making your own.

In any case X100 probes are MUCH cheaper that repairing a blown up channel on the CRO and over voltaging the input is a sure way to do just that.

ASIDE: In the day job I have a 1.5GHz Analog Bandwidth CRO which will tolerate only 100V max on the input (or 5V RMS with the 50 Ohm termination selected). To fix that would cost thousands.

Cheers,
Ian

[km6xz]

I am really surprised by the number of people who say having time and amplitude information is not very important in design, construction or repair of amps. I do not think I could recommend such a person to work on or build an amp for my worst enemy without feeling guilty. Maybe it is the new macho claim like old time electricians who used only their fingers to measure 220 and bragged about being clueless.
Why would anyone intentionally give up having such valuable information to quantify and qualify circuit operating conditions that a scope provides? A good used scope costs less than a quad of tubes that that person is likely to burn up due to not knowing the status of the circuit under test.
Sometimes this forum scares me.

Warning to readers: If you take your amp to someone for repair or modding and they do not have the proper test instruments to analyze the circuit effectively, walk....no, run....away quickly, they are the electronics equivalent to a watchmaker with only a ball-peen hammer and an hourglass as tools. They can't know much about the conditions of the circuit that is going to hurt your wallet and equipment.

[Chuck H]

I agree that in repair it's important to have access to the information a scope can provide. As it happens, WRT typical tube amps, it isn't usually critical information. Many vintage AND modern designs didn't idealize circuits based on such information. So what good is it when a scope shows the user that things aren't ideal??? Most of the failure symptoms in tube amps are very indicative of the problem to most experienced techs. Also, there are other relevant parameters that can be measured with a standard DMM that will indicate if a tube is working correctly. AFAICT there is almost nothing a scope can tell me about a tube circuit that can't be ignored as long as you can gather other info with a DMM. And as design goes, again, typical tube guitar amp circuts are so well known that it's easy for a designer to keep things safe. And since guitar amp designs are typically tweaked by ear, for the tonal distortions desired, there's not much in this regard that a scope can show me that I can't hear better.

I DO think that it's entirely possible, WRT tube guitar amps, to get by without a scope. But, I don't ever want to be caught unprepared. What problem might a tube guitar amp have that can't be solved without a scope?

I wouldn't want to though. I use mine a lot and find it expediates things greatly. I also get a warm fuzzy when I can look and see that everything is right. And I do use my scope every time I do a repair and every time I tweak a new design. Could I get by without it? Yep. Would I want to? Nope.