Analog vs digital scope

[ToddW]

New member here and I am looking to get my first oscilloscope.  Last time I touched one was in school in the early 80's, sad.

Anyway, I have a vintage Fender guitar amp and it needs work.  I am going to replace the tubes, recap it, the standard stuff.  Might do more on tone controls, hum/noise reduction and who knows what else.  I just got a bug to buy a scope to help in this project and if for nothing else for the fun of it and just have around for other projects I might get into.

Since its been a long time since I have used one, I could use a little advice.  I thought about going to ebay and just getting a cheap old analog scope.  Then started looking at the modern DSOs and holy cow the things they can do nowadays.  I saw some under $400 that did FFT, wave sampling and storage, and well I am sure you all know.

So my question is:  which is more appropriate for working on an old tube amplifier, analog or digital?  An old analog ebay scope might run around 200 with shipping but 200 more gets me a DSO with so much more functionality, but it might not be the best tool for the job.  I am definitely leaning towards a DSO unless there is some real limitations with them for tube work (upwards of 500vdc).

I am not asking which scope I should buy, this is a more fundamental question - which type.  And once I know which type, I can search this forum for threads on particular ones. 

Sorry, maybe I should have searched for the "analog vs dso" already, so I will do that now.  But if you have thoughts or a link to a discussion it would be appreciated.

[WattsThat]

Welcome!

Digital scopes have replaced analog scopes in every way, shape and form. Sure, some still prefer analog and that’s fine but there aren't many upsides to older analog scopes that are getting long in the tooth. Big, noisy, hot and no storage. I still like my thirty year old 2245A and still use it. But, I took it out of the box, new, all those years ago. My daily driver is a TDS220, maybe second gen digital. Not great but okay. I use them both, it just depends. The newer stuff is way better with respect to performance, I don’t understand why they they still fetch $200 and more on eBay. I guess there’s a different flavor for everyone.

IMO, If I had no scope and was going to do amplifier work, I’d buy a modern digital and a differential probe to work on tube stuff. The bigger screens, compact size and overall performance make it a no brainer.

I’ll also add that at 700 volts, I wouldn’t trust my life to a $8 Chinese probe. Don’t go cheap when it comes to probes and safety.

[bob91343]

I have both and sometimes one is better than the other.  Certainly analog is more intuitive and when working on audio gear is probably the simplest way to go.  For more sophisticated measurements the digital can give a new functionality.

Some waves are difficult to display on either but I have had more success with analog, especially when trying to view an amplitude modulated signal.  And of course the gremlin called aliasing only exists on digital units.

[james_s]

I'll always have a soft spot for my Tek 465, and the XY mode still beats that on any DSO I've ever used. Despite that, I'd say digital hands down, there are a very few niche applications (like XY) where analog is superior, for everything else digital is the way to go. I also still see some value in a beginner playing with an analog scope but if you already know how to use a scope then there's not much to be gained

The one reason I'd say to maybe go with an analog scope is price, if you're on a tight budget sometimes you can find an analog scope for peanuts, occasionally free. If the price is right, it will do the job you're trying to do.

[james_s]

Digital scopes have replaced analog scopes in every way, shape and form. Sure, some still prefer analog and that’s fine but there aren't many upsides to older analog scopes that are getting long in the tooth. Big, noisy, hot and no storage. I still like my thirty year old 2245A and still use it. But, I took it out of the box, new, all those years ago. My daily driver is a TDS220, maybe second gen digital. Not great but okay. I use them both, it just depends. The newer stuff is way better with respect to performance, I don’t understand why they they still fetch $200 and more on eBay. I guess there’s a different flavor for everyone.

I don't understand that either. I'd consider paying $100 for a good condition TDS220 because I'm a long time Tek fan, but $200+? That's just nuts, at that price I'd toss in another $150 and get a 4 channel Rigol with a nice color display, factory new.

[WattsThat]

And of course the gremlin called aliasing only exists on digital units.

That occurs when you don’t have enough samples to display a digitized waveform. That’s not going to occur at audio frequencies with a modern 70MHz digital scope, pretty much the bottom end of any decent scope in today’s market. Looking at any modulated waveform, for example audio, without a consistent amplitude and or frequency, is a perfect example of where digital storage comes to the rescue with being able to examine  any part of the waveform at a desired sweep speed. So long as you capture the event with enough samples in memory, you can look at it fourteen ways to Sunday. That’s not going to happen with an analog scope

[Fungus]

I thought about going to ebay and just getting a cheap old analog scope.

 

There's no such thing. Any that are still working are hoarded by Gollums.

[AndyC_772]

Look at probes first, *then* worry about what scope to connect them to. Any normal bench scope measures voltage with respect to ground, which in turn is connected through to mains earth, and I wouldn't personally want to have an earthed lead flapping about inside a high voltage amp even if I was measuring an earth referenced signal.

I have one of these, which I'd use to protect both myself and my scope when measuring anything with more than about 50V on it. It's good for 700V; you may want more.

https://www.picotech.com/accessories/high-voltage-active-differential-probes/25-mhz-700-v-differential-probe

[jxjbsd]

The requirement of audio measurement to oscilloscope is very low. Any oscilloscope with 1MHz input bandwidth or above can be competent. To measure the electronic tube amplifier, only a 100:1, 1kV pressure probe is needed, which is competent.
Analog  scope： Tek 2245A 2246A 2247A 
digital scope  Rigol1054Z   

[tggzzz]

So my question is:  which is more appropriate for working on an old tube amplifier, analog or digital? 

Start by understanding how to use scopes safely. There are voltages around tubes that can destroy a scope, and which could seriously harm a scope's user.

In particular learn how about different probes, and look at the prices of them. That may or may not limit your options. FFI, have a look at the safety references in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Sorry, maybe I should have searched for the "analog vs dso" already, so I will do that now.  But if you have thoughts or a link to a discussion it would be appreciated.

The key question isn't "which is better". More useful question are "which are good enough" and "what else do I want to spend my money on".

If looking at audio, understand the consequences of 8-bit digitisers and a scope's linearity specification w.r.t. noise and distortion and your measurements.

[Gandalf_Sr]

The simple answer to the original question is digital.  WattsThat has already explained why aliasing isn't even a consideration at audio frequencies.  Rigol have some great deals right now and you could pick up a DS1054Z for a little over $300 and have a 5 year warranty. You could hack it but there's no point for what you're doing, 70 MHz bandwidth is just fine and they give you all the extra options such as serial decoding for free.

The comment about using a high voltage differential probe adapter is VERY good advice which I would follow. If you want to cheap out, you can set your Rigol probes to x10 (a tiny slide switch on the side of the probe) but if you accidentally switch that back to x1 and overvolt the scope input, you'll be SOL.

[MarkMLl]

A few weeks ago I had a hankering for an analogue 'scope that I once owned. I needed to display two signals with a fixed phase offset plus jitter, and this 'scope was a true dual-beam with a delay line in one of the analogue inputs.

Now I broadly agree with what everybody else has said about the superiority of digital 'scopes, which of course apart from anything else have the overwhelming advantage that you can usually capture a pretty picture for your notes. But just now and again...

MarkMLl

[Gandalf_Sr]

The HV differential probe suggested seems overpriced - you can get the Micsig DP10013 for $179 on Amazon and they also have several x100 HV passive probes for around $25 (yes, you can set a x100 multiplier on the DS1054Z on the channel menu).  The DP10013 will offer better protection IMHO and get you away from ground loop issues but the x100 HV probe will also get the job done for a lot less money.

[Fungus]

you could pick up a DS1054Z for a little over $300 and have a 5 year warranty.

I'm in two minds over this recommendation. The Rigol is an awesome deal but the OP specifically wants to do audio work and look at hums and distortions so a good FFT is important. FFT is the Rigols's achilles heel.

Cue the resident Siglent salesman ...

(although after watching this video the Siglent doesn't really look like a stellar performer either)



Another option is the Analog Discovery II. The AD2 will destroy both of those 'scope in FFT performance as well as doing frequency sweeps, Bode plots, etc. With the right probes there's no reason not to use it with a high voltage tube amp (consult here if you're unsure of any part of how to probe that).

The AD2 also (a) Cheaper than a DSO, and (b) Highly recommended for the sort of people who "just got a bug to buy a scope to help in this project and if for nothing else for the fun of it and just have around for other projects I might get into."

The only real downside to the AD2 is that it needs to be used together with a PC/Laptop.

[Gandalf_Sr]

I'm not a big FFT user but having stuck with the Siglent FFT video for the first 10 minutes, I then cross-compared with the Rigol DS1000Z video below; seems to me that the Rigol's FFT responds faster and the video is pushing MHz into the scope.

[Electro Detective]

Pushing past for a minute... 

Reading through OP ToddW's requirements, I would roll with a cheap 100% working generic badged 20 Mhz CRT scope that has its combined owner/service/cal manual supplied,

and a decent affordable high voltage differential probe (buy once, cry less)
saving yourself the ordeal of learning about oscilloscope safety and pesky earth/ground/neutral relationships
with a half chance of escaping the tragedy of taking out all your gear on your first serious probing session          

This humble setup is more than good enough for the requirements described, SAFE/R, and FUN!


Any future upgrade path may be the ADDITION of a pricier DSO that can do the few things most CRT scopes can't


But if the CRT scope gets it done, don't blow money on a novelty scope you can do without, for now 

[Fungus]

Reading through OP ToddW's requirements, I would roll with a cheap 100% working generic badged 20 Mhz CRT scope that has its combined owner/service/cal manual supplied,

And you get that mythical beast, where...?

[Fungus]

I'm not a big FFT user but having stuck with the Siglent FFT video for the first 10 minutes, I then cross-compared with the Rigol DS1000Z video below; seems to me that the Rigol's FFT responds faster and the video is pushing MHz into the scope.

Yep. There's very few videos showing their FFTs but judging by those two there's not as much difference as people claim.

[jxjbsd]

If you want to study noise distortion and FFT, a 196k / 24bit sound card is a good choice

[nctnico]

I'm not a big FFT user but having stuck with the Siglent FFT video for the first 10 minutes, I then cross-compared with the Rigol DS1000Z video below; seems to me that the Rigol's FFT responds faster and the video is pushing MHz into the scope.

Faster response isn't always better. Sometimes you want more details which is where the Rigol doesn't deliver at all. Getting the FFT faster on the Siglent is likely a matter of selecting a shorter memory length (if possible). But for audio analysis there are better tools anyway.

To get back at the OP's question: definitely DSO combined with a differential probe. One of the things you can do with a DSO is sweep an amplifier through a frequency band look at the response of the tone control. Or look at the low frequency response; a analog scope is pretty much useless below 20Hz or so. Another problem is that analog scopes are so old nowadays that most of them are restauration projects.

[Gandalf_Sr]

The Norwegian Blue prefers kippin' on 'is back!

[free_electron]

If you want to study noise distortion and FFT, a 196k / 24bit sound card is a good choice

tube amps , guitar amps. they are all about distortion ! the more the better. Some guitarists prefer their amp to sound like a cat being milked using pliers ...

if all you want to do is recapping , invest in a good soldering iron . A scope is useless. you can look at signals. so what ? what do you think you will learn from it ? Scopes are used to troubleshoot or characterize during design. Recapping is neither.

[2N3055]

One big advantage of digital scope that is rarely mentioned is capability to create and save reference waveforms, and easy saving of screen shots for documentation purposes.. That, together with single shot captures, makes digital scope more useful.
Only thing that analog scope excels at is a scope art..

[Fungus]

tube amps , guitar amps. they are all about distortion ! the more the better.

Sure, but engineers want to measure it.

A scope is useless. you can look at signals. so what ? what do you think you will learn from it ?

I imagine the same sorts of things everybody else learns when they own an oscilloscope.

[ToddW]

Thanks to all for the feedback.  Think I will get some kind of DSO around 300- 400 bucks.  Also looking into safe ways to measure high amp voltages - iso transformers, diff probes, 100x probes, understanding mains, grounds, neutrals, etc. 

I would like to use a scope on my tube amp to see the various AC/DC voltages and forms, ripple, signal inputs, preamp stages, noise at different locations and different modes, effects of changing RC circuits, reverb/vibrator stuff, etc.  Scope could be overkill but the new DSOs are just so damn pretty.................

I also fly RC helicopters.  I have one bird that I can't even get off the ground because it goes into ground resonance funky chicken mode before I get enough power to lift off.  Wonder if I could rig a sensor to it and hook up to scope to see an FFT to get a better understanding of mechanical frequencies going on.  There is the tail rotor, main blades, drive shaft, electric motor etc.

Anyway, my first project is the tube amp.  I will play around with the scope on it and sure there will be other uses in the future.  Not much risk for a few 100 bucks.

 Pro-AA763-schematic.pdf (499.54 kB - downloaded 79 times.)