Author Topic: A digital scope for audio circuits, what do i need?  (Read 18262 times)

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Offline MrSadman

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A digital scope for audio circuits, what do i need?
« on: May 31, 2013, 08:55:36 am »
Long story short - I want a new oscilloscope.
I currently have an old Soviet analog scope, 10MHz, single channel. While it sort of works, it simply doesn't compare to the gear I have access to at school and work and I have been thinking about upgrading.

Main use is audio, including tube designs. I do some repairs but mostly analyze and modify circuits and build DIY projects. Also dabble with small scale automation.

Option one would be to upgrade to a better analog scope, however there is one drawback with analog scopes.
I write about the schematics in a blog and also document some case studies for uni or myself. Getting images of signal shapes is a pain with analog scopes, atleast compared to the DSO's That is actually the main reason I am interested in a digital scope.
 
So, what would you say I need for working with audio?
Picoscope has some PC scopes that have a high vertical resolution, enough memory and decent sampling rates around the 1000euros price range. They also include a signal genertor(I currently use a PC soundcard as a signal source), and my workspace is small, so saving space would be a big plus. However if they are anything like the PC scopes I have used, they are more of a logging device then a scope for a workbench. The no buttons thing usually means it takes a long time to get the desired image on screen. Also I would be worried about working with tube designs and high voltage.
 
Should I spend some more serious cash on a higher end digital scope with some extras, like Agilent 2000 or 3000 series(We use Agilent at work, I could probably get more bang for buck from a different brand), or go for a Rigol 1052E etc. and a separate signal gen? How imortant it actually is to get vertical resolution higher then eight bits and are there any scopes in under 3000 euros pricerange that actually offer this without sacrificing other functions?
Or is the answer that there is no place for digital scopes in the world of audio?
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #1 on: May 31, 2013, 11:36:04 am »
For audio work, an analog scope is cheapest and best. They are almost impossible to damage with high voltage overloads at the input, can display magnified baseline views of waveforms that exceed the graticule limits that digital scopes cannot except by distorting them to give you a false display.

Analogs are quicker to set up to the right vert and horiz settings and trig with just knobs and switches, all around easier to use. What could you possibly want memory storage for if you are working with tube amps?

Analog scopes are even easier to trigger on signals, especially with low-level signals that are smaller than 3 sub grat lines, the lower trig limit of any waveform displayed. On the DS4024, when I use mains (line triggering) I can always get a stable display, but  unlike on the analog scope, the DS4024 does not allow me to adj the trig level with line triggering.

A good digital scope that can get close to match the displayed fidelity of the 20 dollar used 20-MHz dual-chan analog scope I bought cannot be found.

Digital scopes are easily overloaded and severely distort signals displayed that have an envelope outside their graticule range. You cannot get a clear magnified view of signals around the 0-V point when their amplitude is way off  the graticule range.

In audio work you may want to get a magnified view of crossover distortion, those signals so often seen in audio work that are most important. Digital scopes cannot do this if you try to magnify the crossover point.

No digital scope I have tried is able to show the true waveform shape with input overload.

 At the tiniest V. level, the signal you may want to see is hidden by HF noise and unexpected ground loop effects not seen on a low freq analog scope.

And playing around at the 450+ V of tube amps...one single mistake hitting the range switch of a 10/1 probe and that 3000 buck digital scope input is trashed.


Digital scopes for some strange custom, and in normal operation, will always display a ghost image of the last waveform that was Norm triggered until the next trigger and so you to have to be very careful to check if you are really watching a live waveform at all. This odd behavior perhaps pays homage to old sampling scopes, but it cannot be turned off. Digital scopes do not seem to obey their own persistence settings with NORM triggering. An analog scope displays a waveform only when there is a waveform to display in NORM trigger mode.

When trying to see some instantaneous slow phenomena, the digital scopes most often fail to do this, requiring that a full 14-div trace is processed by the scope before displaying. So what you see is history. Only with Roll sweep, and only at certain time/div settings can you hope to see instantaneous things like microphonic effects displayed in real time of a circuit you might be testing.

Digital scopes, except for the most expensive ones, do not have anywhere near the vertical and horizontal display resolution of a cheap analog scope(essentially infinite), so waveforms are misrepresented time and voltage-wise. Waveforms are distorted because firstly, the display horiz and vert resolution is too low and secondly by the 8-bit digitization vertical limitations of all but 6000 US buck + high end digital scopes. Some high-end digital scopes require you to jump through hoops to achieve a 10 to 12 bit by non-real time averaging or the most expensive offer higher vert resolution, but that requires a special setup and a fat wallet..who has the money in one case or the time in the other?
« Last Edit: May 31, 2013, 06:28:56 pm by Paul Price »
 

Offline zibadun

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Re: A digital scope for audio circuits, what do i need?
« Reply #2 on: May 31, 2013, 01:12:36 pm »
Paul

I think what you are saying is "the digital scopes have a limited dynamic range".   DS2072 is 8 bit which may be slightly low, but it has a hi-res mode at 12 bit.   If that is not enough there are some 16-bit scopes out there. 

but how about some waveform pictures showing an ancient 20 Mhz outperforming a modern DSO  :scared:
 

Offline jahonen

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Re: A digital scope for audio circuits, what do i need?
« Reply #3 on: May 31, 2013, 01:32:24 pm »
For audio work, an analog scope is cheapest and best. They are almost impossible to damage with high voltage overloads at the input, can display magnified baseline views of waveforms that exceed the graticule limits that digital scopes cannot except by distorting them to give you a false display.

Digital scopes are easily overloaded and severely distort signals displayed that have an envelop outside their graticule range. You cannot get a clear magnified view of signals around the 0-V point when their amplitude is way off  the graticule range.

In audio work you may want to get a magnified view of crossover distortion, those signals so often seen in audio work that are most important. Digital scopes cannot do this if you try to magnify the crossover point.

No digital scope I have tried is able to show the true waveform shape with input overload.

Jim Williams has a slightly different opinion on his AN74:

Quote
Analog and digital ‘scopes are susceptible to overdrive. The classical sampling ‘scope is the only architecture that is inherently immune to overdrive.
Thus, I wouldn't trust blindly overdrive behavior on any ordinary scope blindly, whether analog or digital.

Regards,
Janne
 

Offline Lightages

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Re: A digital scope for audio circuits, what do i need?
« Reply #4 on: May 31, 2013, 04:34:47 pm »
I would have both, an analog and a digital. Get a good used analog scope for cheap and something like the Rigol DS1052E tht is still fairly popular and very cheap. This way you get the best of both worlds. The analog scope will be more useable for its lower noise inputs and other reasons, and you will have the digital scope for its ain benefits. There is no need to choose only one type!
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #5 on: May 31, 2013, 05:22:36 pm »
Zimbadum:

Here are two traces, the first on a cheap Chinese knockoff 20MHz 2-chan scope displaying the baseline (0-v) part of a 15-V peak rectangular pulse with both scopes at 50uS/div and 200mV per division, all views are with DC coupled input, 10x probe.
At 5V/div both scopes display the same pulse correctly, but hiding the baseline important view of  diode conduction that I am interested in viewing. Both probes have been correctly adj'd for input x10 compensation using the built-in calib. out.

This is a view of the catch diode conduction in a SMPS right at the schottky diode and the inductor in a step-down convertor.

The 20-MHz scope trace is accurately displaying the diode conduction portion of the pulse near the baseline. The DS4024 also showed a 1cm displacement positive of the baseline, increasing the distortion and meas. error.

No happy with this, I thought it was because I didn't buy a Tektronix DPO4000 series, costing over twice as much. So I requested Tek tech support to send me a video of the same pulse simulated by a sig generator. They sent me a video and the result was another remarkably distorted but different exponentially decaying waveform that was nowhere close to the actual waveform shape displayed on the 20MHz analog scope. I then made a simple voltage  window clamping circuit and viewed the same trace with the same probe going to the input of the DS4024 at 200mV/div and then only saw the correct waveform, closely matching that on the 20-MHz scope. ( Both the analog and digital scopes are both set to 20mV/div but with 10x probe=200mV/div display.)

« Last Edit: May 31, 2013, 05:51:48 pm by Paul Price »
 

Offline free_electron

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Re: A digital scope for audio circuits, what do i need?
« Reply #6 on: May 31, 2013, 05:32:49 pm »
turn off interpolation and vectors. see what the pulse looks like then... what you are seeing is an artefact of the data processing.
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Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #7 on: May 31, 2013, 05:42:45 pm »
Turning off vectors and interpolation doesn't  change the waveform displayed.
I am not using averaging or any other special feature of the DS4024. The problem (as explained to me by both Rigol and Tektronix tech support engineers) was that with even high end digital scopes,when displaying signals beyond the graticule limits,  will overload and saturate. and then the input amp cannot recover in time to correctly display the baseline.
« Last Edit: May 31, 2013, 06:06:08 pm by Paul Price »
 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #8 on: June 01, 2013, 04:20:45 am »
First of all, I would like to say that I am planning on doing as Lightages suggested. Get both. The problem is, that the used market here is rather thin, for 20 bucks I can't really get anything. A single trace dual channel 20MHz scope that is not broken costs in the ballpark of 200 bucks. For that price I could just go for GW Instek lower end scopes. Or spend some more and get a dual trace one.

So far I am getting a message from Paul, that I should forget digital and get a camera to record the waveforms.
 

Offline Lightages

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Re: A digital scope for audio circuits, what do i need?
« Reply #9 on: June 01, 2013, 05:12:13 am »
I think we need to make it mandatory for people to indicate where on the face of the earth they live. It is very hard to give advice about some things if we have no idea where you live!
 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #10 on: June 01, 2013, 05:58:04 am »
Fixed that part of my profile. Location: Estonia, what we get in the used market is mostly old soviet scopes and some Velleman toys. Strangely the Soviet stuff that works costs quite a lot. The stuff that doesn't work or has no probes etc. is cheap or even free, but some suitable replacemenet parts are a pain to find and finding separate probes is close to impossible.

There is always the chance to go fishing on eBay but my final purchase shipped by a certain courier has left me a bit mistrusting when shipping something fragile.
 

Offline Pat Pending

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Re: A digital scope for audio circuits, what do i need?
« Reply #11 on: June 01, 2013, 06:16:36 am »
I work developing high performance audio encoders for BLU-RAY, DVD and CE goods - most likely, you've listened to audio 'manged' by my code,
I'd suggest what you really need is a good multi-channel sound card with S/PDIF  (TASCAM, RME, M-Audio), sound editing software and headphones. 
But even then FFTs and Matlab are your tools of choice - pro-audio has a lot to do with math and measurement.

The scope (any scope) is useful for checking sound card phase alignment - still, you'd be surprised at the time wasted by 'pro audio' sound cards that can't even get the basics right!
« Last Edit: June 01, 2013, 06:42:16 am by Pat Pending »
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #12 on: June 01, 2013, 12:29:30 pm »
MrSadmad,

Digital scopes have their good points, they are so good for storing a single trace onto a USB memory stick.

My DS4024 can save 70 mega-points of a single waveform trace to an USB stick in less than 2 1/2 hours in CSV format. Not a problem, once saved by the scope, my PC could get it into Excel in just a few seconds.

At first it was a problem that the instruction manual says don't use a memory stick with a capacity greater than 4GB, so I had to hunt in the attic to find one small enough to work.

Since I couldn't use the scope while it was saving to USB, I had a chance to go to lunch, take a nap and read the newspaper.

The ease of use of an analog scope has been lost with digital scope, with so many, less or unneeded functions crowding the controls of their face.

Any new digital scopes you buy will also  may also have the same chance of damage by that certain courier, so take a short vacation to the USA, Denmark, Britain, Germany or France.  The weather is nice and used scopes abound on local used-buy-sell websites in these countries, not to mention Ebay but be sure to email the seller and tell them how you want them to package this thing so it is not trampled by elephants on the way to you. Else, have a pleasant summer journey and carry one home as a souvenir.

The last analog models made by Tektronix, like those in their 2600 series are still the best scopes around.
« Last Edit: June 01, 2013, 01:33:37 pm by Paul Price »
 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #13 on: June 01, 2013, 02:25:22 pm »
I actually agree with everything said in this topic about the analog vs digital subject. If you give me any analog scope, I can instantly use it and I know what most, if not all of the buttons do. I also know that most of the time what I see on the screen, is the actual waveform. For debugging I will most likely be using an analog scope for the foreseeable future.
With digital scopes there are menus and buttons, sometimes organized in not so intuitive ways. Also getting a waveform on the screen in a pleasant way may take some more time and button pressing, and, at times it may even be impossible to display the correct waveform, as Paul has demonstrated. For me the main reason for starting this topic, is the ease of getting images of signals. For most of the other upsides of digital scopes, I have been able to use scopes at work or at school. However, dragging something like a 15kg amplifier along with me for some screencaptures is something I'd rather not do.

But my original question, what are the minimum parameters for a digital scope, if I mean to use it for audio designs, is still mostly unanswered.
 

Offline KJDS

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Re: A digital scope for audio circuits, what do i need?
« Reply #14 on: June 01, 2013, 03:03:18 pm »
10 bits is enough for audio, unless you're extracting the data and doing a complex analysis to check for distortion and frequency dependent performance. Even then, 10 bits is probably enough and 8 bits usually will do. A reasonable computer screen until recently was 640 by 480, so only 10 bits high. If you're just looking at the waveform on a small screen you'll not see much beyond 8 bits.

I've just checked and the shipping cost of sending a scope from the UK to Estonia is £40.80 so it's not surprising that products are expensive out there.

Offline don.r

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Re: A digital scope for audio circuits, what do i need?
« Reply #15 on: June 01, 2013, 03:11:51 pm »
Bandwidth. You will need more than you think, especially when trying to look for noise. Luckily, many of the cheap Chinese scopes can be hacked for higher bandwidth so you need not spend a lot. A 60Mhz Hantek DSO that sells for about Euro300 can be hacked to 200MHz for instance. I do a little audio work as a hobby and my two most used scopes are old Tek analogue models, 2225 and 2245A. The 2245A is 100MHz and 4 channel, great for seeing how various stages change a signal, and the 2225 has 500uV/div for tracking noise although its limited to 50MHz.
« Last Edit: June 01, 2013, 03:14:07 pm by don.r »
 

Offline ivan747

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A digital scope for audio circuits, what do i need?
« Reply #16 on: June 01, 2013, 04:07:04 pm »
Something to note about the DS2072: it has a 500uV/div setting. That and the persistence display makes it much more useful for audio. With the 500uV setting and a FFT you can analyze the spectrum of the noise in your high gain amps.

Dave has said not many scopes have such a high gain. And it has the function generator you wanted. I'm not sure if it does sweeps, though.

And of course, all what I mentioned except for the FFT is not available in the DS1052E. Dave has a comparison video.
« Last Edit: June 01, 2013, 04:24:29 pm by ivan747 »
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #17 on: June 01, 2013, 05:30:06 pm »
I can not afford to buy a scope very often, so I did some research before I bought one.

How I made my buying decision:

Rigol offers the best performance/quality from all aspects of operation v. price, if you need 4-channels, good for debugging digital circuits. Rigol has thought out fairly well how to ease setup using multiplexed rotating controls with switches,  and choosing switches that provide a nice feel to the push and  a nice feel when turning of knobs, etc.

Owen is the second best in quality and ease of use and limited to 2-chan. Good enough for a fast overview of your audio signals.

The more you pay for either brand, the closer you will get to nice-feeling, easy to use controls and a digital scope that emulates a cheap used analog scope for audio use.

I found the Rigol DS4024 to be the closest thing to best value for both digital and analog use, and having fairly good fidelity of waveform display compared to an analog scope when used within its graticule range of input signals. 4 chans makes digital troubleshooting possible.

If you get the cheapest digital scope model of either brand, you will be able to see what's going in and out of any audio apparatus, with some distortion caused by display limitations and have to deal with some limitations on vertical amp dynamic range, but any digital scope will be so so much better than what you have now.

If you have the bucks for an Agilent, the more expensive low-end models include a function generator as an option, good for testing audio equipment, though I have easily built my own little circuit  using a battery or two in a small box to do generate sines and triangles and squares good enough for audio work. (It doesn't really matter if a sinewave is slightly distorted at the input if you can get the amplifier to match the same waveform at the output. The squarewave and triangle waves are almost perfect.) Two channels are a must to do this and does the trick.

I have watched all the videos on YouTube.com about reviewing scopes, and all reviews have not seemed to find a better use for an oscilloscope than to display sinewaves from a function generator, and they all restrict their reviews to do this than to show any other digital trouble-shooting or for any advanced use or for any other purpose one might imagine to use a very expensive 500MHz scope for.

Watch the YouTube reviews because they show how nice the knobs and buttons are and then go on to show a few other inconsequential things.

Tektronix and Agilent and Rigol have videos that show little somethings about how their scopes work, usually with sinewaves again, are very careful to avoid displaying signals that are distorted by their scopes, of course, in all ways avoiding showing their scopes to reveal limitations and faults .

For use, isn't viewing sinewaves all about what you want a scope to do?

Rigo and Tek and Agilent tech support were friendly and honest and rapid. Rigol support was limited mostly to "that's the way it works, we are working on these problems", and replies direct from Rigol engineers about more technical aspects were very short and in Chinglish.

On the other hand, Tek and Agilent were all too eager to bring by a demo model of my choice and willing to converse in great detail over the phone. I also took a trip to a local technical university and took a look at what they were using in their instructional labs.

That's how I've come to the conclusion about buying the Rigol DS4024 and not an  Agilent or a Tektronix, or an Owen or some Signalent, or an Instek or Tenna or else getting one of those many who-the-heck-ever-heard-of-this-company models.
« Last Edit: June 01, 2013, 06:24:01 pm by Paul Price »
 

Offline zibadun

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Re: A digital scope for audio circuits, what do i need?
« Reply #18 on: June 01, 2013, 10:45:06 pm »
I'd like to make sure I understand the problem with DSOs.

here are three pictures from Rigol 2072 (using its test output) 

1. Full scale signal
2. Maximum "zoom" on picture 1 (so this not captured in real time)
3. Real time capture with most of the signal off graticule range.

There is a difference between 2 and 3. 
Are you saying waveform #3 is not accurate because the input exceeds screen limits?
Is #2 accurate then?  Or both 2 and 3 are wrong?

I don't have an analog scope anymore to compare. :(


« Last Edit: June 01, 2013, 10:48:03 pm by zibadun »
 

Offline NiHaoMike

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Re: A digital scope for audio circuits, what do i need?
« Reply #19 on: June 02, 2013, 03:28:56 am »
50MHz should be enough for audio work, unless maybe if you're into the digital interfaces or amplifiers with very high carrier frequencies. (With SiC FETs switching faster, that can happen pretty soon.)

For actually digitizing and analyzing audio, an AFE (can be DIYed) and good sound card can work nicely. If you also use an external ADC and an optical link to the PC, you'll also have a very well isolated interface.
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Offline vk6zgo

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Re: A digital scope for audio circuits, what do i need?
« Reply #20 on: June 02, 2013, 04:08:53 am »
In fairness to digital 'scopes,analog instruments do show errors due to overload when the signal is very much greater than the vertical scale.

Some 'scopes have a hardly discernible problem,while for others it is more evident.
The higher grades of instruments are usually better!

To the OP--I wouldn't throw away the old Soviet beast.
If the new one ever dies,you will be glad it is still there to help you fault find! ;D
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #21 on: June 02, 2013, 06:22:07 am »
Zibadum:

Without an intermediate voltage setting view, it is difficult to see any detail of the baseline waveform in the 500mV/div top waveform, and then you jump in view from 500mV/div to 5mV/div. How about one more view at 50mV/div? That would reveal what is really going on.

Having an analog scope would help to resolve this question, but it looks to me like both 5mV/div pictures are severely distorted at the baseline since this is a calibration waveform that should have a clean shape and fast fall time.
There also seems to be a D.C. shift in the baseline, a difference between views two and three at 5mV/div.

However, the calibration signal should be putting out a clean square wave  and the short horiz. voltage excursion on the falling edge, in the middle picture does not match either the top or bottommost view, so it looks very likely distorted.

 

Offline MrSadman

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Re: A digital scope for audio circuits, what do i need?
« Reply #22 on: June 02, 2013, 06:46:26 am »
I've been looking around and so far the Rigol 2000 series just seems incredible bang for the buck. I'll contact the local retailer who carries some Rigol stuff and see, if I can order through them. Probably have to find a cheap function generator from someone as well, or just build something.
 

Offline Paul Price

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Re: A digital scope for audio circuits, what do i need?
« Reply #23 on: June 02, 2013, 06:49:36 am »
MrsSadMan,

Although you may have had a bad experience with 'THE CARRIER", you must not think your luck will always be bad.

But probes should be no problem.

On ebay you can find sets of 2 or 4 ea.  100mHz 10x/1x probes cheap and I've bought them and they work well, most importantly, they are not likely to be damaged by shipment and the cost of shipment will be much less.

Attached: Here's how I built a small function generator for audio testing from just one chip in small project case, a cheap solution to testing audio equipment.
« Last Edit: June 02, 2013, 07:10:36 am by Paul Price »
 

Offline larry42

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Re: A digital scope for audio circuits, what do i need?
« Reply #24 on: June 02, 2013, 10:19:08 am »
As a sidenote I would like to mention probes.

I'm sick and tired of chinese crap - and when testing tube designs with 500V (e.g EL34) - invest in some reasonable probes.

PMK make 100:1 and 10:1 probe that are high quality and reasonably priced and reliable. I have a pair of PMK PHV1000 1000V CAT II for that purpose.

A differential probe (one of the cheaper 1:20 25MHz units 700V CAT I rated) might also be very useful.

When I did my tube amp work (Mullard 5-20 type amp) I was using an old HP 180A and the 100MHz plugin. Unfortunately I had a dodgy probe and later discovered a dodgy channel in my scope (the wiring to the input hybrid had failed) - which made fault-finding the tube design a bit more difficult!


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