Time to get an O-scope...tips?

[Matt T.]

I think it's about time I got an o-scope. I've seen some threads here where folks have taken pix of a handheld Fluke device that appears to have o-scope capabilities. Is this the best route? Or should I look for an older (and much larger) used o-scope? I don't know enough to make a good decision...I don't need it have a bunch a features/capabilities that I'll never use while working on guitar amps (pretty much the only thing I intend to use it for). But I also don't mind spending a little extra for something newer and much smaller.

Is there anything else I'll need? Do I need a signal generator...or is that part of the o-scope? Any special leads or anything?

Any products/features I should avoid?

Thanks for any suggestions.

[stokes]

You dont need to get too extravagant,dual channel is nice.I've been using a Tektronics T922 for about 10 years,paid about $100 used.Before that I had an old RCA tube unit along with an electronic switch to give it 2 channels.The Fluke Scope-meter you mention is quite expensive,I've never had one,couldnt see spending that much.Check eBay,you'll see some of the costlier ones with 100MHZ inputs,but a 10MHZ is fine.You will need a signal generator to inject a signal for testing.Get one that will give you a sine wave,the RF type are not for guitar amp use,they are for radio.You can find some older tube units that will give you a sine wave and square wave at a wide range of frequencies,but all you need is something that will give you a 1K signal at a 1/4 volt or less.You will also need a couple of "dummy loads" to check your output.I have 2 8ohm 50 watters that I can use individually or in series/parallel to get 4 or 16ohm.

[bob p]

I've been on a test equipment buying spree over the past few weeks, so I'll offer my two cents on the subject. There are a few important topics that should factor into your decision:

1. Analog vs. Digital

Don't buy a ditgital sampling scope unless you have special needs for that type of equipment, ie: interfacing the scope iwth a PC. Otherewise, buy an analog scope.

2. Bandwidth

If you plan on audio work and only audio work on amps, that tremendously diminishes the bandwidth of the scope that you will need. Just about any scope that you are likely to find in working condition is going to have bandwidth that is far in excess of what you will actually need. So the first decision that you need to make is whether or not you want a mulltipurpose scope for audio, TV, radio type stuff (or even logic circuits), or just a plain old audio band scope.

If all that you want to do is to work on tube amps, then don't feel the need to buy a high bandwidth scope as it would be money wasted. For exampke, I bought an old 500 KHz Heath 0-10 vacuum tube scope on eBay for $0.99 that needed trivial repair work. Its a single channel scope, but its still plenty of scope for tracing audio signals in guitar amps. You read that right -- the bandwidth is only 1/2 MHz.

3. Number of Channels

Most people recommend a dual trace (two channel) scope for tube amp work. In some respects, its more of a luxury than a necessity. If you can remember what a sine wave looks like, its easy enough to do your circuit tracing with a single channel scope. A second channel is helpful for direct comparison between two signals, but I think of it as more of a luxury than a necessity for audio work. 4-channels scopes are fun to work with, but those extra channels are nothing but an expensive luxury for tube audio work.

4. Delayed Sweep/Dual Sweep Time Base, External Triggering, Y-Mode Alternate Triggering, Signal Summing, etc.

More advanced scopes offer these features. If you're just doing audio work, you may never use the features.



In a nutshell, then, a single trace or a dual trace scope of just about any bandwidth will be fine for tube audio work. You don't have to spend a lot of extra money on a scope with features that is primarily intended for repairing TV, radio, or logic circuits.

There are lots of brands to choose from in old gear. Some of the vacuum tube scopes are becoming costlier because of collectability. I have both solid state and tube 'scopes, and for reliability, I'd recommend an inexpensive solid state scope.

Your best bang for the buck is problably and older solid state scope that's in good operating condition. Tek scopes tend to cost more than other equivalent scopes because people are willing to pay a premium for the brand name. The same appears to be true for BK, though the premium appears smaller.

[bob p]

As an example of buying used scopes, I bought a 60 MHz, dual trace, delayed dual/sweep timebase scope on eBay for $54. It was made by Hitachi, who I think did the ghost manufacturing for BK. Looking at other eBay auctions, the equivalent Tek scope would have cost at least twice as much.

Watch out for buying scopes on eBay. If the scope doesn't come with the full compliment of probes and manuals, its either military surplus that may or may not have been trashed, or it could be stolen merchandise. There is no shortage of disreputable people selling dead/untested gear on eBay. Beware of gear that is "untested but pulled from a working environment" or offered for sale "As-Is."

I have also encountered no shortage of people selling perfectly good working scopes without their manuals and probes included in the deal. Making matters worse, these people have absolutely no idea how to use the equipment or to demonstrate that it works. Caveat emptor, as their wares could be stolen merchandise. I would only buy from a reputable seller who offers a money back guarantee.

If you don't want to hassle with shopping for used gear and your budget allows for it, buy an inexpensive new scope. I found someone on the web that was giving deep discounts on BK's entry level scope: BK-2120B. I almost bought one, but ended up buying something else.

Personally, I don't like the portable scopes by Fluke. For one, they're incredibly pricey. Second, I just can't stand to look at an image of an analog sine wave that is poorly approximated on a low-resolution LCD display. To me, a real CRT is preferable. YMMV.

HTH.

Don't forget, when you buy a scope, that's only the first step in your gear acquisition. After buying the scope you'll need some sort of signal generator (see my thread in music electronics). You might also want to buy a frequency counter. As @stokes mentioned, you'll need load resistors, etc. ... Just in case you haven't thought about it, somebody should warn you that the decision to buy a scope is just the beginning...

[Matt T.]

Thanks so much for the responses.

bob - Scope GAS? Say it ain't so!

[Ray Ivers]

Matt,

What's your budget?

My B&K 2120 'scope died about a month ago (after almost 20 years of service), so I've been using my first-generation Fluke ScopeMeter until I figure out what I want to replace it with; I'm leaning towards a Tektronics 2004B right now. FWIW, I favor a bright, rock-solid presentation down to 0Hz, low latency, two or more channels, wide input sensitivity range, etc. - but your budget will be the biggest determinant of what 'scope to look at IMO.

Ray

[Satamax]

http://search-desc.ebay.fr/tektronix...Z2QQsofocusZbs or the TDS 210 and so on series

http://search.ebay.fr/ws/search/Sale...&fcl=3&frpp=50

[bulldogguitars]

I purchased a Tenma 20Mhz dual trace scope from MCM in one w/a built in sig gen for under $300. You might want to check them out. www.mcminone.com

[Matt T.]

Thanks for the links/suggestions. I like the fact that that one has a built-in signal generator.

Ray,
I hadn't set a strict budget but I didn't want to spend more than a few hundred. It appears that I have options at or below that price point.

[bob p]

If you're willing to spend a few hundred, here's the link for brand-new BK 2120B for only $350. That's a smokin' deal on a new brand-name scope. (Ray, this might interest you too!)

http://www.testequipmentdepot.com/b+...sion/2120b.htm

I've seen one fellow on eBay that sells used 2120 (and similar) scopes with a 30-day warranty for about $125. That's not a bad price, considering the merchandise is warranted for 30-days. And it makes the idea of buying a used scope on eBay a less dangerous proposition.

I dedided to watch ebay because I wanted to find a killer deal. It took me a couple of weeks, but I picked up a 60 MHz multi-channel scope with two probes, manual, and all the bells and whistles including dual time bases and delayed sampling, and a big CRT for all of $54 or so. That just goes to show that if you're willing to do your homework, there are good deals to be had. The bad news? The scope was such a good deal that I had to pay almost as much for my HP dual channel frequency counter/timer and my HP function generator as I paid for my scope!

[Mark Lavelle]

I'll second all of the above, but I'm surprised that nobody mentioned that you must get "10x" probes for high-voltage amp work. That's what most scopes come with, but you'll want to be sure.

And while I agree that two channels aren't absolutely necessary I would also say the odds are good that some day you'll want that 2nd channel (unless your scope and your sig. generator both have an additional trigger/sync input & output, respectively). That's the only way to see the phase differences at different points in your circuit...

[Steve Conner]

I would go one step further than Mark, and say that a x100 high voltage probe is well worth having. The voltages in tube amps are towards the upper end of what regular x10s can stand: if you try scoping the power tube plate voltage of a SVT cranked into a dummy load, you could get a very nasty surprise indeed. A x100 should be safe with any voltage in any tube musical instrument amp.

What's more, the input impedance of a x10 probe can load tube circuits enough to change the very waveform you're trying to measure. For example, if you scope a grid circuit with 1M impedance, the x10 probe will change things by 10%, the x100 by only 1%, and clipping direct to the scope would cut the signal to half its original amplitude, and that's not counting the capacitance of whatever lead you clip it with.

I have a few scopes, and a lot of other test equipment, since I'm an EE and often do consulting work from home, besides messing with guitar amps. I used to use a little battery powered 20MHz scope that I was given as a freebie since it was broken. Later I got an old Tektronix R7603 from a military surplus dealer on eBay.

[drewl]

The good news is this stuff is very cheap nowadays.
I used to work on test equipment long before (well a few years before) I got into working on tube stuff, you can get a good Tek scope for $100 that listed new for $1000!
if I'd only known.....we literally had boxes of thousands of tubes! I got some for my amps, but I could have had the mother lode!

[bob p]

Mark's comments about probes are good points. (Mark, I don't know how I managed to overlook probes when I made that earlier post -- it completely slipped my mind!)

Steve's post has me seriously rethinking my decision to use a pair of 1:10 probes. Darn. More stuff to buy...

When I was shopping for a scope I asked for guidance from a friend who is a retired tube-era design engineer from Motorola. He told me that as a rule of thumb, test gear has always sold on the used market for about 1/10 of what it costs new. That's been my observation on eBay as well. The biggest exception seems to be when you buy used gear where the model is still in production. Prices for gear that is still in production tend to be higher. Once a piece of gear is no longer in production, its price on the used market really drops, and it seems to follow the 10% rule.

HTH.

[Matt T.]

Well I did a little shopping. I've found that the prices can be all over the place and the issue of "fully tested/fully functional" doesn't always mean you're paying way more...although that often seems to be the case.

I've found some e-bay sellers that have a ton of feedback with no negatives and are selling older SS 'scopes for reasonable and the auction says "fully functional...price refunded if DOA" or words to that effect...but they have no probes (maybe arouses some suspicion). It also has a cal sticker that was only a few years old. So I decided to take a chance and try one...it's only 20MHz but it's a dual...after shipping it'll be $90.

All the Tektronics scopes that were "fully tested, fully functional" were in the $300s and rising...I never saw a Tek that was cheap except for the ones where it seemed very likely to get a non-functional unit.

So I needed probes. It was extremely uncommon to see a 10x/20MHz probe so I assume I can just go higher in bandwidth? My bandwidth will be whatever the 'weakest link' is...correct? Then I noticed that many of the 10x/100MHz probes I was looking at were only rated for 300v...but I found some rated for 600v and got them (I got the 1x/10x combo...I didn't know if I'd need the 1x for pedals but it wasn't much more for the 2-pack than just a single 10x probe). See, bob is already right about the 'scope-GAS'.