Newbie intro and oscilloscope question

Hi all,

I am new to the forum. I'm currently saving up for either the VTA-70 or -120 and am in the research phase. I don't have much of an electronics background so I have been starting off small and moving up. I've built a few guitar pedals from kits and have a Bottlehead headphone amp kit on order that I plan to build before I move up to the VTA kit. I'm sure I will have a number of questions when I finally place my order and begin the project but I have one right now about test equipment and oscilloscopes in particular.

I would like to slowly start picking up test equipment since I'm very interested in tube audio and want to learn what exactly is going on inside those circuits. One of the obvious pieces of equipment is an oscilloscope. My question is, for someone just starting out and wanting to have equipment that will last a while, would you go with a new digital scope like the Rigol DS1054z or an older analog Tektronix scope? I have a lead on what looks to be a good condition 2246 that is fairly close but I am thinking that it might be better for me to go with the new unit first since, if anything goes wrong with the older one I don't have the skills yet to get in there and fix it myself. What do you folks think?

I have a Rigol 2-ch scope at home and a 4-ch. version at work.  The model you selected would be great for a beginner scope.  Just keep in mind the supplied scope probes and/or input to the scope may have voltage limitations that prevent you from measuring ripple on the 500V supply of say the ST-120.  But no issue on smaller signal levels.  You may be able to purchase different probes with higher voltage ratings. 

Also the lower price scopes don't have a tremendous amount of bandwidth (these days) which would make it difficult to accurately measure risetimes on digital clocks.  The 50Mhz bandwidth rating is the -3dB response so it is already rolling off at that point.  In general you need 5x the bandwidth for the speed of the signal measured.  But for audio and general digital stuff looks good!

https://www.rigolna.com/products/digital-oscilloscopes/ds1000Z/ds1054z/

rdonahue wrote:Hi all,

I am new to the forum. I'm currently saving up for either the VTA-70 or -120 and am in the research phase. I don't have much of an electronics background so I have been starting off small and moving up. I've built a few guitar pedals from kits and have a Bottlehead headphone amp kit on order that I plan to build before I move up to the VTA kit. I'm sure I will have a number of questions when I finally place my order and begin the project but I have one right now about test equipment and oscilloscopes in particular.

I would like to slowly start picking up test equipment since I'm very interested in tube audio and want to learn what exactly is going on inside those circuits. One of the obvious pieces of equipment is an oscilloscope. My question is, for someone just starting out and wanting to have equipment that will last a while, would you go with a new digital scope like the Rigol DS1054z or an older analog Tektronix scope? I have a lead on what looks to be a good condition 2246 that is fairly close but I am thinking that it might be better for me to go with the new unit first since, if anything goes wrong with the older one I don't have the skills yet to get in there and fix it myself. What do you folks think?

A couple of things:

a) Tektronix scopes are generally of excellent build quality, last a very long time, and are very reliable. For an analog CRT-based scope, you cannot do better. Writing entirely for myself and based on my experience, for a newbie, I would only recommend Tek scopes if CRT-based.
b) If you can afford it, look for a good, used or new, FLUKE solid-state LCD-based scope. New, they are out-of-sight expensive, but often can be found used at a reasonable cost and within the range for general audio use. They are also truly auto-ranging so that you will not need a multiplicity of probes. Fluke scopes are far less hungry for real-estate, and are portable as well.
c) And, if you (eventually) wish to move on to FM tuner alignments and such, you will need a scope at a higher MHz level than for general audio use. So, consider that in your equation.
d) And last, A good scope, well kept, will retain its value more-so than most other pieces of test equipment. Meaning that you should be able to find a home for it (sell it in the future) should you either get out of the hobby, or decide to upgrade.

One observation: I have been in this hobby for over 40 years now. I can count on one (1) hand the times I have really *needed* a scope to diagnose or correct a problem. As they are expensive, awkward, and greedy for real-estate, you might invest in other things first. If you are serious enough for a scope, then you will want a first class workbench first, and the tools to go with it, also first. Start with an Isolation Transformer, preferably one wedded to a Variac (AKA: IsoVariac). ALL mains-based, and even some battery fed tube equipment has lethal voltages inside. Some over 500V. An isolation transformer gives you some protection. Then, tools: I keep some very nice tools, not one of which is from China. Yes, I paid a bit more, but some go back with me almost 50 years - my ChannelLock box-joint linesman's pliers for instance. I also keep an VOM, LCR, and ESR meter, also all US-made. Excellent lighting (three (3) articulated lamps, one with an optical grade magnifier) and such. Third hand, soldering station, solder dispenser, and so on and so forth. I keep a very good tube tester, as well... the very last thing I acquired for test instruments.

Point being that an expensive, complicated, and needs training, care and feeding item may want to fall back on the list until you are more firmly grounded in the process. Maybe, not certainly. The balance to that thought is that if you get used to, familiar and adept with a scope, you will be well ahead of most newbies in short order - for those tasks that need a scope.

Hey guys, first I wanted to apologize. You both supplied me with great responses and I didn't thank you. I had just signed up for this forum and put the wrong email address in, so I wasn't alerted of any replies.

I will probably go with the new Rigol OScope. It is easily hackable up to 100MHz and should suit me for my beginner projects.

Peter, I am looking at variacs as well. I was thinking of going with the Circuit Specialists 20 Amp, 2000VA Max, 0-130V Output variac that is on Amazon. I also have an Alinco DM-330MVT switching power supply. It is meant for ham radio duties so I would need to rig up some type of Power Pole to standard female plug outlet. It probably wouldn't be ideal but in the interest of saving money I think it should work for now. Let me know what you guys think.

You won't need more than 20mhz to do home audio unless you get into the new fangled digital stuff. For analog gear, I've got a nice little Tektronix 2205 here that's more than capable of doing what I want it to do ...

If you do go used, pics of the scope in action are nice to verify the screen's good and can focus properly. Also a BIG plus if it has the original or similar probes. The cheepo Chinese probes you find with a lot of used scopes are basically crap on a stick - I paid more for a pair of decent TPI probes than I paid for the scope itself, but it's a good investment if you want accurate results.

* Never saw a use for the x10 switch on a probe. That just adds another failure point that really isn't needed for the "classic" bench.

A few things:

It is extremely unlikely you need a 20A variable autotransformer (Variac is a trademark of General Radio, now ISE) for bench use, unless you intend to isolate the entire bench. And, if this is your actual attention,  you may be setting interactions between test instruments and working instruments to create differences in potential "above" the isolation. All this means is that you will need to be careful to check for and/or avoid those conditions, not at all difficult.

Switching power-supplies can be noisy. Just a thought, if you are focused on analog audio.

I keep a single iso-variac, the HEATH IP5220 (a search will give you lots of pictures and descriptions). However, there are those made by VIZ, and any of several other makers that are as good or better. The virtue of this device and those similar is that they have very excellent, and very fine-pitch meters such that differences of only a few watts may be discerned. Keep in mind that Variacs per-se are useful only for dimming lights, unless metered accurately - and it is this metering that is something of a pet peeve of mine.

Enjoy!

Pay a dollar per megahertz for a low end CRT-based B&K scope on Craigslist. If you find it indispensable and underperforming you can always sell it for what you paid and upgrade to a low end color LCD storage scope that will make your bench look cool.

Buy the B&K and spend the money you save on a NAD (or similar) receiver or integrated with external pre/power jumpers for your bench. I think you will make frequent use of that.

Picked up a 5a Chinese variac some years back for a real decent price.



Conservatively rated and I haven't been able to kill it ... yet anyway.

One small issue that has not been mentioned : when measuring on tube poweramps one may
fins 1000V peak on the output plates.
Ordninary 10:1 probes ( and scope inputs) are limited to 500V. Thus a pair of 100:1 probes are
needed ( they should be classed for > 2000V ) .

I am going to take a small risk here, and post one of my Variac/Isolation Transformer Rants from another NG.  It is related to vintage radios, but equally applicable to audio equipment. Nor is it directed at any one in particular, but at diagnostic techniques in general.

RANT WARNING

I am wild-guessing that you are using a variac to bring the radio up to
operating voltage, as much because this is "what is done" as for any
expectation that the variac is doing anything effective.

A Variac is in most cases like this WORSE THAN USELESS unless it has both
voltage and current meters that indicate in meaningful increments the actual
voltage and current being delivered to, and used by the radio. By "meaningful"
I mean in very small increments of an amp, or one-or-two watts. By worse than
useless, I mean that it gives a false sense of security to its user in that
there is a belief that a gentle start is somehow better than a straight
plug-and-play.

1. A tube-rectifier radio does not usually pass DC current until the voltage is
somewhere between 70 and 90V. Assuming that the B+ is anywhere up to 400V, the
filter caps get hit with 300V (or so) right from the git-go.... i.e. a Variac
does not help to "re-form" caps in-situ. Period. This must be done with a bench
DC supply, the proper dropping resistor, and great care. Some cap testers have
this capability, but not all of them.

2. A Variac (again, without meters) is no better at protecting a radio than the
wall plug itself. Scenario: A new AA5 has just arrived at the scene. With
careful attention by an unmetered Variac, it is brought up to operating voltage
and appears to play nicely, and hum-free.... The radio *should* be drawing 55
watts. In fact, it is drawing 75 watts due to several partially shorted
low-value caps on the output side. Now, 20 watts of heat is being dissipated
somewhere in the radio where it should not.

Replace an AA5 with a nice 11-tube Zenith console. Should take 85 watts, is
pulling 115 watts (example: The 11S474 that just got off my bench). Again,
hum-free, pretty sensitive.... That 30 watts was being dumped right into the
output transformer, and coming through the typically marginal power
transformer.

A dim-bulb device would indicate the second of the two described radios,
kinda-sorta, as 30 watts is a lot. But were it 15 watts, not really. And that
path to failure would simply have been a little longer.

If the point is to attempt to gain meaningful information about the condition
of a radio by applying current, then make at the very least an inexpensive
dim-bulb device. If the point is to reform caps, then get a Bench DC
power-supply, or one of the more capable cap testers. If the point is to dim
incandescent lamps in your house, then by all means get a Variac.

One can make a Black- Box that allows current draw
to be measured with a VOM across a precision resistor of known value. THIS with
a Variac can be useful. This box can also be fused (and one can install a
dual-element fuse... another rant) for additional protection of the radio.

Anyway, if ironmongery is really wanted on a bench, invest first in an
isolation transformer... THAT will do some good, at least. Otherwise, hold out
for a metered Variac that will convey useful information.

Any of you that have been thrown across a room, or worse, by hitting B+ on some piece of tube equipment will appreciate why the Left Hand in Back Pocket rule does not apply safely. And why I am such a strong advocate for isolation transformers for tube equipment. They will not protect against deliberate idiocy, but they will protect against many other conditions that would otherwise be dangerous or potentially fatal.

In any case, when building a work bench for high-voltage electronics, this is an important, nay, critical tool.

To be honest, I am very much a beginner so I am figuring out what tools I will and will not need for a successful build. Over Christmas break I have fifteen days of vacation. Since we aren't traveling this year I was planning on building either a VTA-70 or VTA-120 (still determining which I want to go with) as well as a Fender 5E3 tweed Deluxe amp kit. I will be doing the 5E3 kit first since it is much more straight forward and will be good practice. I am hoping to finish that one before my vacation so I can dedicate plenty of time for the VTA build. I just want to make sure that I have all the tools at my disposal that I will need so that I don't need to pause the build to wait for another delivery.

By the way, I mine as well say that my bench right now consists of a Hakko adjustable soldering iron, a lower end DMM, a big role of Kester solder, a solder sucker and soldering wicks, and some flux. That very well may be all I need to start off, you guys would no much better than myself.