Maybe I need another 'scope?

200v limit sounds most unlikely. Enjoy frying yourself. It sounds like you will at some point.

NT

An expert who has no knowledge of voltage dividers :-)

Most tube amps are serviced in our shop with a digital voltmeter...not a scope. Really all that you need to know are correct are the plate and cathode voltages, and if they are off then you check the screen and grids for proper bias - after testing the tubes for proper operation.

Most common failures are electrolytic caps, then inter-stage isolation caps followed closely by plate resistors (they overheat with bias goes wrong) and then cathode resistors, bias circuit, grid resistors, and then - perhaps - a damaged output transformer.

Virtually all of that can be tested quickly with a voltmeter once you know the average plate voltages. If the plate voltages are good then the amp is fed a known AC signal of a set level and you then follow the AC signal through the amp using your AC setting on the voltmeter. I check first the input level, if good I then check the high side of the volume control and if is on spec I then go to the output of the phase splitter tube(s). Divide the amp in half, then halve the remaining sections to find the problem.

If you are doing high end amp work where distortion is a factor then you will break out a slow dual trace scope to find the issues, but for regular tube amps like guitars and jukeboxes we usually don't bother. Not that we don't have scopes (have almost ten in various sizes and speeds), but they rarely help - at least for tubes!

John :-#)#

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I agree. Forget about the scope if you want to just measure voltages. I have the same scope and would never try to make accurate voltage measurments with it.

You can always get a 100 to 1 probe. However, you can get one of the 'free' Harbor Freight meters and do better for what you want.

For just DC measurments you can just make your own 100:1 voltage devider.

** Wow, that is contrary to how most audio amplifier techs do it.

With a scope probe, you can check DC levels and AC waveforms at the same time - so the first step is to connect an audio generator and dummy load and see if there is any clean output.

** OK, maybe having a good tube tester changes how you do things.

** I see amplifiers from the 1960s up to a year or so old and faulty electros are not the most common parts to be replaced - faulty tubes are. Electros always get replaced if my ESR meter says they are bad or there are visible problems like leaking electrolyte. ** Only rarely need replacing, IME. ** I find I use the scope and DMM in combination until the amp is all working properly. This will usually take longer on an unfamiliar example where no circuit is available.

.... Phil

Everybody should have another scope now and again. I have 11 at the moment, which is none too many. ;)

(On average I pay about 3 cents on the dollar, which makes new scopes an easier sell.) ;)

Cheers

Phil Hobbs

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no

If you're going to get one you don't need, consider making it a pocket scope. Quite handy, even with modest specs.

NT

Find a mirror. Put it at eye level and stare into it for one (1) full minut e. Then repeat ten (10) times:

I don't need a scope to service tube amps.

Repeat as necessary.

Your isovariac will be adequate for the work. That, reasonable caution, com mon sense, understanding of what tubes are and do (and do not), and patienc e. None of which are common characteristics.

On tube testers:

YES, they are handy if you have a good one that tests for shorts and gas. A nd if you are going "full audiophile", one that allows you to match is also useful. But the brute fact of the matter is that there are few testers out there that actually give information that is both good and useful. That a tube has no shorts and is not gassy is good to know, but not particularly i ndicative of its function-in-use. I keep two - one a fairly simple Simpson emissions tester, good for about 90% of my needs, and a Hickok 539B, which is allows proper matching with additional equipment (2 x VOM). Sadly, tube testers that have been restored, calibrated and are reliable are getting sc arce and expensive. The amount of *stuff* that crosses my bench in a given year justifies the need.

I also keep a very good scope - that I have used twice in five years to fin d that niggling problem, and that was on solid-state devices. If I ever get another scope, it will be a "pocket" device as they take up a LOT of real- estate otherwise.

Peter Wieck Melrose Park, PA

Eric,

I'd suggest using a voltmeter (analog or digital, whatever you prefer)for the DC voltage measurements. I like my old Heathkit VTVM for troubleshooting, as you are unlikely to cause any serious damage to the meter, as could happen with a digital.

The scope you have is all you need, as what you need it for is the AC part of the waveform. Set the scope to AC coupling at the input and you'll be looking at the ripple on the supply. If the ripple exceeds

400 volts (8 divisions at 5 volts each is 40Vp-p and with a 10x probe that gets you to 400Vp-p) then something is seriously wrong with your power supply or measurement technique, or you don't have a good ground between the amp and the scope.

Regards, Tim

Eric,

I'd suggest using a voltmeter (analog or digital, whatever you prefer)for the DC voltage measurements. I like my old Heathkit VTVM for troubleshooting, as you are unlikely to cause any serious damage to the meter, as could happen with a digital.

The scope you have is all you need, as what you need it for is the AC part of the waveform. Set the scope to AC coupling at the input and you'll be looking at the ripple on the supply. If the ripple exceeds

400 volts (8 divisions at 5 volts each is 40Vp-p and with a 10x probe that gets you to 400Vp-p) then something is seriously wrong with your power supply or measurement technique, or you don't have a good ground between the amp and the scope.

Regards, Tim

And if you are going "full audiophile", one that allows you to match is al so useful. But the brute fact of the matter is that there are few testers o ut there that actually give information that is both good and useful. That a tube has no shorts and is not gassy is good to know, but not particularly indicative of its function-in-use. I keep two - one a fairly simple Simpso n emissions tester, good for about 90% of my needs, and a Hickok 539B, whic h is allows proper matching with additional equipment (2 x VOM). Sadly, tub e testers that have been restored, calibrated and are reliable are getting scarce and expensive. The amount of *stuff* that crosses my bench in a give n year justifies the need.

ind that niggling problem, and that was on solid-state devices. If I ever g et another scope, it will be a "pocket" device as they take up a LOT of rea l-estate otherwise.

Maybe it's time to make some new tube testers.

While we're here, an iso won't help if you connect an earthed scope to the thing. It will bite you if you touch, and that's how HT is.

NT

It is usually easier to just pop in a good tube instead of testing unless looking for a matched set.

I have bought a $ 300 Hantek scope. It is suppose to be good to 200 MHz and rated for 600 volts peak. Good thing about it, you can set the screen up for waveform and digital volt meter. YOu see the waveform and good resolution of voltage at the same time.

Sometimes it may be better or easier to just isolate the test equipment from the ground and not the unit under test. I don't know how much the solid scope draws without going to check, but it could probalby be powered by 2 transformers of 24 volts at 2 amps or so placed back to back.

s. And if you are going "full audiophile", one that allows you to match is also useful. But the brute fact of the matter is that there are few testers out there that actually give information that is both good and useful. Tha t a tube has no shorts and is not gassy is good to know, but not particular ly indicative of its function-in-use. I keep two - one a fairly simple Simp son emissions tester, good for about 90% of my needs, and a Hickok 539B, wh ich is allows proper matching with additional equipment (2 x VOM). Sadly, t ube testers that have been restored, calibrated and are reliable are gettin g scarce and expensive. The amount of *stuff* that crosses my bench in a gi ven year justifies the need.

find that niggling problem, and that was on solid-state devices. If I ever get another scope, it will be a "pocket" device as they take up a LOT of r eal-estate otherwise.

There's plenty of them out there, and they're very simple circuits. I pro bably have half a dozen or more, the only one that's kind of a dead end is a very nice one that checks the four pin tubes that has a dead meter. Othe rwise, the standard recap and check for off value resistors and maybe repla cing a selenium rectifier. But very simple to restore.

These testers are very handy and I do use them, but the best way to check a ny tube is to observe it's behavior and measure it's performance in the cir cuit itself.

This is one of several currently available. They are not cheap. If one is into tubes at a level to justify a new tester at this level, I would posit that the need is beyond the hobby level.

I tripped over my 539B when an old friend closed his business. The Simpson I found at a garage sale in Tamaqua, PA.

"Popping in" a new tube is OK for those of us with a couple of thousand tubes in a closet. Not so much for those working on operating inventory only. I do, also, like to screen tubes prior to installation.

Peter Wieck Melrose Park, PA

Thanks for the reply John. I think my tongue in cheek "expert" comment fell flat but you saw through my poor attempt at humor. Since I will be interested in looking at the AC signal through to the speaker to see how it changes, when clipping starts, a scope will be necessary. From what you say it looks like using a VOM for the DC and a scope for AC would be a good way to see how the amp is working. Thanks, Eric

I know what a voltage divider is and have used them for some DC stuff, but I don't know enough about AC signals to know if one would affect the way the signal looks on a scope. TYhjanks, Eric

Thanks Ralph. Eric

rom the ground and not the unit under test. "

"Floating the scope" is not good practice and may be illegal in a commercia l setting. I do it but I am the type who works on live wiring and all that, there is specific and unmitigable set of rules for that which I will not a ttempt to list here. You have to REALLY know HOW to respect electricity, ES PECIALLY with tube equipment.

Pretty sure all scope manufacturers advise against it, some might try to vo id the warranty, maybe. Thee are other dangers, such as any ground fault in the test equipment you float can damage the DUT.

It most certainly can create insidious shock hazards as well, not just the ground and metal case of the scope, but ANY and ALL other test equipment co nnected to the same circuit(s).

I'll do it but I do not recommend it. Save that for when you have no choice .

See that's exactly the kind of advice that I can't be told often enough. Even though I did already know about this. My isolated variac does not have an isolted ground so I bought one of those plug/socket adapters that doesn't pass the ground through but instead provides a tab to connect to ground. Thanks, Eric