Test Equipment for tube amp repair question(s)...

fmena

Junior Member
I'm ashamed to admit it, but I have been sitting on a oscilloscope (Tektronix 745D 500Mhz DPO) for quite some time and have used it very little. Now that I am semi-retired, I have a lot more time and wish to use it for fault finding for DIY tube based amps under 15 watts (both push pull and SET). I don't have any other test equipment other than a Fluke digital multi meter, but figure a function generator (presently looking at a B&K 4003a function generator) and isolation transformer would be the next items I should look at. Would this be a good start to get a function generator and isolation transformer? Has anyone used downloaded software for a function generator instead (what software worked for you)?

I presently have 2 probes :100Mhz probe and a 10x 500Mhz probe. These came with the scope when originally I got it. Would you recommend other probes ? From my limited reading I am unsure of what a differential probe is and if I should look into getting one. I've read you can get away without using an isolation transformer if you have a differential probe.... not sure about this.

My goal is to learn more about how to fault find in tube amps. Eventually I would like to use a distortion analyzer ... but that is for another day down the road. If you have some suggestion(s) I would welcome your response(s) as to what test equipment would be most useful or what you have found most useful. I realize everyone's situation is different and my needs might be different from yours. I have lots of learning ahead of me.

Thanks
Frank M
 

shoshin

Site Supporter
Site Supporter
Hi Frank, there’s a tiny bit of info here. COVID happened, work became increasingly demanding, and I have not explored the topic further, so have little to add. The fine folks here will be able to help you out, I’m sure.
 
First thing I'd suggest is a few dummy loads. 4-, 8-, and 16-ohm. Signal generator is, in my opinion, a must. A few years ago I picked up a PC based scope. I got a Digilent Analog Discover 2, and I seem to use it almost exclusively these days (on SS gear). In addition to scope and signal generator, it also includes a spectrum analyzer, network analyzer, impedance analyzer, in addition to features you probably don't need, like logic analyzer, pattern generator, and references.

The differential probe is useful for directly measuring voltages across devices that aren't return (ground) referenced.
 

shoshin

Site Supporter
Site Supporter
First thing I'd suggest is a few dummy loads. 4-, 8-, and 16-ohm. Signal generator is, in my opinion, a must. A few years ago I picked up a PC based scope. I got a Digilent Analog Discover 2, and I seem to use it almost exclusively these days (on SS gear). In addition to scope and signal generator, it also includes a spectrum analyzer, network analyzer, impedance analyzer, in addition to features you probably don't need, like logic analyzer, pattern generator, and references.

The differential probe is useful for directly measuring voltages across devices that aren't return (ground) referenced.

I like the idea of a quality PC- based scope, though I have a few questions. Is the low input voltage of many PC-based scopes an issue when working with tube amps? Does having a single PC-based set of devices affect the practical convenience?
 

Punker X

Junior Member
Site Supporter
Hardly ever work on tube amps, but I would recommend in addition to the previously mentioned signal generator, probes and dummy loads here is a list of other things that haven't been mentioned yet.

Multimeter
Tube tester
Distortion analyzer
Variac
LCR meter
 
I was concerned about the lower input voltage of PC scopes at first. Mainly, if their input protection is as robust as a bench scope. Turns out, some, but not all, do have decent protection. If you have an input that is 5V (max, not per division), then a X10 probe gets you to 50V max in. Good for outputs, but not internal measurements. A X100 probe then allows probing up to 500V. If that's still not enough, go for a X1000 probe, although these are typically huge and unwieldy, about the size of an artificial inseminater for cattle. You could get a X10 BNC/BNC attenuator to go between the X100 probe and scope input to give you X1000.

I find the all in one aspect of PC gear to be a nice coninience. I mounted a decent sized flat panel display just above where my older Tek scope sits, and really love the huge display of waveforms right in the thick of things. For me, the addition of the network analyzer makes it all worthwhile, for this is an indispensable tool in the way I approach bench work.

If you were to jump straight to getting a distortion analyzer, you could probably skip getting a signal generator as most have a good sine wave oscillator built in. Although, if you want square waves you'd still need to get a signal generator. Alternatively, I used a CD player and preamp for many years with a CD filled with many test tones.
 

paul_b

Active Member
Site Supporter
A dedicated signal generator isn't 100% necessary. I don't use mine all that often, but sometimes it's nice to have something that can generate a really clean square wave to shoot into stuff. For general debugging, an old cell phone with a headphone jack, an 1/8" TRS to RCA cable, and a signal generator app is good enough. It takes up less space and if there's a fault in your gear, you toast an old phone rather than a piece of test gear.

For measuring output power and performance, an inexpensive pro audio sound card can be used with ARTA to measure THD pretty easily and for not a lot of money.
 

MrEd

Senior Nobody
Site Supporter
Great thread.
I was gifted an older but functional scope and need to start the learning / measuring of my projects.
 

fmena

Junior Member
Thanks for the suggestions. I have already made some dummy loads for the amps and speakers with 50W resistors and heat-sinked them. The small hammond project boxes I had from other failed projects came in handy. Pete Millett 's web site has a really good write up on dummy loads as well as a really intensive description on making a regulated high voltage power supply, perhaps a little over the top for the the hv supply in terms of involvement but I think that's for another day.

One thing I'm really bad at is organizing all my small parts (resistors and caps) and knowing what I have before re-ordering. I think a labelled plastic bin shelf with small drawers will help save on time although seems like a lot of work up front.

Cheers
Frank M
 
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