Scoping a Dumble

[Structo]

Hi guys,
I have a Tek 466B scope and was wondering where I should set my controls on my amp in order to check the waveform through the amp?

I have a signal generator and a dummy load.

Just curious where I should set the knobs on the amp?

Thanks!

[jelle]

Depends on what you want to see...

You have a Dumble on your bench? First Dumble for you? Congrats!

[Structo]

No, it's the 100w ODS I built.

[crbowman]

It does depend on what you're looking for but, that said, the first thing people usually look for is cross over notch. Set your signal generator for a 1K sine wave. At this point you want to adjust your amp so that you are seeing a 'clean' sine wave on your scope. This usually, for me anyway, involves turning up the bass, turning down the treble, and adjusting the midrange until you have the cleanest sine wave you can get. You'll want to turn off the bright switch and then just make whatever amp adjustments are necessary. If you can't get a clean sine wave then something is up. If there is a bump around the mid point of your sine wave, this is cross over distortion usually resulting from the amp being biased to cold.

[jelle]

What are you trying to do?

[Structo]

I just want to experiment because I haven't used a scope in a long while.

So just look at the different stages output and possibly look for any parasitics.

Thanks

[LeftyStrat]

I haven't built a Dumble yet. I finally have boards and I am waiting for the next run of CE chassis. But I'd love to see some scope shots of the onset of distortion along the various stages. Scoping the express was quite educational, and I love to learn something from what is going on in a Dumble, especially the ODS section.

[ic-racer]

You might want to also 'listen' to each stage with a audio probe.

[Structo]

You might want to also 'listen' to each stage with a audio probe.

So that is a probe with a cap on the end and the other end goes to a "listening amp" with headphones or speaker output?

[ic-racer]

A few years ago I read about how Ken Fisher could diagnose an amp in a few minutes and find exactly what was wrong. The person writing about that gave me the impression he used a sound probe and, starting at the input jack, followed the signal by ear until he found the problem.

I actually was set up to make an audio probe to diagnose my new HRM build, but I found the problem on the HRM board and did not need it. Still considered doing it just to actually hear how each stage influences the sound in the Dumble circuit.

Here is a thread about it: https://tubeamparchive.com/viewtopic.ph ... sc&start=0

You want to hear a full guitar chord so you can hear distortions or other problems. A signal generator is of no help, it does not represent a full spectrum of sound. Put your oscilloscope away too, it's of no help either.

Quote from the Hoffman amp site linked below.
http://www.el34world.com/Hoffman/tools.htm

[Chris333]

I bought a cheap old scope several years ago. Don't know how to use it very well for troubleshooting, but it's interesting to look at waveforms. The signal probe, on the other hand, is for sure the most useful troubleshooting tool I've come across.

[Structo]

I agree that a amp stethoscope is a great tool for troubleshooting but I am more interested in learning how to use my O scope for looking at waveforms.

[katopan]

For just experimenting and looking at stage waveforms, there isn't any particular amp settings to use. The point would be to play with the settings and see what happens! Just jump in Tom. Scoping amps can be educational if you keep in mind the limitations of what it's actually showing you.

[jelle]

+1! And keep in mind what the settings on your amp actually sound like with a guitar plugged in. Then compare that to what you see on screen. Have fun!

[cpollack]

The scope is a useful tool. Sometimes it's the best (perhaps only) way to quickly and accurately diagnose ultrasonic and subsonic oscillation issues as well as noise and RFI gremlins. It can see things we can't hear, and for the less experienced tech it's not always easy to translate audible symptoms into root cause. Watching a square wave at 1KHz can tell you a lot about how the amp is behaving. The power supply is worth checking, too, not just the signal path.

Kenny had 1000X more experience than most of us and could do amp triage in his sleep, though I'd bet he had a scope, too.