Scope-less Diagnostic ideas...

[Justin Thomas]

Hey All, since I don't have a scope, I was wondering if this would work... I know it's not ideal, but it might help a bit.

I have an amp that is essentially a clone of a 6G5 Pro Normal channel. Since I also have a 6G12-A Concert, I was thinging of using the Concert as a power amp to figure out where in my build I'm getting distortion, etc.

Figuring I can put my signal into my amp and then use a shielded patch cble to inject into my Concert at various points & vice versa. Kinda like the old "FX Loop Send & Return" trick, without the jacks. I know it'll probably be a little noisy, but I'm chasing distortion & what seems to be excess gain, not noise.

I realize it may be a little silly or otherwise, but as long as I'm not going to blow stuff up (barring obvious stupid mistakes) yhen I'll probably go ahead anyway. Just wanted to throw it out there!

Thanks, and hope everyone might get a laugh at my expense!

Justin & Jusrin

[The Dude]

You'll want to use a series cap to isolate any DC from your source. And, it would be good to know the expected signal level at each point signal is inserted. If you provide a signal level wildly out of expected range, you won't get an accurate sense of what's happening. It's certainly not the best way to do this. It would be easy to see the distortion on a scope. But since you don't have a scope, you may learn something.

[dmeek]

I have a scope with a small amp and speaker connected to the Ch1 output. I often don't even look at the scope when I can hear what I need to know.
As The Dude says you will need a capacitor like .01/600V in series with the input probe and you will be using the Volume control a lot so set up the amp within arm's reach.

[glebert]

I was thinking it would be cool to make a type of electrical stethoscope with a probe and piezo headphones.

[J M Fahey]

Hey All, since I don't have a scope, I was wondering if this would work... I know it's not ideal, but it might help a bit.

I have an amp that is essentially a clone of a 6G5 Pro Normal channel. Since I also have a 6G12-A Concert, I was thinging of using the Concert as a power amp to figure out where in my build I'm getting distortion, etc.

Figuring I can put my signal into my amp and then use a shielded patch cble to inject into my Concert at various points & vice versa. Kinda like the old "FX Loop Send & Return" trick, without the jacks. I know it'll probably be a little noisy, but I'm chasing distortion & what seems to be excess gain, not noise.

I realize it may be a little silly or otherwise, but as long as I'm not going to blow stuff up (barring obvious stupid mistakes) yhen I'll probably go ahead anyway. Just wanted to throw it out there!

Thanks, and hope everyone might get a laugh at my expense!

Justin & Jusrin

Why would anybody?
It´s just a Signal Tracer, and has been suggested here many btimes.

No need to tie an expensive bulky Tube amp for that, any cheesy 10W SS amp "free with your $99 guitar purchase" is fine.

Or straight build this:

https://www.velleman.eu/products/view/?id=9360

even if you don´t buy that particular kit (I would, just to avoid making the PCB), download and read schematic and User Manual, it will give you a few ideas.

And you can always build a $2 fixed attenuator and use it with your freeware PC Scope.

[g1]

No problem with your approach.
But I think you would be quite surprised how cheap you can get a usable scope for. And sometimes nothing else will do (or it takes a heck of a lot longer, cuz I know somebody will chime in that has gotten away with it forever without one ).

[Mick Bailey]

At one time a scope was a super-expensive and bulky item and just too costly for many repair shops. My first scope, a Cossor, weighed about 150 pounds and came with its own trolley. It took up the same floor footprint as a filing cabinet. So at one time audio signal tracers were the norm and the electronics magazines often featured signal tracer circuits. You could also buy a ready-made unit with a built-in amp. I still use what's now termed an audio probe to this day, especially on FX units or SS preamps. I find that listening to a signal is quicker and easier for me than glancing over at a scope.

The usual method of using just a series cap has a big drawback where you're likely to encounter widely differing levels, or where there's any DC present. The cap charge/discharge can create an unpleasant high-level spike. I would use a fixed-value resistor (try 10K) in series with the cap and a pair of back-to-back zeners across the audio feed to clamp the voltage to a reasonable level. Bear in mind that clamping the signal can introduce distortion. Not a real problem with SS signal levels, but with a tube amp you could have several tens of volts (which will also distort the input stage of your listening amp) , so the maximum signal level needs to be limited - maybe with a (say) 1M pot. So my setup would be resistor/cap/pot/zeners.

[mac dillard]

diystompboxes.com/pedals/debug.html Take a look. May be just what you want. Been around a long time. I use it. It works.

[Justin Thomas]

Thank you for all the suggestions! I won't say whether I'll go through with my crazy idea or not yet but perhaps a laptop scope is best. My main reason for not going that way is, if I blow up my laptop, well... Letsjust say part of the reason I'd rather use a tube amp is vuz they take the abuse AND I can fix that in the cheap!

Juan, I actually don't own a tiny SS amp! My 66 Jet is the smallest amp I got, and that's one of the ones I'm trying to figure out if it works right or not, as it has two triodes mysteriously wired (to me) and only one gain stage before going into the paraphase inverter? I'll worry about that later on.

The Dude, thanks for the reminder on the cap!

Justin

[Mick Bailey]

diystompboxes.com/pedals/debug.html Take a look. May be just what you want. Been around a long time. I use it. It works.

You use it on tube amps?

[Justin Thomas]

If he's talking the signal tracer, yes. I have one I built.

Justin

[mac dillard]

You use it on tube amps?

Yes I use it on tube amps. Actually I use it as a listening device plugged into a ss amp and as a signal injector with it plugged into a CD player. Quick and cheap way to find where you are dropping your signal.