Audio Probe Simple Question

[sonolink]

I bought a signal tracer by TH Custom that works very nice with low voltage gear.
https://www.musikding.de/Signal-Tracer-Injector-Pen-V62-kit

I was wondering if I can put a 100nF/600v cap in front of the probe to trace tube amps?

Something like: Tube amp -> added tip with 100nF/600v cap -> Signal Tracer Tip

Thanks
Sono

 

[ccaudle]

When you touch a capacitor to a high voltage you have a high frequency signal for a short time until both sides settle. If you touch a cap to a 300V point, the other side of the cap will jump up to 300V or close to it for a very short period of time, and then fall down to 0V based on the RC time constant of the impedance on the 0V side.
So you should be sure to have current limiting resistors, and some kind of voltage clamp on the low voltage side (maybe zener diodes, or TVS devices).

 

[sonolink]

Thanks for your reply, ccaudle
What resistor and zener values, would you suggest?
Would they be inline with the probe? (Tube amp -> added tip with 100nF/600v cap -> Current Limiting Resistor -> zener diode -> Signal Tracer Tip

Thanks
Sono

 

[JohnRoberts]

What are you going to connect this probe to?

Perhaps a cap and a resistor in series.

JR

 

[sonolink]

What are you going to connect this probe to?

Perhaps a cap and a resistor in series.

JR

The probe is a signal tracer for low voltage gear that I bought. It's an audio probe with built in signal generator. It's meant to be used to trace guitar fx pedals. But I'd like to be able to use it with tube amps too, so I need to protect it from high voltage to be able to use it to trace a tube amp. My initial thought was to place a 100nF/600v between the amp and the probe to avoid the risk of toasting it, but as Ccaudle mentioned I should probably also add a current limiter resistor and a zener or a transient suppressor.
I have many resistors and zeners, so my next question is what resistor and zener values I should go for?
The original signal tracer was meant to be used with fx pedals working at 9-18v.
Should I use a 9 or a 18v zener then?
What about the resistor? How can I determine its value?

Thanks for your kind help
Sono

 

[sahib]

The build manual states that the maximum input voltage is 12V but 9V nominal.

A simple, switchable voltage divider between the probe and the tracer will also serve you well.

However, also make sure that your probe has sufficient insulation to withstand higher voltages.

 

[abbey road d enfer]

Should I use a 9 or a 18v zener then?

You would need to use two zeners back-to-back (in series), in order to protect both positive and negative peaks.
I would start with 6.2 or 6.8v zeners.

What about the resistor? How can I determine its value?

The resistor value must be high enough to limit current at a reasonable value, and not too high as it would act as a low-pass filter and increase noise. Knowing/assessing teh input impedance of the tracer would help making a choice.
I would start with about 50 kohm.
I sustain sahib's recommandation of an attenuator when testing tube power stages.

 

[sonolink]

Thanks to all for your replies.
I've decided to build a dedicated signal tracer for tube amps inside a small Hammond enclosure. Following Sahib's advice, I will use a HV scope probe with attenuator, and following Ccaudle and Abbey's suggestions I've come up with this for the aluminium enclosure:



Would that work ok?
Thanks
Sono

 

[abbey road d enfer]

The zeners must be connected at the output (To Amp).

 

[sonolink]

Oooops......
Like so?

 

[radardoug]

The zeners should be a shunt to ground!

 

[radardoug]

If you are using this with a 10:1 scope probe, you need to consider what is in the scope probe.

 

[sonolink]

The zeners should be a shunt to ground!

You mean they should be ACROSS the output jack terminals? Sorry, I misunderstood they should be in series with the incoming signal.

 

[ccaudle]

You mean they should be ACROSS the output jack terminals?

Yes, so that if the signal exceeds the input capability of the signal tracer device (which seems to be powered by 9V, so presumably +/-4.5V centered around signal ground would be the absolute maximum it could accept) the signal amplitude will be clamped by the zener diodes.

 

[sonolink]

Yes, so that if the signal exceeds the input capability of the signal tracer device (which seems to be powered by 9V, so presumably +/-4.5V centered around signal ground would be the absolute maximum it could accept) the signal amplitude will be clamped by the zener diodes.

I think I begin to understand it now

Like so then, right?



Thanks a lot for your help and patience
Sono

 

[abbey road d enfer]

This would be more correct:


Note that if you use an attenuating probe, it won't give the correct attenuation, because a probe typically wnts to see 1Megohm; here it sees about 60kohm.

 

[sonolink]

Thanks a lot Abbey!

 

[ccaudle]

The advantage of Abbey's connection method is that the load impedance to the audio circuit side does not change as much when you change the attenuation setting.
Check the data for the signal tracer pen to see if the ~7V clamping voltage is low enough. If the tracer runs from 9V and centers the input at the mid-point, then it may need the input signal limited to less than 4.5V, in which case a zener voltage of just under 4V would be appropriate. Or maybe the signal pen includes the limiting already and you do not need to worry so much about that (although your box would be more generally useful with that in place for using with other devices, e.g. soundcard).

 

[sonolink]

I whipped up the box with some spares I had around







I'll try it out tomorrow. Now time to get some sleep. Good night everyone and thanks again for your help!
Cheers
Sono