Signal Tracer DIY

Hi all.
I would like to know if it is advisable to use a litlle Guitar amplifier as a signal tracer, or should i rather get a dedicated amplifier/ speaker for this. What type of circuitry will I need front- end to add this to my amp? What type of input Impedance will I need? I want to be able to probe valve amps etc.
Any advice please.

I used a 1M resistor in series with a 1000V 0.1u cap connected to two back to back parallel 1n914 diodes grounded at one end as an input circuit to a simple headphone amp when I used to signal trace power amps. Worked fine.

You may want to drop the resistor value for smaller signals.