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Can someone advise whether there is a risk of adding noise to a circuit or other issues when using permanently connected external test points on a SET amp?
The amp will be direct-coupled and it might be a good idea to mount some banana jack test points to the outside of the amp for measuring driver operating points. The jacks will be wired to both ends of the driver plate resistor and both ends of the driver cathode circuit. This will allow me to perform MM readings of the V-drop across plate R and the Cathode V while I tweak the current through the driver cathode circuit using a pot wired as variable resistor.
Can anyone see any issues with this?
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Follow Ups:
I'm with DAK on this one. My SET amp has 1 ohm resistors in the cathodes that I brought out to the front panel without issue. Remember, the cathode is a low impedance point so it would take a very large interference signal to affect it.
Thanks guys.
Based on your comments:
- I won't implement the jacks to the plate resistor (not really needed). this will help avoid the potential for noise in a high impedance part of the circuit.
- I'll add a 10 Ohm resistor to the driver cathode resistor chain and wire jacks to both sides. This will be in a low impedance part of the circuit and still allow me to measure the V-drop and calculate the driver stage current, which are the main reasons for the test points.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
I wouldn't add wire to any point that's high Z. In fact, the voltage across the anode resistor shoudn't need to be physically metered. Unless there's something unusual about the circuit, the voltage can be calculated if you know the cathode current (a low-Z point easily measured) and the anode resistor value.
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Buy Chinese. Bury freedom.
Although not exactly what you are doing, my fixed bias se amp has test points on the cathode's 10 ohm resistor with no problems. I suppose in a high emi/rfi environment there is a possibility of some extra wiring picking up an external signal but that would be mostly on the input grid.
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