> 800VDC peak to peak rated as maximum input voltage...
You do NOT want high voltage inside your 'scope.
In general, it is a BAD IDEA to be poking power plates. Nearly all you need to know can be inferred from secondary (speaker) voltage and turns-ratio. On my last build, I never went near the plate, even though I was very curious about voltage and efficiency.
Yes, in the past I have probed power plates with my 'scope. And yes, I have a melted probe where 800V arced the capacitor inside the probe. And I am a little lucky it did not ZAP the input stage of the 'scope.
While there exist high-voltage probes with accurate response, you do NOT need to be this fancy around guitar amps. Plate impedances are just a few K, so you do not need high impedance. Guitar questions are mostly under 10KHz, you do not need MHz response.
Use a 1 Meg 2 Watt resistor and a 10K resistor from plate to ground. Be SURE the ground clip is secure before you poke high voltage with the 1Meg! Put the scope across the 10K to ground.
This is a 100:1 divider(*). Your plate may sit at 400V, wobble to 100V and 700V. After the divider, it is 4V, down to 1V and up to 7V. Perfectly safe for any general purpose 'scope.
(*) It is really 102:1 ratio. But in guitar amps, we don't need great accuracy. "About 400V" is good enough, we never need to prove 408.1V exactly. We are more concerned with the shape of the wave, and relative to the supply voltage. If the plate can pull-down 80% of supply voltage, the load impedance is not too low; if it only pulls-down half the supply voltage before going flat, the tube is unable to pull the load. Anyway, all your voltages will change 2% or more from home to bar A to tavern B, from noon to midnight, etc.
The 10K shunt resistor into a typical 'scope cable and input gives frequency response over 100KHz. Actually the stray capacitance across a 1Meg resistor may keep sorta-flat response much higher, but who here cares? We are not tuning a radar modulator. (Not when off-work.)
This is for power plates. Voltage amplifier (12AX7) plates will be loaded by the 1 Meg. The stage will work, but the measured voltage and overload point may be 10% off from the unloaded point. This is still usually good enough. You can also learn a lot by probing the next grid (near zero volts DC) instead of the 200V plates.
The 1Meg:10K divider was also handy back in days when cars had ignition points. The primary spark voltage exceeds 400V, and with a Mark IV ignition module, sometimes a lot more. The shape of this wave probes many parameters of cap, points, coil, wires, plugs, and chamber. The 25 cent divider allowed me to study the ignition waveform on a standard 'scope.
Still in all, I suggest you stay away from power plates. Any distortion at the plate is also shown at the load and in the cathode current (1 ohm cathode resistor). If you just want to compare overload levels at preamp, driver, and power stage, reading power stage secondary tells you when it overloads, you do not need to know if plate-swing is 300V or 330V.
Topic: word on HP O-scope 130C (Read 9981 times)