Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: paintballnsk on February 08, 2021, 06:01:13 pm
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Hey,
So I bought a scope and a signal generator. They'll be here Friday.
I'm fairly new to using scopes other than like checking digital traffic and stuff like that. I've never had to probe around a high voltage box.
I'd like to get a look at my gain stages and see what they're doing.
So for example, how do I safely plot the grids of V1B and V1A? I want to compare the signals, make bode plots based off the input signal, stuff like that.
I've attached a schematic. Can someone walk me through where I should be putting my probes and how to safely do it? I understand how to bleed caps between tinkering. I assume I need to always make sure the probe ground is connected to ground since the amp is also mains grounded right?
Any help would be great.
Thanks :)
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VERIFY your scope probe spec's!!!
I'll assume 300vdc
I use 880hz test signal, hot so 80-100mVac_rms
put that into your scope, once you're happy, know what the scopes telling you;
I probe input on CH1 and leave it there
measure plates of the pre amp tubes (AFTER you verify it doesn't exceed the probe spec). Leave the PI for now till you're happy
probing grids can be problematic, so do the plates, know what you're seeing, then;
probe the grid AND plate at the same time, compare to the "only plate" image, did the scope skew it?
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Generally it is unwise to probe plates. The high DC voltage is bad for the probe. The _AC_ you can see at the subsequent grid, or some point on the grid network which is at low DC voltage.
The Bode plots you propose will be trivial, boring. You could plot them on a strip-club napkin. Try it.
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The golden rule is to never probe a power valve anode. A special high voltage 100:1 probe is required there.
And as the scope and amp chassis are grounded (you’ve checked that, yeah, right back to planet earth?), it’s best to have a probe with a removable ground clip, as 99% of the time it’s an unnecessary hinderance.
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Should I be using an isolation transformer?
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Isolation transformer is for suspected faults, shorts, unknowns. my understanding;
you have a good amp and you want to "spec" it out.
that requires test equipment and the understanding of the equipment as it relates to the "amp" in question
also requires understanding of the amp
in short, don't probe willy-nilly, least you destroy both test Equipment and amp
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Scope chassis etc - grounded
Amp chassis - grounded
Benefits of isolation transformer - none (at least that I can perceive)
A light bulb limiter is probably the most useful item of test kit, ahead even of a decent meter.
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I suggest you sit down with scope and sig gen and learn some scope basics by looking at the sig gen before you ever connect the scope to an amp.
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^^^^that´s how i did it, although i first used it on a boost pedal i purchased not working: signal gen on input and then trace through the circuit until the signal stops (turns out whoever built it has as much trouble reading those little blue metal film resistors as i do... 470r to ground on an op amp input instead of 470k). once comfortable with the scope in a low voltage situation i did what the others mentioned above. if you´re probing for signal, make sure your scope is set for ac coupling!