How to scope secondary side of a transformer?

[katastic]

Do you HAVE to use either an isolation transformer, or differential probes?

I've got a cheap tube guitar amp with tons of noise and I'd like to be able to probe if the power supply is the culprit, which stage the frequencies are coming from, etc.

Now, I did some googling and I read you can ground one side of the secondary. So I tried that, and then added the scope ground there. Flipped the switch... and immediately blew a fuse in the socket housing. So clearly something I don't understand is going on. ... should I have tied one side of the secondary to neutral?

Can you just clip onto one side of a (no center tap) transformer? I don't want to blow my scope doing something stupid before asking someone.

Thanks,

[TimFox]

Many tube amps with a power transformer have a center-tapped secondary with two rectifier diodes for the plate supply.
Others use a non-ct secondary with four diodes in a bridge rectifier.  You cannot ground either end of such a secondary winding.
What exactly did you connect to what when “grounding one side of the secondary”?

[katastic]

I THINK I FIGURED OUT the oopsie! It looks like someone before me... already wired ground to one of the taps??? (and maybe I grounded to a different side, or location)

I've got a Epiphone Valve Jr. There's apparently many different versions, mine is extremely cheapified (3 main capacitors, no RF choke, no additional speaker transformer taps, no heater filament rectifier, etc compared to other schematics)

Low quality pic of mine []. Note the blue crimp around a green wire (top right) that does not match any of the other wires.

bottom of this site has the PDF schematic
https://sites.google.com/site/stringsandfrets/epiphone-mod
direct link:
https://drive.google.com/file/d/1EqlnqAVVJAd6PTHE_ARHUW2XmKmNjUYx/view?usp=sharing

[katastic]

Maybe this is stock...? Odd it's a different style crimp but the layout I guess makes some sense.... I don't know, I don't have the experience to know for sure, I've just got guesses at this point.

The transformer outputs, red ~260V, goes to the four diodes on the right. Top two diodes go to one red wire, the other two go to the second red wire. And the left side of the diodes appears to be against ground? I'm more used to the chassis being grounded and completely disconnected from the mainboard and neutral.

We've got power into the primary side, output to 260V secondary. But then the "zero" is earth ground. Which feels like it's floating since neither primary/secondary taps are tied to ground. But then the circuit (and those giant caps) is definitely referenced with negative to earth ground.\

[edit] I guess if I had bothered to look at the very bottom-right of the schematic, I'd see earth ground clearly being the reference. I don't do much mains electronics so this topology is a new one for me!

SO. Back to the original question, THEORETICALLY, if I simply put my scope ground on earth ground, my scope should be referencing to everything correctly and not blowing fuses?

[TimFox]

From the wiring diagram, the HV secondary is not to be grounded and feeds a four-diode rectifier bridge.
One terminal of that bridge goes to one terminal of the heater rectifier bridge (also from a 6.3 V ungrounded secondary), and they both go to ground.
Note the drafting error in the lower right, where the power-supply "ground" is a different symbol from that used in the rest of the circuit.
Since the cathodes of all tubes are returned to the normal ground signal, that must be the negative end of the power supply as well.

[Harry_22]

Would you like to redraw the circuit diagram of your valve guitar amplifier?
Understanding the scheme is much more interesting than just guessing.

[J-R]

Have you considered using two scope channels and a math function to act as a differential probe?
https://www.eevblog.com/forum/testgear/oscilloscope-operation-differential-voltage-measurement/

[xvr]

Do you HAVE to use either an isolation transformer, or differential probes?

You need any of these for sensing mains connected circuits. Secondary side of transformer do not connected [internally] to primary (to which mains connected). If other part of schema do not connected to mains you do not need any special.