What is Reddit's opinion of Eurotubes Bias Probe for Octal Base Power Tubes.?
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I haven't had the time to open the chassis and verify this schematic for myself (found afterward), but the one I linked to should be the exact same amp. I measured the transformer output voltage a while ago before finding it... now that I think about it, probably improperly (and somehow didn't kill myself).

As for retesting plate voltage, would this work? https://www.amazon.com/dp/B019PAVXPG

It's designed for measuring plate current, but I should be able to use it for voltage by connecting the red probe to a multimeter and covering the black probe... right? It would certainly save me a lot of time opening the amp again.

Would this be ok to use? https://www.amazon.com/Eurotubes-Bias-Probe-Octal-Tubes/dp/B019PAVXPG I remember seeing a youtube video years ago on biasing that used this. When I was looking into it recently, everyone only said about using a multimeter. I'm assuming I just put it under each tube, power on and adjust. Is there anything else I'm missing or should know about?

I think I see the resistor you are talking about (it looks discolored). Is this something I could and should replace? I'm not too experienced soldering.