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How much B+ ripple is too much?
#1
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Hi guys, have been out of the homebrew amp community for probably >10 years; I used to be fairly active on AX84 and ampage.  

Quarantine has me resurrecting an old amp I never finished with all the equipment from the office I now have at home.  It's a complex beast: channel switching, EF86 first stage, 2x6V6 power stage with power scaling a la London Power, reverb/tremolo.  AND I cant find my old schematic, so I'm tracing everything by hand.  Send me prayers and good thoughts!  

I've got signal all the way through one channel to the speaker and actually sounds rather good, but i'm hunting down a couple of buzz sources.  

The first one is in either the PA or the poweramp. My question for y'all is, how much B+ ripple is too much on power tube plates/screens?

I measure almost 2-3V on each power tube plate.  It's in phase, so in theory it cancels when the signals are combined, but I'm sure that's not a perfect cancellation.  So, how much is too much?  

(here's my plates, scaled down to about 250VDC so I don't blow my 300V scope inputs)

[Image: 49923970757_09605f4553_w.jpg]
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#2
Ok, well it's not the ripple on the B+ going to the plates, I can confirm this.

This new scope trace is of the plate of one side of my PI.

[Image: 49924222656_6f89e4f96e_w.jpg]

The other side shows similar spikes, but not quite as severe. When I AC ground the PI plates through a big capacitor and 10ohm resistor, the buzz disappears and the plate looks clean. When I AC ground the PI input, it doesn't *quite* go a way, but it gets better. When I hard ground the PI input (which I can do because it's ~0VDC), it does go away entirely. Therefore it's getting in inside the PI.

The PI B+ looks clean.

The period of these spikes seems like 120Hz rather than 60, so it must be from a rectifier somewhere, right?

PS documenting the troubleshooting here helps me work through it logically on my own haha, whether or not ya'll have any insight.
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