Question re grounding

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RhythMick
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Question re grounding

Post by RhythMick » Tue Aug 27, 2019 7:15 pm

Hi all

My amp is 3 stage, each is valve rectifier into LCLC. Transformer coupling between stages. Each stage uses entirely separate mains transformer secondaries, and each stage is grounded to star earth exactly once, at the bottom of the cathode resistor. Each circuit is arranged with chokes in both + and - power connections.

I now want to experiment with things, including driving all 3 stages from s single mains transformer winding. Each stage would retain separate rectifiers, LCLC etc.

Am I right to think that I should still only provide one 0v reference (star ground) ? If I continue to ground each stage at the cathode resistor, each will be at different voltages with respect to the transformer so that can't work.

Someone set my thinking straight, or tell me I'm right.

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Nick
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Re: Question re grounding

Post by Nick » Tue Aug 27, 2019 7:41 pm

Not sure what you hope to achieve. Sharing a single secondary with three sets of rectifiers is going to be strange. Even stranger if you add chokes into the 0v lines.
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simon
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Re: Question re grounding

Post by simon » Tue Aug 27, 2019 8:38 pm

Yeah, if I had three separate PSs the only change I would consider in your situation is three separate power trafos for galvanic isolation. Which may or may not be better.

But if you're going for a single PS then I'd use one rectifier, probably a single LC, and then further LCs to each stage. But you'd need to be sure the rectifier and power trafo winding has enough current rating for the whole amp. And the chokes, like for like, will probably have less inductance if there's more current through them. And you'll perhaps be adding series resistance to reduce some of the voltages.

Good luck, interested to hear what you find.

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Nick
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Re: Question re grounding

Post by Nick » Tue Aug 27, 2019 9:11 pm

three separate power trafos for galvanic isolation
Three separate secondaries will do the same thing.
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simon
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Re: Question re grounding

Post by simon » Tue Aug 27, 2019 11:16 pm

Yes, true, I was a bit sloppy.

I've never had a trafo with separate secondaries to be able to try this. But certainly giving the driver stage a whole separate PS (trafo, rectifiers, LC filter) was a notable improvement over a shared PS when I tried it with the GM70 amp.

Whether it's better than separate secondaries would be interesting. My gut feeling is it should be, and Broskie talks about the benefit of separate heater trafos. But I've never had the chance to compare, and any improvement may be marginal at best.

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