The Nemesis 813 PP Amplifier

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Nick
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#181 Re: The Nemesis 813 PP Amplifier

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Worth trying the filters that worked for Mike, just because I never got them to work doesn’t mean you won't.
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#182 Re: The Nemesis 813 PP Amplifier

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Thinking about the hum pots not making a difference (evening dog walk), that may point to the problem being a signal introduced into the cathode WRT amp ground, via the SMPS input from earth and or line neutral. unlike indirectly heated valves, you can't just reference the output directly to 0v.
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#183 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Sun Dec 09, 2018 7:27 pm Thinking about the hum pots not making a difference (evening dog walk), that may point to the problem being a signal introduced into the cathode WRT amp ground, via the SMPS input from earth and or line neutral. unlike indirectly heated valves, you can't just reference the output directly to 0v.
OK, I read this about 10 times and don't quite get it. Are you saying as the SMPS output isn't ground referenced but elevated, that the SMPS could be introducing noise into the ground or the cathode some how?

I bypassed the cathode to see what would happen and it did nothing.

If so, could that also be the problem I'm having with AC? The only other person I know who has done this and says their amp is slient uses fixed bias and that would mean the cathode circuit is grounded.

If that's not right, would you mind trying to explain that again?

Cheers,
Stephen
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#184 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

I've been reading Andrew's DHT reg thread. Skimming really as it's very long. I was hoping that maybe some of the participants could give me a management summary to a few questions.

Is there a version that could do 10V @ 5A for the 813? It seems paralleling the filaments isnot such a great idea with this reg solution.

I see Mark was using an SMPS. Do I understand that was feeding the regs? Is that an option and was the HF hash an issue?

What is the voltage drop across the reg? I've read it's 4V. Is that right?

I'll keep on reading the thread as well.

Cheers,
Stephen
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#185 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Sun Dec 09, 2018 7:09 pm Worth trying the filters that worked for Mike, just because I never got them to work doesn’t mean you won't.
Always worth a try. Only one way to find out! As I am fond of saying. I'd be interested if it makes any difference to the hum. You never know (without trying) :D
 
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#186 Re: The Nemesis 813 PP Amplifier

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izzy wizzy wrote: Sun Dec 09, 2018 8:22 pm
Nick wrote: Sun Dec 09, 2018 7:27 pm Thinking about the hum pots not making a difference (evening dog walk), that may point to the problem being a signal introduced into the cathode WRT amp ground, via the SMPS input from earth and or line neutral. unlike indirectly heated valves, you can't just reference the output directly to 0v.
OK, I read this about 10 times and don't quite get it. Are you saying as the SMPS output isn't ground referenced but elevated, that the SMPS could be introducing noise into the ground or the cathode some how?

I bypassed the cathode to see what would happen and it did nothing.

If so, could that also be the problem I'm having with AC? The only other person I know who has done this and says their amp is slient uses fixed bias and that would mean the cathode circuit is grounded.

If that's not right, would you mind trying to explain that again?

Cheers,
Stephen
What I meant is that the SMPS may be injecting noice into the cathode because there will be effective capacitance between the SMPS output (cathode) and the mains input to the SMPS. Your 0v line in the amp will be refenced to the same mains lines, so will see that signal across the hidden caps. May not be true, but I was trying to find way sthat noise can get into the amp without the hum pots affecting that noise.

Not sure Mark has a 10A capable reg yet, you asked what I did, and what I did was to use linear current source regs. But that was for a 211 at the time so a simpler job.
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#187 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Sun Dec 09, 2018 11:39 pm
What I meant is that the SMPS may be injecting noice into the cathode because there will be effective capacitance between the SMPS output (cathode) and the mains input to the SMPS. Your 0v line in the amp will be refenced to the same mains lines, so will see that signal across the hidden caps. May not be true, but I was trying to find way sthat noise can get into the amp without the hum pots affecting that noise.

Not sure Mark has a 10A capable reg yet, you asked what I did, and what I did was to use linear current source regs. But that was for a 211 at the time so a simpler job.
OK thanks Nick. I think I have to try fixed bias to at least remove a variable.

I wouldn't need a 10A reg but a 5A one if possible? However that's a load of heat to get rid of via a mighty heat sink. 4v drop min at 5A x 2 is 40W of heat. What sort of drop did your current source jobbies need?

Whatever happens I reckon it's going to get a bit more complicated.

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Stephen
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#188 Re: The Nemesis 813 PP Amplifier

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Should have said Andrew not Mark. I know he is working (when he has time) on a high current variant, but not ready yet. 5A should be do able, I did 3A for a gm70 using a three pin reg as a current source, that worked ok, but did generate heat so needed a big heatsink. I had similar startup problems when cold. I got around by bipassing the CCS with some power zeners to start current flowing, then shutdown and let the CCS take over once is was underway. I n theory LT1584 should do the job.
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#189 Re: The Nemesis 813 PP Amplifier

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The fact that you are using a interstage helps a lot for fixed bias, so worth a try.
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#190 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Voltage drop was slightly over 4V I think , maybe 4.5V. I seem to remember 23.8V in from a 24V/5A SMPS and 19.2V out, when running GK-71's. However I don't think 4.5 was needed from the SMPS input, which is relatively smooth, it just came out that way.
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#191 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Sun Dec 09, 2018 11:39 pm What I meant is that the SMPS may be injecting noice into the cathode because there will be effective capacitance between the SMPS output (cathode) and the mains input to the SMPS. Your 0v line in the amp will be refenced to the same mains lines, so will see that signal across the hidden caps. May not be true, but I was trying to find way sthat noise can get into the amp without the hum pots affecting that noise.
Interesting point. Image

If it helps (but may not) both my SMPS's have "floating outputs", neither is normally connected to mains earth, except where I've added the link for it which is also via an earth lift resistor.
 
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#192 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Mon Dec 10, 2018 12:01 pm Should have said Andrew not Mark. I know he is working (when he has time) on a high current variant, but not ready yet. 5A should be do able, I did 3A for a gm70 using a three pin reg as a current source, that worked ok, but did generate heat so needed a big heatsink. I had similar startup problems when cold. I got around by bipassing the CCS with some power zeners to start current flowing, then shutdown and let the CCS take over once is was underway. I n theory LT1584 should do the job.
I found that datasheet and it looks like 3V would be a likely drop at 5A so about 1.8V drop for the reg leaving roughly 1.2V for the ref. That's getting more manageable. More food for thought.

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Stephen
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#193 Re: The Nemesis 813 PP Amplifier

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I'm trying to make my mind up which way to go with the fixed bias circuit and have two examples below. The first is simple, taken from Altec A260 but I think could be not as safe as the second taken from Turner Audio re-engineered Cary 2A3. Certainly plenty of commercial examples of the first but that's no recommendation in itself. I'm thinking open wiper scenario if that's a thing with multiturn trimmers these days which example two is safer from i.e open wiper pulls the neg bias down. Other than that, what do people think about one option over the other so leading them to construct that one? Real world experience welcome of course.

813bias.jpg

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Stephen
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#194 Re: The Nemesis 813 PP Amplifier

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The first gets my vote as its going to be simpler to adjust each valve on its own. You can make them failed pot safe by adding a 1M resistor between each wiper and the -ve rail. What the 180k resistor doing BTW?
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#195 Re: The Nemesis 813 PP Amplifier

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Nick wrote: Sat Dec 15, 2018 7:48 pm The first gets my vote as its going to be simpler to adjust each valve on its own. You can make them failed pot safe by adding a 1M resistor between each wiper and the -ve rail. What the 180k resistor doing BTW?
Thanks. I was leaning for the first but the second confused things in my mind.

It's a good question about the 180k. As I said, I nicked both circuits without too much further thought other than what I said. I don't see a reason for them, so let's get rid. Probably more specific to the circuits I took them from i.e they were capacitively coupled; first with a grid choke so maybe to tame a resonance.

The 813 has a max grid resistance of 30k so maybe lower than 1M otherwise possible runaway on wiper failure? With some other altered values to accomodate. Does that make sense?

cheers,
Stephen
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