The Nemesis 813 PP Amplifier

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pre65
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#211 Re: The Nemesis 813 PP Amplifier

Post by pre65 »

izzy wizzy wrote: Sun Mar 24, 2019 7:06 pm

So last week I bought 2 Meanwell 10A 5V supplies, connected in series with the centre tap taken to ground like Silvercore do. On first turn on, I was very disappointed as the valves didn't light up. Then slowly they came alive. It has a slow start. Woohoo.


Stephen
Hi Stephen, not sure I fully understand the way you wired those two 10A 5V supplies, a little diagram might help.


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#212 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Do you mean centre-tap to cathode resistor ? (Stephen)

Very pleased for you - must be a big relief after all of the mods you've tried.
How does the top-end sound, though, with no HF filtering ( I assume these are SMPSs ) ?
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#213 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

pre65 wrote: Mon Mar 25, 2019 11:41 am Hi Stephen, not sure I fully understand the way you wired those two 10A 5V supplies, a little diagram might help.
Thicko country bumpkin. :lol:
I didn't get it until I saw it either.

Circuit like in this thread https://www.audioasylum.com/cgi/t.mpl?f ... y&m=199885

The 833 amp looks like this http://knipschild.net/projekt833c1.htm

Cheers,
Stephen
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izzy wizzy
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#214 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Mon Mar 25, 2019 11:56 am Do you mean centre-tap to cathode resistor ? (Stephen)

Very pleased for you - must be a big relief after all of the mods you've tried.
How does the top-end sound, though, with no HF filtering ( I assume these are SMPSs ) ?
See above for pics. I got fixed bias now.

Relief is part of it. I so wanted AC to work so a bit disappointed it took me this long and that I had to use SMPS but the other options were LCLC or linear regs and the cost, weight, heat etc made it prohibitive. If the SMPS don't have significant downsides, then it is a very elegant solution. But still very happy I have a silent amplifier.

Haven't made a comparison between channels yet to see pros and cons. I intend to try common mode choke at some point.

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

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Ah, fixed bias, missed that.
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#216 Re: The Nemesis 813 PP Amplifier

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IslandPink wrote: Mon Mar 25, 2019 11:56 am How does the top-end sound, though, with no HF filtering ( I assume these are SMPSs ) ?
So I went to compare channels to see if there was something to your question. Tried a few records and was falling for the SMPS channel on soprano and solo piano. Thought I'd made my mind up on the top end in favour of the SMPS amp. Then thought I'd give Roy Orbison a go. Oh dear. The AC channel was so much more of everything and had more bass where the SMPS channel was shy. Tonight I rolled the amps around to swap the speakers they were driving. The difference didn't follow the amps so swapped the crossovers over and the sound difference followed the crossovers.

So now I got to find out why the crossovers sound different. Maybe it's time to do the crossovers properly or at least a bit better than they are currently which is a bit of a lash up from many years ago in an attempt to keep the crossover semi original or maybe not to do the work I needed to do. Hard to tell now :oops:

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Stephen
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#217 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

I've been having loads of trouble from bad connections in the phono chain lately , so do try & disconnect & re-connect anything relevant, just to make sure there's nothing cruddy in one channel or other. I had speaker swaps & tests that just weren't making sense.
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#218 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Thu Mar 28, 2019 9:47 pm I've been having loads of trouble from bad connections in the phono chain lately , so do try & disconnect & re-connect anything relevant, just to make sure there's nothing cruddy in one channel or other. I had speaker swaps & tests that just weren't making sense.
I started the comparison by monoing the cart and swapping it back and forth, then eliminated the phono stage by only using one channel and swapping the amps on the line out and so on until I found the crossover issue. Pulling them apart I found a wire had fallen off a resistor which loads the tweeter autoformer. Oh dear. But it's too late now to make a big noise. Another go tomorrow. I think the bass might be something to do with the speaker locations being slightly different but moving them is a major PITA. They need castors ike the amps have ;)

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Stephen
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#219 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Thu Mar 28, 2019 9:47 pm I've been having loads of trouble from bad connections in the phono chain lately , so do try & disconnect & re-connect anything relevant, just to make sure there's nothing cruddy in one channel or other. I had speaker swaps & tests that just weren't making sense.
Phono signals are so low I think they can degrade with dirt and corrosion on the connectors. Probably a good thing to wiggle everything around once and a while. Maybe a timely reminder to do that now and again.

So going back and forth with a bunch of different records has me gravitating to the SMPS channel. I keep forgetting to compare when listening to that channel. I'm not hearing any SS stuff that would normally put me off; the greyness and coarse sound that goes with SS in the chain. The AC channel is a bit crude, a bit vague, timing is a bit off, not so much colour in comparison. And the SMPS channel fades into the background rather than a hum. I want to try more music with the SMPS channel and get bored with the AC channel. I'm not hearing anything in the top end that's annoying. There must be a downside somewhere but I'm not getting it.

So I'll put SMPS supplies into the second channel.

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

Post by izzy wizzy »

SMPS into 2nd channel and it hums :roll: Not the same hum/buzz as before. Thanks goodness I did the other one first. There is one difference I know of between them as this one has a choke in the HT to the driver and maybe it's picking up some stray field. It is quite close to another transformer.

At least I have one that is quiet so can swap things back and forth to eliminate but that will have to be much later in the weekend.

So close yet ....

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

Post by izzy wizzy »

Swapping things around has revealled where the hum is coming from. I did the easy thing first, swapped the 46s over and the hum went with them. Doing some more swapping found the culprit.

Wasn't expecting that and so not sure why that would happen. The 46s don't have any grid stoppers, ferrites or anything. This is my first experience with DHTs so is this a thing?

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Stephen
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#222 Re: The Nemesis 813 PP Amplifier

Post by IslandPink »

Cor, this is a journey.
I must admit I haven't had much hum trouble from DHT's though I haven't used a lot of old ones like 46's. I have however used 300B, 2A3's ( Sovtek ) , 6B4G's of various vintage, 45's occasionally , 4P1L, GK-71 .
I would mainly expect HF oscillation from omitting grid-stoppers, but maybe others have different experiences.
It must be a relief to find that you've taken the SMPS out of the equation for the hum though.
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#223 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

IslandPink wrote: Mon Apr 01, 2019 8:35 pm Cor, this is a journey.
I must admit I haven't had much hum trouble from DHT's though I haven't used a lot of old ones like 46's. I have however used 300B, 2A3's ( Sovtek ) , 6B4G's of various vintage, 45's occasionally , 4P1L, GK-71 .
I would mainly expect HF oscillation from omitting grid-stoppers, but maybe others have different experiences.
It must be a relief to find that you've taken the SMPS out of the equation for the hum though.
My first thoughts were to try the grid stoppers and found a bag of ferrites in the man cave that would be suitable to slip over wires. After what Nick said, I removed all stoppers and only have a multi loop ferrite on the 813 grid and would go forward on them when I needed. Could be the time for that.

You're right about this bloody journey thing. Be great when I get to some kind of destination for a rest.

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

Post by izzy wizzy »

This has taken a turn I wasn't expecting. Rang Langrex for starters to get some more 46. They only have 1, maybe more but they're not sure. Billington, none. VacuumTubes.net, none. Can't find any online in the few places I looked. Oh dear.

Any ideas of other places? Once an unknown, now unobtanium?

Which means I may have to choose another input/driver :shock:

All ideas welcome.

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

Post by Mike H »

I'm not surprised TBH. I thought they were already unobtainium. I'm guessing Langrex want silly money for the one they've got?
 
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