The Nemesis 813 PP Amplifier

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izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Fri May 10, 2019 9:40 pm

Cressy Snr wrote:
Fri May 10, 2019 8:51 pm
it’s in the best boat anchor tradition. :)
Very true. It needs to be split., top half and bottom, otherwise I can't lift it or work on it and on casters so I can move it. But it's lighter than I thought it would be :)

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sat May 18, 2019 8:08 pm

Last week I fiddled to try something out and it didn't go well and blew the plug fuse coming up on the variac. Upsetting and a step backwards. Realised it was built so badly I couldn't get to things which was a contributer to the problem. The rebuild for Owston wasn't going to make it so tried an idea planned; the second tier. This has worked out well and it's all go again :)

2nd tier contains 4P1L supply, 4P1L CCS regs, 813 bias supply all laid out way better than perched round the edge. The other amp has the top deck a lot lower and the final layout will be much narrower as well. About as narrow as the bottom level transformers now.

IMG_20190518_195210153_compress73.jpg

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun May 19, 2019 3:05 pm

Latest audio circuit

IMG_20190519_145950313_compress53.jpg

steve s
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Re: The Nemesis 813 PP Amplifier

Post by steve s » Sun May 19, 2019 4:58 pm

Looking forward to hearing it at Owston looks a good circuit.. very 40s ?...
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Re: The Nemesis 813 PP Amplifier

Post by Cressy Snr » Sun May 19, 2019 5:24 pm

izzy wizzy wrote:
Sun May 19, 2019 3:05 pm
Latest audio circuit
Look like it’ll kick some serious booty! :)

izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun May 19, 2019 5:49 pm

Latest power supply circuit


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izzy wizzy
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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun May 19, 2019 6:12 pm

I started this journey in April 2003. It came about as an amplifier to drive my then Acoustat 1+1s. I had heard an 813 PP amp and wanted something like that myself. It morphed into something very silly and my conclusion was, if that was what I needed to drive the Acoustats, then something was wrong somewhere. I ditched the Acoustats and went down the Tannoy route. Being way more sensitive made the design very difficult and many amplifiers were built and there was much disappointment. At one point my wife thought I needed to be banned from the shed as it wasn't a happy place.

So this time I went about things in public so had to be more structured in the approach and it has worked. I have arrived here with an amplifier that works how I want and I think it's better than I thought it would be. My wife sat down for a listen last night and unprompted thought it sounded lovely. That's never happened before.

I think I get the direct heated valve thing. I knew it would have effortless power from the first amps but then there's this something else. I guess it's what draws people to using these DH valves. What a PITA they are but it is worth the effort.

Thank you to everyone that contributed. It has helped enormously in getting the thing to this point.

This isn't a destination but the first part of the journey is over. I've done what I set out to do in the first place. It's just taken a bit longer than planned.

Cheers,
Stephen

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Re: The Nemesis 813 PP Amplifier

Post by IslandPink » Sun May 19, 2019 8:06 pm

Well done for sticking with it and working through all the problems. Looks great on paper and I'm very much looking forward to hearing it. It's a while since I've heard class A PP. Have been mulling over a 'Tabor' build myself in the background on email, this amp has got me thinking about it again.
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Wed May 22, 2019 8:17 am

The power feed to the first stage has been giving me a rising concern. If anything went wrong or I forgot to put the valves in and so on, HT2 could rise to HT1 and the HT2 decoupler, rated at 400V would go boom. So I've replaced it with 3 x 220u 400V in series so it can now easily take full HT of about 1000V and I can sleep better.

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Re: The Nemesis 813 PP Amplifier

Post by Paul Barker » Wed May 22, 2019 1:00 pm

Sensible yet the single small cap is cheaper to get quality of a quality. You chose safety or quality. Or figure out a current sink which would pull down the voltage in the absence of valves pulling current place parallel with the last ps cap, for which you could use power resistor like in Flesh & Blood power supply. Or more complexes but proving some regulation a pull down regulator with 3055’s which could be paralleled or a tube shunt regulator.
I can do it right or I can do it now, but I can't do it right now.

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Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy » Sun Jun 09, 2019 5:45 pm

A question for the panel. In looking to take this further, I'm reviewing everything to see where I might travel so 813s might be subbed for GM70. What's the impression out there about these things? Not necessarily vs 813 but in general. They certainly seem cheapish and available. Pros and cons?

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Stephen

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Re: The Nemesis 813 PP Amplifier

Post by IslandPink » Sun Jun 09, 2019 6:30 pm

Even the graphite ones are very good. I'd say better than an 813 for sure, others like Nick / Paul have more hands-on experience with those vs. 211 & 212 , I know they did a big comparison once. I saw your comments re. GK-71's , that's OK, and I'd say GK-71's will be very similar to graphite GM70. My only proper comparison at home was GK-71 vs. expensive KR 300B's , equal, except better in the bass, at 1/10th the cost !
"The bass is the king of the instruments - it has no known natural predator" (Wobble)

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Re: The Nemesis 813 PP Amplifier

Post by Nick » Sun Jun 09, 2019 8:57 pm

Test with graphite gm70's then get a set of copper for best.
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Re: The Nemesis 813 PP Amplifier

Post by simon » Sun Jun 09, 2019 9:02 pm

Supposedly the real goodness of GM70 starts at 1000V. That's too much for me - the amp you heard at Owston has around 550V HT.

I've read that some poeple prefer the graphite to the coppers, and they do sound different - the graphite's bass is bangin' man, but once I'd heard copper GM70s I lost interest in trying other valves.

I thought your 813s sounded really nice though.

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Re: The Nemesis 813 PP Amplifier

Post by simon » Sun Jun 09, 2019 9:13 pm

Blimey. I've just looked on ebay - seems I've missed the boat on coppers...

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