The Nemesis 813 PP Amplifier

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simon
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#331 Re: The Nemesis 813 PP Amplifier

Post by simon »

Thanks chaps. As a pentode with the bypass cap I might get away with it but otherwise I'd need to add a stage. More thinking required.
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izzy wizzy
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#332 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

Time for some theory on the OPT. Back in the the day I selected the opt from Sowter coz it said it was for 813pp and as I didn't know any better, bought it. A few days ago decided for the first time to try a different output tap. For 8R out, the primary is meant to see 7k a-a. I tried 4R secondary so therefore 14k a-a. Didn't like that at all. Very dull and boring.

IMG_20190801_215847232_compress23_crop_54.jpg

Thought I'd try and draw some curves reflecting that. I hope I've drawn some composite curves right and plotted the two load lines for 7k and 14k. If I look at these lines, the steeper one looks better as it hits the composite curves. But could I have picked that as the best place if designing from scratch? What exactly is a good line? I see people claiming they like a line but it's always in SET designs. Is pp that different? More questions than answers.

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Stephen
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#333 Re: The Nemesis 813 PP Amplifier

Post by RhythMick »

izzy wizzy wrote: Sat May 04, 2019 12:02 pm
IslandPink wrote: Sat May 04, 2019 11:56 am OK, so one side ( 'E' ) is connected to the 440R resistor ( or a suitable value for 4P1L's ) and you can connect one Lehane supply across E-F ?
I think things have moved on a bit from that post due to Paul's comments. I will need 2 Lehane regs but quite how I use them is up for grabs at the moment. See Rod's drg further up and Nick's comments.

Cheers,
Stephen
Sorry for the comment on an old post, I'm catching up a bit (work has dominated my life way too much recently).

For my all DHT 26-71A-45s I use Lehane boards and was able to compare 1 board driving the valves in a pair in parallel against separate boards. No comparison, the separate boards were much better. Both of these amps I've built (Geoffs and mine) use 12 Lehane boards for 12 valves.

I sketched out the following to get my head around the separate connections - probably no help but you never know. I never implemented the idea of the switch to reverse the +/- connections, deciding I could just as easily switch the wires over on the terminal blocks. However I've never done that either.
sketch-1564728822292_compress62.jpg
It may also be worth noting that I later switched from the shared cathodes to separate cathode resistors with bypass caps.
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Paul Barker
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#334 Re: The Nemesis 813 PP Amplifier

Post by Paul Barker »

Sorry to be very late into this thread. Have you studied the Altec 260A? What a gem!

Image

Both HT power supplies choke input with bearly any capacitance compared to modern power supplies. Both rectified from one tapped winding, simultaneously.

The 813 b+ rather high for the average modern diy tube amp builder but the value of the circuit would work at lower voltage.

I love the look of it for the old school ideas and practices.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Nick
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#335 Re: The Nemesis 813 PP Amplifier

Post by Nick »

The choke linked grids are rather interesting. Not seen that before.
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izzy wizzy
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#336 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

It was as amplifier that was inspiring. It also had me utterly convinced I could use AC for the 813 fils but no matter what I did, I couldn't get the noise down. It's got a lot of interesting features.

I tried grid chokes but it has that part on a common core. I read somewhere that this kind of arrangement can force/help balance. IIRC, the term iron current mirror was used as an analogy but I didn't look further. I could be talking nonsense though.
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Nick
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#337 Re: The Nemesis 813 PP Amplifier

Post by Nick »

Yep, I suspect its to allow A2 and provide a low impedance path to DC, and will also add to balancing.
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#338 Re: The Nemesis 813 PP Amplifier

Post by JamesD »

Similar to the Iron Concertina that Jeremy Epstein discovered in RDH v4 http://home.earthlink.net/~jeremyepstei ... rtina.html. Exceot its not a split load inverter...

There is also some discussion of this on Intact Audio http://www.intactaudio.com/forum/viewto ... 3cfb39aca6 - mostly a repeat of Jeremy website info with Dave Slagles view

I've used this is a couple of designs and its very good.

ciao

James
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izzy wizzy
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#339 Re: The Nemesis 813 PP Amplifier

Post by izzy wizzy »

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Nick
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#340 Re: The Nemesis 813 PP Amplifier

Post by Nick »

Similar to the Iron Concertina that Jeremy Epstein discovered in RDH v4
I disagree, its nothing like that.
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Nick
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#341 Re: The Nemesis 813 PP Amplifier

Post by Nick »

I guess its a bit like a push pull parafeed.
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#342 Re: The Nemesis 813 PP Amplifier

Post by JamesD »

I was about to reply that I would argue that the iron concertina is two things on a common core - its a split load inverter on the 'input' side and that is not the Altec circuit and a PP common core choke on the secondary side and that similar to the Altec circuit so excluding the split load inverter its similar and that was what I was getting at...

But then I thought - a PP common core choke - how does that work? And I didn't have an answer as the balance part of the two signals magnetic field cancels leaving the unbalanced part of the signal as the net magnetisation and I guess as that appears as common mode it cancels in the pp differential action leaving the balance signal to appear as output from the stage...

If that is correct then this second part is common to the two designs so they are similar... the iron concertina is maybe better in that the input to secondary coupling that remains as common mode would also cancel in the secondary stage... so maybe better balance...

I need to sit down and study the Altec design some more as it wouldn't surprise me that I'm missing something - so please fire away at this :-)

James
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Nick
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#343 Re: The Nemesis 813 PP Amplifier

Post by Nick »

Yep, its why it seemed different to me. In the iron concertina the signal that drives the two grids is generated in the would component. But in the Altec, the signal is generated and applied to the inductor. You can see the difference if you remove the inductor from each amp (in AC terms at least). In the one case you get nothing, in the other case it keeps working. Its the fact that the effect of the part is opposite that makes me say they are not in any way the same.

I can't decide for certain how its going to work with the applied signal, should play with it in spice. But I still feel there is some magic there to handle grid current better than a grid resistor (certainly a 180k one as in effect there is).
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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Paul Barker
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#344 Re: The Nemesis 813 PP Amplifier

Post by Paul Barker »

I recon it’s a beauty but beyond do reach to be fair. But a dressed down version might be as good. I especially like the opt feedback winding.
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pre65
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#345 Re: The Nemesis 813 PP Amplifier

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Paul Barker wrote: Mon Jun 29, 2020 4:52 pm I recon it’s a beauty but beyond do reach to be fair. But a dressed down version might be as good. I especially like the opt feedback winding.
I know where you can get a pair of mains transformers for it. :lol:
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