N43 pentode amp design

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JamesD
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#1 N43 pentode amp design

Post by JamesD »

Hi Chaps,

Back to audio after a few mad years. Retired, more or less, through ill health but getting much better now. Time to design and build a new system as over the years I have given up my old system. Running on some old ss amps with Ruark speakers. Its OK but nothing special. I've been going round in circles over what to build, 2A3/6B4G, PT15 or GK-71 main amp... even dreams of building the 833 again and no nearer a decision than when I started so I decided to do something simple and relatively quick. Looked through my valves and beside the above I have the usual selection of EL84, EL34, 6L6, 5881, 6V6, etc, etc but want to do something different... Then I came across four Osram N43 pentodes and looked up their datasheet. Nice audio specific pentode with nice linear curves for a pentode! In fact almost a Spud as 3V in will fully drive it... add a 1:2 step up input transformer and its done :-) 250V HT is easy and 4V at 2A heater - simples...

So how could I change it up a gear and make it interesting? How about running it in grounded grid - get a bit of extra gain that way too... OK but then it has a low input impedance so the step up transformer presents a very low input impedance to the previous stage and that could be a problem... OK a simple Cathode follower driver stage would be nice between the input transformer and N43 cathode... and so was born the following circuit.
EF184CF N43 pentode.png
EF184CF N43 pentode.png (6.8 KiB) Viewed 6817 times
Must add some values but quick summary is 5K OPT, 20H 90R dcr cathode choke and a ll1676 input tranny in 1:2 config.

Just need to order the iron - thinking of trying Toroidy - anyone any experience of them?

ciao

James

p.s. its great to be back!
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ed
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#2 Re: N43 pentode amp design

Post by ed »

welcome back....good to see you....glad you're well.
There's nowhere you can be that isn't where you're meant to be
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pre65
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#3 Re: N43 pentode amp design

Post by pre65 »

JamesD wrote: Sat May 09, 2020 3:12 pm
Just need to order the iron - thinking of trying Toroidy - anyone any experience of them?

ciao

James

p.s. its great to be back!
I understand that NVA now source their toroids from Toroidy, but one of the partners is Polish.
The only thing necessary for the triumph of evil is for good men to do nothing.

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G-Popz THE easy listening connoisseur. (Philip)
JamesD
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#4 Re: N43 pentode amp design

Post by JamesD »

Hi Ed,

Thanks for the welcome - hope you and yours are well and staying safe.

Phil,

Thanks nice to know - they are very well established for power transformers and several companies use their audio transformers too but I haven't found much from western europe diy community there are positive reviews from eastern europe diyers :-)
simon
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#5 Re: N43 pentode amp design

Post by simon »

Hey James, great to see you again, will reply when I get a bit of time.

I ordered some mains toroid with LV secondaries from Toroidy for, blimey, what was it? Think it might have been a Salas simplistic folded phono and his pre amp too. I think they do a few different levels, mine had the epoxy centres. Seemed really well made to me and very quiet, no hum or buzz. I thought they were good value at the time too. Not tried their OPTs though.
Cressy Snr
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#6 Re: N43 pentode amp design

Post by Cressy Snr »

Hi James and welcome back.
It’s been a long time.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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IslandPink
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#7 Re: N43 pentode amp design

Post by IslandPink »

Nice to see you James, I did get a couple of days sneak preview on this and your circuit, of course !
So the N43 is driven via its cathode ?
"Once you find out ... the Circumstances ; then you can go out"
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Ali Tait
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#8 Re: N43 pentode amp design

Post by Ali Tait »

Hi James, nice to see you around again. :D
JamesD
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#9 Re: N43 pentode amp design

Post by JamesD »

Hi Chaps,

I cannot tell you how nice it is to be able to post again! When I was feeling my worst I couldn't wait to be well enough to think about audio and to post here again. It became the goal of my mental recovery from being ill and the welcome here is really uplifting - thank you.

James
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#10 Re: N43 pentode amp design

Post by JamesD »

Hi Mark,

Yeah the N43 is cathode driven - I like the sound of cathode driven valves but most of them have been class A2 so a bit different from this one. Really looking forward to how this sounds as its in pentode mode and cathode driven :-)

ciao

James
JamesD
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#11 Re: N43 pentode amp design

Post by JamesD »

Hi Simon,

That's good data too :-) I'll definitely try them this time.

No worries on the reply - after all mine took a year or so!

ciao

James
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Ray P
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#12 Re: N43 pentode amp design

Post by Ray P »

Hello James.

I've bought a few mains transformers from Toroidy and they've been excellent and the prices are good too. They do three levels of mains transformers, standard, audio and supreme, which is audio grade potted in a nice polished stainless steel 'can'. All of mine have been audio grade. I've not tried their OPTs as I tend to build amps without them.
Sorry, I couldn't resist!
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Dave the bass
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#13 Re: N43 pentode amp design

Post by Dave the bass »

Blimey, a blast from the past, welcome back JD!
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steve s
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#14 Re: N43 pentode amp design

Post by steve s »

Dave the bass wrote: Sat May 09, 2020 7:32 pm Blimey, a blast from the past, welcome back JD!
I'll second that. Great to see you back James.
I have a large box of 1930s output pentodes I'm watching with a vested interest how you get on.
Good project..
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
JamesD
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#15 Re: N43 pentode amp design

Post by JamesD »

Hi Ray,

Thanks for the confirmation on Toroidy! It seems a real thumbs up for mains transformers and the company so I'll try out the OPT and chokes for the full set.

Hi Steve,

Lots of treasure amongst those 30s valves - I was really surprised by the N43 curves and how optimised they were for audio work :-)

Hi DTB,

Played anymore Hawkwind recently? I love Masters of the Universe and learned the bass lines after your demo!

ciao

J.
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