N43 pentode amp design

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Dave the bass
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#16 Re: N43 pentode amp design

Post by Dave the bass »

JamesD wrote: Sat May 09, 2020 9:01 pm Hi DTB,

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

ciao

J.
Not 'arf. Still a firm fave here.

Aww, thats nice you learned a bass part, I'm pleased. I had to search in the A-T memory banks for that vid, I'd forgotten about that.
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Greg
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#17 Re: N43 pentode amp design

Post by Greg »

Great to see you back, James. There was so much missing without you here. Really looking forward to your future contributions which I’m sure will be fun.
Max N
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#18 Re: N43 pentode amp design

Post by Max N »

Welcome back James, very glad you are feeling better. Looks like an interesting project.
Will also be interested to hear what you think of the toroidy OPTs
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#19 Re: N43 pentode amp design

Post by JamesD »

Thanks Chaps :-) I'll try my best to keep it interesting...

I have a RIAA preamp design using LRs only for the eq section and solid state to talk about and my new speaker design to run through too as I'm redoing the complete system except the DAC. I bought a Khardas Toneboard a couple of months ago and I'm enjoying that as its a step up from my old EMU 0202 that I've used for donkeys years....

I did a little work on the design early today - starting with the Output stage and re-assessing the operating point. I had looked at 250V and 40mA for 10Watts dissapation and 5K load that yields about 2.5W at pushing 4% distortion mainly 2nd harmonic but still to high. To get it down with a 5K load requires 50mA and thats over the max Pa so I need to back it off a bit. Having had a play with the graph it looks like 200V and 50mA would work for 8W dissaption and a 3.6K load for 2Watts at under 2% distortion but dropping below 1% at !.25 Watts and with the distortion rapidly dropping off below that. I try both when its built but expect this op point to work best. Grid would be at -3.5V so cathode choke dcr needs to be 70R (ignoring the CF current for now) - so thats the OPT stage done.

The cathode follower is dc coupled to the cathode of the N43 so it needs to bias at the same -3.5 V this could be a pentode or a triode but it needs to have decent Gm and to provide a decent amount of current to drive the N43 - as luck would have it I quite quickly found that the EF184 in triode mode biases nicely at -3.5 volts and 9mA with the B+ at 240V and I can arrange that from the psu - although if I find something better I'll change this. d.c. current through the choke is now 59mA so required dcr drops to 60R...

Must update the drawing!

James
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#20 Re: N43 pentode amp design

Post by JamesD »

Taken the plunge and ordered the Toroidy Iron :-)

Better order some psu caps from AskJanFirst next...

J
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#21 Re: N43 pentode amp design

Post by Scottmoose »

Missed this before, but excellent -welcome back, James. :)
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#22 Re: N43 pentode amp design

Post by JamesD »

Hi Scott,,

Good to hear from you too!

I've been thinking about you as I've been working through a whole bunch of possible speaker designs investigating vented designs with relatively low group delay and comparing them with sealed boxes.O f course the answer as what meets the brief best is heavily dependent on the drivers but I settled on the SB Acoustics SB29NRX75-6 or SB34NRX75-6 as suitable drivers so sealed boxes won out in my thoughts - now I have just to buy and build something but at the moment I'm using a Ruark Log Rythum sub woofer and have just ordered a pair of Sonido 145 s for satellites with it but the plan is to put then with the SB Acoustics bass drivers before deciding on refurbishing the AERs or not for the Quasars....

ciao

James
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#23 Re: N43 pentode amp design

Post by Scottmoose »

Likewise James, sorry it's taken a while to get back.

The SBs are good units; sounds like a plan to me anyway. :)
'"That'll do," comes the cry of the perfectionist down the ages.' (James May The Reassembler)
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#24 Re: N43 pentode amp design

Post by JamesD »

Just got a delivery from AskJanFirst of some bits for the amp:

Top Cap (Grid) and Old British B7 base - picture of N43 with a 6AQ7 alongside for scale
N43 versus 6005.jpg
N43 versus 6005.jpg (12.77 KiB) Viewed 5784 times
And some 70s AEG PP in Oil 83uF caps with the same 6AQ7 for scale
AEG PP in Oil 83uF cap.jpg
AEG PP in Oil 83uF cap.jpg (10.81 KiB) Viewed 5784 times
Just waiting on the custom transformers that Toroidy are shipping today...

James
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#25 Re: N43 pentode amp design

Post by IslandPink »

Oooh, the N43 is a whopper !
Parcels are always nice.
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#26 Re: N43 pentode amp design

Post by JamesD »

It is a whopper with a nice big square anode.

Its interesting that the 6AQ7 is more powerful at Pa=12W than the N43 Pa=10W although the N43 has twice the Gm...
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#27 Re: N43 pentode amp design

Post by JamesD »

To say things are going slowly is an understatement...

But here is a first pass layout on the wooden chassis...
N43 amp layout - resize 1.jpg
Lots of Toroidy iron :-)

Changed the psu caps for a big old delta cap - pure PP - no oil, three 165uF sections used as two 165uF caps with one common terminal

Wonder when I'll get the wiring done by....

James
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#28 Re: N43 pentode amp design

Post by IslandPink »

It's important that the parts are moving closer together. Entropy is being reduced.
"Once you find out ... the Circumstances ; then you can go out"
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#29 Re: N43 pentode amp design

Post by JamesD »

agreed - one has to optimise the rolling resistance...
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