Sigh, not the three threads myth again. Three threads in the 1600s was a tax dodge - since the tax on strong and very strong beer was the same, you could dilute a barrel of very strong beer with two barrels of low-tax weak beer, and end up with three barrels of strong beer whilst paying much less tax. Porter didn't come along until a generation later, and was made simply with brown malt. They didn't have much in common other than being affordable beers.
@
Tobor_8thMan - it depends on what you mean by Newcastle Brown. Which means looking at some of the proper British beer historians :
Martyn Cornell :
http://zythophile.co.uk/2011/03/31/why-theres-no-such-beer-as-english-brown-ale/
Roger Protz :
https://www.morningadvertiser.co.uk/Article/2011/03/23/Heritage-means-little-to-the-marketing-men
The local paper :
https://www.chroniclelive.co.uk/news/history/newcastle-brown-ale-20-fun-12791435
So it was created in 1927 by a brewer with a background in porter, as a less bitter competitor to the likes of Bass. "Newcastle Brown was for many years a blend of two beers, a young ale and one that had been matured for several months." (Protz)
"mixing a low-gravity beer brewed at about 1030 OG (and sold separately for many years as Newcastle Amber Ale) with a matured, darker (from crystal malt and caramel) high-gravity beer to produce a blend with an abv of 4.7 per cent. The high-gravity beer gives fruity notes to the blend, and a final colour that is much the same as or only a little darker than many traditional English bitters...The sweeter, maltier characteristics are more forward than you’d find in a bitter/pale ale, and there’s less of the hop apparent than would be found even in a Burton: bitterness, I believe is 24 IBU." (Cornell)
Protz writing in 2011 says they stopped blending "some years ago" (anecdotally this seems to have happened by the time production moved out of Newcastle in 2005 and possibly before) and they stopped colouring it with caramel in 2015 (which usually means adding a touch of black malt to maintain the colour).
Dog in the
1920s was over 6% with OGs of 1060ish and apparent attenuations in the high 70s%, in the
1950s OGs were 1049-1053 with apparent attenuations touching 80% for ABVs in the 5.1-5.8% range and colour of 40-50EBC (ie 20-25 SRM). I suspect they were using more adjunct at that time. Now it's 4.7%.
So when you say you "want to brew a Newcastle Brown ale" - which one do you mean? The original blended ones at >6%, the blended ones of the 1990s heyday, the unblended ones with caramel of 2005-15 or the current unblended recipe coloured presumably by black malt?
Forget the recipe, the first thing to get right is the yeast. The S&N yeast is a very fruity northern English yeast of a kind that's not readily available from homebrew suppliers. Brewlab F40 allegedly came from them but that's not easy to get hold of. I'd use the most characterful British yeast you can find, maybe add a pinch of hefe yeast like Munich Classic to ramp up the esters, and ferment pretty warm, try 21°C for your first batch but be prepared to go higher.
I suspect they always used 10-15% invert #2, and I imagine that Heineken have added more adjuncts - rice, maize etc. Start with UK pale malt (ordinary pale, nothing expensive like Maris Otter, or cut Otter with some US 2-row or pilsner to dilute it) and that 10-15% invert, then I guess they're using a fairly dark crystal to replicate the old ale - but not much of it, mebbe 2%? Maybe 1-2% chocolate malt? And a pinch - 0.5%? of black for colour. 22 IBU of British hops at 60 minutes, then a modest amount of some British hops (something cheap like Phoenix or WGV) at 10-15 minutes.
There's no real substitute for ageing to get the right character if you want that old ale taste, as I say I suspect they're using a bit of dark crystal to get somewhere close, maybe a pinch of Special B.