How do I get a ph of 5.6 ?

[Nobby Novice]

When brewing my latest AG, I ordered some ph test strips to check the sparge and run off as my beer tastes okay but lacks head retention. I treat my water (Thames water) with half a campden tablet as I have been advised through this forum.
I was surprised to note that the sparge and the run off tested less than 4.6. I tested the wort after boiling, Still less than 4.6.
I have read the section on this in the Art of craft brewing and Graham Wheelers book (my bible) and I am uterly confused.
Can someone please tell me what I need to do to get to 5.6? What do I add ?
I was reasonably good at chemistry in 1969 but the years have not been kind to my head retention either.
Any simple advice would be grately received.
Thanks
Nobby

[Wolfy]

The list below is BrewLab's suggestion of the pH you should expect through the brewing process, and a pH of 5.6 hardly gets a mention.
As far as I was aware the most important pH is that of your mash, and you want that somewhere between 5.2 and 5.5, but you didn't say you measured the actual mash pH.
Comparing the numbers below to what you did measure, seems to indicate that you may not be too far off what is expected.

Part of brewing process Normal pH range
Raw liquor 6.8 - 8.0
Treated liquor 6.5 - 7.0
Mash 5.2 - 5.5
1st runnings 4.8 - 5.2
Last runnings 5.4 - 5.6
Wort pre boil 5.1 - 5.4
Wort post boil 4.9 - 5.3
Final product 3.9 - 4.2

[Horden Hillbilly]

As far as I was aware the most important pH is that of your mash, and you want that somewhere between 5.2 and 5.5.

Correct, it's the mash ph you should be checking. Also, if your beer tastes ok but lacks head retention, you could add some torrified wheat to your recipe.

[Nobby Novice]

Thanks guys. You are right, I didnt check the mash only the run off. I will check all on my next AG. and have some torrified wheat standing by.