Fri Jan 23, 2015 6:45 am
Yes, it is wise to raise your temps a bit for a diacetyl rest to help clean up any buttery flavors that may be present before racking off the yeast cake into your keg for lagering. I usually like to start slowly raising the temps a couple degrees per day after the krausen starts falling back into the beer to keep the yeast active and happy. With a lager, I will usually get those temps up to the low to mid 60's (F) for about 3-4 days for a proper diacetyl rest before racking to a keg for long term lagering.
As for building up the yeast for a dopplebock you will definitely have enough to brew one with this yeast cake from your pilsner in your primary fermenter once you rack the beer off. Your best bet would be to brew your dopplebock the next day after you rack your pilsner into your keg as the yeast will be most viable and ready to ferment a strong lager. If you plan on storing the slurry, then 2-3 days in your fridge is about the time span you should try to repitch it into your dopplebock. Be sure to check Mr. Malty's website for slurry repitching calculations so you are saving enough yeast for your big lager.
If you don't plan on using the saved slurry within 2-3 days, then you will need to make a starter (or even a stepped up one) to get your yeast ready to ferment the dopplebock. I would try to use up the slurry within 2 wks if it is going into a starter, but have been successful storing it for longer (6-8 wks), although most definitely fresh is better. Your best bet is really to repitch that saved yeast slurry the day after kegging the pilsner IME.
"A bad man is a good man's job, while a good man is a bad man's teacher."