Anyone have a solid working technique for poor man's preinfusion on the Gaggia Classic (no PID)?
Looking around the boards there doesn't seem to be a definitive guide. Maybe there are too many variables?

On my specific machine, I found the pressure of the water on my pucks causing 2mm holes and therefore (?) channeling. With the cooling flush (wait til light out, then back on) and a reheat of the water temp for 25secs to bring to just over brew temp (quirk of my machine I think as a 22 sec reheat brings me to ≈200F), opening the steam tap for about 5secs while pulling the shot led to those holes disappearing and a better tasting shot.

I'm still finding the perfect mottling elusive however, despite Mazzer grinder, even (read anal) dist and tamp and VST basket etc. I'm grinding 16.5g in my 18g basket so as not to cause channeling by the puck hitting the screen.

Opening the steam tap seems to reduce the temp of the water (confirmed with styro cup & thermometer) but I can't seem to get the balance between reheat time and steam tap open time for ideal brew combo.

Just wondering if anyone has mastered this technique at all?

Cheers.

I put a 600w dimmer switch between the brew switch and the pump. I can brew at a v low flow rate for a few secs then click the dimmer off (not engaging the 3 way valve) then wait for 10-15 secs then click dimmer on and ramp up flow rate to full.

This means control over flow rate, length of preinfusion and soak. This also allows you to dynamically compensate flow rate during pours that go wonky just by turning the dimmer.

This also means no temp loss and in fact allows the heater to heat up again during the soak period while the dimmer is clicked off.

I have found that by decoupling the pump from the 3-way valve I am able to activate the pump just before the valve and thus eliminate the "steam bump" that occurs when the the valve opens and the puck isn't yet wet and creating back pressure. I don't how much the steam bump effects taste, but with a fast temperature probe, it shows up as several seconds of excessive temperature in the coffee.

Not sure if this is similar but I flush quickly at low flow before the first shot and without portafilter to get the steam out.

IMO, flushing and allowing steam to be created just create additional variables that change temperatures. But they may be necessary if you don't have PID.

Computer control of the pump is very effective for preinfusion and for creating a slow pressure ramp up.

I'm liking the idea of the dimmer. Great idea and makes sense to me. I'm not particularly handy with such DIY things but in my pursuit of the god shot, I'm prepared to step out of my comfort zone!
Could you poss take a short video showing us the dimmer and how it's connected happycat? That would be very useful and much appreciated.
Having said that, I descaled my dispersion screen today and got a great shot. I think it was firing pin prick-jets at my puck. Also, my steam valve timing is getting pretty close to optimum pressure loss combined with bringing the temperature down to ideal temp (I over heat the water to compensate for heat loss while having the steam valve open). It was my first ever shot with no hint of sourness or bitterness, just creamy, chocolateyness with that lovely mottled crema.
I'm def getting there! The bottomless PF and WDT have brought my shots up a level...this is fun!

See this thread

Gaggia Classic dimmer switch pressure profiling

where I link to this site (not me)

http://www.samplett.com/node/21