28 February 2012

Laney Gut Project. End Days


So, It's been a while since I posted but I've been away doing this and that and working and all that good stuff. I say good, but that's questionable... alright it was mostly good.

Things with the amp have moved on a bit, so I'm going to try and catch up over the next few days with some retrospective bloggage. Below is the Power Supply board in its testing stage. The Yellow wires are just temporary for a ground connection for what requires one.

What you can see on that board (going front to back) is a good old full-wave rectifier made of UF5408 diodes. The original bias circuit. The faux centre tap and reference voltage source for the AC heaters. Then finally the basic DC heater circuit at the back.

Initial testing showed that I wired the Illuminated DPDT switch backwards, so it would light up as soon as the chord was placed in the IEC socket. That got fixed right away. Schoolboy error.

The bias circuit is dishing out around -30v to -60v if I can remember correctly.
The DC heaters read 3.6vDC at the sockets of V1 & v2, which is also fine. This type of DC heater circuit has a reduced voltage and higher power strain on the wind used to power it, but this power transformer can cope with 2x 12AX7 filaments. The reference for the AC elevation ended up being about 40v, which is awesome.

Elevated AC heat reduces the chances of getting filament hum in the preamp since the AC voltage won't really swing 'negative' in a way that will interfere with the normal operation of the valve. It also prolongs the life of Cathode Followers by reducing the possibility of arcing. That's a pretty basic way to sum it up, so I really suggest researching those ideas a bit if you want to know more.

I used some pretty decent Rifa and Rubycon caps for the main filters. They are pretty large values and super low ESR. They are actually 'snap in' types, meaning they have small but robust pins on the bottom, rather than solder tags. Well... the pins on these ones are robust!

You can also see the INGO soldano style transformers in that photo, and the tone stack module from a previous post poking out the new front panel made by JPF Amplification (Who incidentally make some great amps with a vintage vibe).