Monday, January 18, 2010
Tuesday, January 12, 2010
The tube preamp is working but it took some effort to get there. I bought some better quality replacement parts for it. MKP1837's to replace the yellow MKT's, two big-ass Elna Silmics that barely fit the space and a Burr Brown OPA2134. I replaced the trimpots with Piher trimpots and all resistors were upgraded to 1% metalfim types. The opamp socket was upgraded to a better one, I placed a small decoupling cap (a "104" 0.1uF ceramic) between pins 4 & 8 on the reverse PCB side. Ready-made sockets like this can be bought but they are expensive IMO. Putting the kit together was straightforward. No nasty surprises here. The manual and excellent PCB makes it very easy.
The tubes are very microphonic and must have a damper of some sorts. I chose short pieces of clear silicon tubing. Next the issue of what to do when it's time to replace the tubes arose. I wanted a tube socket for my tubes. Well 5-in-a-row tube sockets don't exist. Bummer! I discovered that the tube hole pitch matched a DIP socket. So off to the local shop and buy a large IC socket to butcher. I ended up with two 5-in-a-row sockets. The tubes fit and tube removal is easy.
Once the preamp was finished I took some measurements to verify it was assembled correctly. I found out that the outputs had a 2.43 volt DC offset. Not good. Long story short, I didn't measure it with a load attached and once I did that DC offset gradually dropped to nearly 0 millivolts. A quick check with some crap headphones and my iPod proved that it did indeed work. Cool!
So now it's time to find a nice enclosure for it and buy the additional items: inputs, outputs, switch etc. I also want to buy some spare tubes, unfortunately the webshop that has them is rebuilding their website after it crashed.