My Alembic F-2B preamp bit the dust a couple months ago (loud hum, low output), and I finally got around to looking at it the other night. I took some preamp p0rn pics along the way to share with anyone who's interested in this stuff. This is my first attempt at repairing any tube-based gear, and so I took some time to read up about it (hence the not getting to it for two months) and try to approach it safely.
This is a two channel preamp inspired by Fender's Dual Showman. At the heart of each channel is a 12AX7A/7025 tube, and I've been using it as it came to me for the past couple years - with a newer Sovtek in channel A, and an Amperex in channel B.
Its my only pre, and I use it for home recording with bass, guitar, and keyboards. With the latter, I usually run both channels in stereo. The different tubes that came in it gave a slightly different character to each channel. That was cool in a way, but for stereo use I decided a matched set might be better. I found a nice old pair of GE tubes on ebay for not much, and decided to give those a spin:
In an effort to avoid barbecuetion, I made a little contraption to discharge the power supply filter caps. Figure I'll get some reuse out of this, since I also want to go thru and recap the tube amp in my old Hammond at some point in the future. I just took an old alligator clip test lead and soldered a resistor inline (I used a 10W, 150 ohm). I clipped one end to ground, and held the other against the + lead of each cap one-by-one ~30 sec. each, and double-checked 'em with a voltmeter afterward to make sure they discharged:
I guess you can also do this with a screwdriver shorting across the leads, but I don't know how bad the sparks would be.
Didn't want to scar up the circuit board.
I found that the failure on this one was due to a broken solder joint on one of the power supply filter caps at the circuit board. The strips of foam glued to the covers that kinda holds these heavy caps in place and keeps them from getting jostled around had disintegrated, and over time the connection probably just gave way:
I fixed her up and threw some new foam in. I also converted it to a grounded 3-prong plug, and cleaned up the AC wiring a bit.
Fire baaaad!
Heat shrink goooood!:
New (to me) toobs:
I wanted to replace those power supply caps while I was in there, just because they go bad over time and if they're the originals then they are wayy overdue for replacement. The only similar spec replacements I was able to find were ~$20/ea from Allied! Decided to wait and look around a bit.
Well it sounds great now, I didn't fry myself, and I got a small glimpse at how deep the rabbit hole goes when it comes to selecting preamp tubes!
Never really looked into any of that before... just used 'em as they came.
