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Alembic F-2B Rehab

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chromium:
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. :o  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.

Aussie Mark:
Nice work.  Hopefully my F-1X won't require an overhaul for a few years yet.  Those Alembic preamps sure sound nice, don't they?

Highlander:
Nice work, Joe... It's horribly deep and dark in there - still gotta go into my Marshall 50 prior to re-capping the Hiwatt...

What's the Hammond...?

chromium:
Thanks guys!

Yeah Mark, it is an awesome pre- I can't get a bad sound out of it with bass.  Even those ultra clear-and-present sounds can be neat for some stuff.  I was testing it out my old DS'd Guild, and man if that isn't a match made in heaven.


--- Quote from: Kenny Five-O on October 08, 2009, 03:07:53 AM ---It's horribly deep and dark in there

--- End quote ---

Yeah the tube thing is insane.  New, old, grey, ribbed, long, short, halos, horseshoes...    it's all a blur.  I get the impression that it probably just makes best sense to try different ones out and find what suits ya.  I do like the way it sounds so far with these GEs, but then again it sounded good to me before too!


--- Quote from: Kenny Five-O on October 08, 2009, 03:07:53 AM ---What's the Hammond...?

--- End quote ---

The Hammond is an old '56-ish M-3 organ (think early Stax stuff).  It has (IIRC specs correctly) a little 10-ish watt class A amp with 12" speaker built into it.  I want to mainly ensure that its not a fire hazard (its not fused - so I'll add one.. or a breaker), convert to 3-prong, re-cap, and also make a couple mods:  I want to install a 1/4" output for recording/processing it.  I got a 10w 8 ohm resistor to use as a dummy load for the power stage - thinking about using a switching jack to engage that resistor and switch off the speaker when you plug something into the line out.  

I also want to add an input to the amp (switched as well, to bypass the organ), and a separate power switch to fire just the amp up without powering all the organ-related-stuff- tonewheels/scanner/etc...  I thought it might be fun to have it serve double-duty as a little tube combo guitar amp for mic'ing/recording, since I don't currently have a decent guitar amp.  Figure it shouldn't be too big a deal to do, since I'm in there anyway...

Hornisse:
Nice work!  I shipped an old F-2B that my friend in Germany, Harald Rost, had purchased on Ebay several years back.  They are nice units.  I may get one like Mark's (F-1X) in the near future. 

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