Scrap Parts Preamp

[RicharD]

Lately I've been servicing an odd assortment of musical equipment.  Everything from guitar amps, to vintage drum machines, to stomp boxes, to Eventide Harmonizers.  I have a cheap power amp and speakers in the lab but no preamp.  It's been a thorn in my side more than once.  Today's project was to rectify this situation.  Here are the rules:

1.  Stereo preamp with a gain of approximately 10.
2.  Make it tube just because I'm stubborn that way.
3.  Build from as many used parts as possible.
4.  Buy absolutely nothing, parts on hand only.

I've had the box kicking around for years.  It's too flimsy to build anything real inside of it.  The power transformers are scrap MR16 halogen lamp transformers left over from some dimming system retrofit.  The combo input jacks are something I purchased by mistake.  It's a single stage 12AU7 circuit.  Yes it inverts but that doesn't matter since this is merely a bench preamp.  Almost all of the resistors & caps have been on my breadboard at some point.  ISOTone ran the drill press, I did the perf board.  Concept to completion < 6 hours including lunch.



R13 & D3 not installed yet.

[eleventeen]

Heh. I like the use of the track-light transformers. The kind of part you throw in the junkbox and never use, except for wondering what you should do with it/them 537 times.

[DummyLoad]

ISOTone ran the drill press

and the chop saw, and the dremel, and the square, and the calipers, and the ruler, and vice brake.... 

word to the wise - paper phenolic perf board is useless - well almost totally. luck had it there was a piece of g10 perf board laying around my lab.

[RicharD]

>I like the use of the track-light transformers.

That was one of the best parts of this project.  I've had a bucket of those things sitting around forever and a day.  I was very happy to use those mis-ordered input jacks and cheapy chassis too.  Really the only part I had reservations about using was the 12V regulator.  I probably coulda skated on the DC filaments or simply used a dropping resistor in lieu of the regulator but that's my "boiler plate" 12V supply so I did it w/o hesitation.

I guess I shoulda said ISO did the chassis work.  Definitely the more difficult part of the project.  Cutting that flimsy metal w/o tearing it is a challenge, especially unibitting the XLR holes.

[kagliostro]

Simply and usefull 

NICE

Kagliostro

[sluckey]

Your scraps sure look better than my scraps!

[FYL]

Ain't no scrap. Nice build.
 

[eleventeen]

I hear ya, man. I used to buy aluminum ice cube trays in thrift shops for a dime to make 1-2 tube project chassis. They worked great, except that they were shallow and it was tough to fit pots & knobs along the side surfaces. Had to be careful, my cruddy tools and inexperienced hands could easily dish them to the point where they couldn't be used.

[DummyLoad]

richard was kind enough to send me care package of parts with the transformers, input jacks & hardware and some of the electrolytics. it seems as thought we both had a bud brand 11 x 7 x 4 chassis lying around and damn those things are really flimsy - they really want be torn rather than machined.

imma knucklehaid so i didn't quite follow richard's plan...

i opted to use a different tube, (mainly since i had more of them than the 12AU7) and because i like to fiddle, + i'm cheap. i opted to use a plate follower with a current source. i added an output level control and RCA out jacks, that way i can drive hi-fi type amps that may come across the bench. the RCA jacks are disabled if a 1/4" phone jack is plugged in. instead of the FWB for the B+ i opted for more B+ and used a full-wave voltage doubler. the voltage regulator i used is the more common LM7812 since i didn't have any LT108x-12, but had a tube full of LM7812s not getting used. the LM7812 made the layout easier at the expense of higher drop-out voltage, but with 4.2A secondary and only a +/-600mA load, this is not a problem - we have 15.5VDC to the regulator input at full load. this circuit makes slightly more gain than buttery's design.

it sounds nice for a one-off without any tweaking. i did get tickled by C6 since the top of the can is unprotected and is at ~150VDC potential - forgot about the (-) connected to the can body - even with modern caps, some are not completely covered in plastic. 

schematic attached.