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Cheap first project


 
1/20/2006 10:21 PM
john
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Cheap first project
I have been thinking about building a small tube amp for a while, but champ style amps are still pretty expensive to build. I came across a schematic for an old magnatone amp that doesn't require a power transformer (one of the more expensive pieces to buy). It uses a 12ax7 preamp, 50l6 output, and a 35z5 rectifier. All tubes are still available. The whole circuit uses 9 resistors and a handful of caps. I'm sure its not going to sound great, but it looks like fun. I don't see any info anywhere about any diy builds on this type of amp - has anyone out there built one of these transformerless amps? if so - any suggestions on the best one to build? (I just stumbled on this magnatone, I'm sure there are more like this). Just want to build this for fun, any input would be greatly appreciated. thanks
 
1/21/2006 4:10 AM
Chris/CMW amps
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Hi John  
 
Don't do that, very unsafe design. If something goes wrong you "feel" the mainsvoltage right away.  
Try to purchase an old small SE amp that does have an output- and a powertransformer.  
 
Hope this helps,  
 
Chris
 
1/21/2006 11:06 AM
ukiahb
I have built one and it sounded OK, but as was said an isolation xfmr is a must for safety....that is about $12 and for $15 you can get a real power xfmr w/ 230V and 6V secondaries from Weber that will power a Champ, it is a WRVVBT IIRC, and is actually intended for a standalone reverb unit.  
Another low cost option is to use seperate 230V and 12V xfmrs, you can get a set for about $25-30, this makes it possible to use cheap NOS octals like 12V6, 12SL7, 12SJ7, etc....
 
1/22/2006 11:33 PM
john
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saw the feedback - saved me some wasted time - and quite possibly my life (seemed to good to be true) - thanks, bro
 
1/21/2006 8:11 AM
DirtyGeorge
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John,  
 
Chris is right. These design are not safe. Most ampagers that work with this type of amp use an isolation transformer for safety. So there goes your cost savings.  
 
Stick with a design that uses a PT. They don't add that much to the cost of a build. It might also be cheaper to go with a push-pull amp since SE OTs are usually more expensive than a P-P of the same power rating.  
 
A 12ax7 pre, like the black face on this site, into a self-split el84 output would make a nice low gain amp. The self-split eliminates the PI so you save 1 tube and some resistors.  
 
DG
 
1/21/2006 6:44 PM
Andy B.
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The ultimate in cheap:  
 
6.3v-0-6.3v 4amp main transformer- $8, one side wired to heaters and the other sent to the B+ transformer.  
 
120vac to 6vdc 1500mA wallwart- $3, took it apart wired it backwards to half of the main transformer supply and then used a voltage doubler for 240-250vdc.  
 
European 220 to 9v 600mA wallwart for the OT-$3.75, (5000:8) good for about 7watts maybe a little less.  
 
Basically used a the microchamp layout with a single 6aq5a power tube and a 12a*7, can use an el90 also these tubes are usually less than $5 on eBay.  
 
I breadboarded it and it sounded good...sorta.- speaker was not in a cab and was leary to push it or play with it too much, but it did sound as I expected.  
 
Fab your own chassis and it could be done for under $30 without a speaker.  
 
I like the idea of using that reverb tranny tho. $15 is hard to beat!  
 
-Andy
 
1/22/2006 11:56 PM
john
email

Thank's for the feedback- saved me a lot of wasted time, and probably my life. Seemed like a good idea at the time (I'm old enough to remember when sears sold this stuff (is that a real poncho, or a sears poncho - FZ). Spec'ing out doing a deluxe - new parts should cost around $200. Have an old bandmaster bottom staring at me with 2 original blue backs sitting idle. I dearly miss my old black face deluxe reverb - bought around the same time as my first car to impress this girl in high school. Car is probably rusted to shit, she's probably all haggered and worn - I like to think that old deluxe is singing as sweet as it ever did. Thank's for the advice, and ride safe - John
 
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