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12AU7 power amp

8/13/2004 8:46 AM
12AU7 power amp
I am planning on using my small stock of 12AU7 taking the dust at the back of my cupboard. I'd like to make a smaller powered version of a Champ using a 12AU7 for the power section. I would be running the triode sections in parallel.  
It seems that I would need an OT with a primary impedance between 5 and 7k. Is this correct?  
Would a standard 220V to 12V/9V/6V wallwart adapter work for this? The impedance ratio seems adequate. I know OT and PT are not designed similarly (and that I could use a reverb driving transformer) but I am not after high fidelity here, I just want to use the parts from my junk box. Any feedback would be much appreciated.  
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8/14/2004 1:15 AM

Yes, you can use it. It also gives you some variety of impedance matching with the 3 different secondary outputs.  
8/15/2004 6:51 AM

Have you checked out Doug Hammond's "Firefly"?  
Several people have built this using 70-volt line matching transformers from Radio Shack for the OT. The catalogue number on one that I have is 32-1031B. The item is discontinued; they were blowing them out for $2.99 a few months ago. You may be able to find them in a few stores.
8/15/2004 9:04 AM
Chris - CMW amps

Hi all  
A customer from me did build a FireFly ver.3 using Hammond transformers and some very small tweaks and it's a pretty cool sounding amp.  
Using the same cathodeR on the 12AU7 but with a cathodebypasscap and a pre/pi like a Moonlight you can make it somewhat louder with slightly more headroom. The Moonlight schematic is also in the Ampage schematics listing.  
Have fun!  
8/15/2004 1:15 PM
I built a high octane preamp with a switchable 12au7 section in PP. I used a hammond 125B and the impedence was 21.5K. I have a schematic and board layout in pdf.
8/16/2004 7:13 AM

Thanks to you all for the input.  
I'd like to build it as cheaply as possible, just for laughs, so I'll go the wall-wart adapter route for the OT. Also, I live in France so there is no chance of scoring one of those Radio Shack transformers. Parts supply is pretty scarce in France for DIYers unfortunately...  
I'll let you know how it turns out.  
8/18/2004 12:45 AM

I just built a 18 watt lite front end onto a 12AU7 output using the RS 70 volt line transformer as an output transformer. I wanted to try the parallel triode into a PI into the 12au7 to see how it worked. I built it with salvaged parts from old tube gear. The inspiration came from Adam Alpern's Lunchbox amp which uses a PI driving the 12au7 rather than self-split. Here's the link to his site-  
I will be tweaking it to get the crunch tone just right it is a little buzzy at the moment.  
You may be able to get a similar 70volt line transformer that will work. Good luck it's a worthwhile project.

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