Friday, November 25, 2011

Son of Judybox Mark II

I completed the second Son of Judybox, a/k/a Vamp tonight and at this point it is an unqualified success and ready for prime time.

If you've been following this saga you know that I was able to find a way to reuse the original Chinese made Ningbo Chaobo power and output transformers that the Champion 600 is equipped with to make this project a lot more affordable-at least $68.00 or so, depending on where you shop.

The problem was that the original power transformer had no 5v rectifier tube winding and the secondaries have no center tap. This was solved with a modified Graetz bridge and a 6X5 rectifier tube as indicated in the Yes You Can posting a few weeks ago, and a good solid 330v of DC and 6.4 filament volts was obtained with everything up and running. You could probably use a 6X4 as many Silvertones did, but it is a lot easier to wire up an octal socket-there's more room to work-and you have an octal socket sized hole ready made for the purpose already where you want it.

The schematics for this setup have been posted here, and the only change would be to add a pair of 100 ohm resistors, one on each leg of the filaments for noise reduction as the original power transformer filament winding is without a center tap either. I wired the filaments with two wires, where the original 5C1 uses the chassis for one side of the filament string.

Another useful thing to remember is to use a grounding jack for the input for noise reduction.

To summarize, then. The original speaker jack was removed and a fuse holder was installed after opening up the hole. Two holes were punched in the chassis to accommodate the two additional octal sockets. Care has to be taken to avoid interference between the 6V6 in the center and the speaker basket or magnet.

A piece of perf board did service for mounting the electrolytics and dropping resistors, and the rest of the components were wired point to point. The lead from the jacks to the preamp tube is very sensitive to interference, so I shielded it with about thirty turns of hookup wire wrapped around a pencil and slipped it over the lead and its capacitor.

At the same time I installed a 1 meg audio pot for the volume, a standard Fender pilot light assembly, and reused the original power switch, saving an extra three bucks there.

The last bit of the puzzle was getting the sound right. I first tried a Weber 6x9 inch ceramic speaker but it didn't get the job done.

Luckily as I was up in Minnesota hunting in a dead TV repair shop last week I uncovered a nice 1957 vintage 6" Rola radio speaker and that does the job just right.

Another good idea is to remove some of the aluminum tape shielding in the cabinet above where the fuse holder sits-otherwise you can get a few sparks if the amp is jounced pretty hard.

You can have tube rectification with all the sag and touch sensitivity that conveys and still retain the original iron which seems well up to the task. You save enough dough that you can go out and get yourself a big hefty output transformer and still have thirty bucks to buy some beers.

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