I finished up the Judybox Revival amp and powered it up today. It got topped off with a nice set of vintage Dakaware chicken head knobs I'd been saving for a special occasion.
Here's what I did to it.
Replaced all the ceramic dropping resistors with a 3w metal film, a 2w metal film, and a 2w carbon comp.
Replaced the ceramic 5w screen and grid resistors with 2w Dale metal film and the grid resistors with 1/2 w carbon film from Rat Shack.
Replaced the pilot lamp assembly with a Fender item. This was broken
Replaced the power receptacle. This was broken as well.
Replaced the 40-22-10 e'lytics with 47-22-22 450v items
Replaced the three ceramic octal sockets with bakelite.
Installed 5 standoffs of the proper length to support the board level and not preloaded.
Revised the ground scheme to a star ground setup
Replaced the chassis screws with new stuff from the hardware store.
Installed a smooth plate Tele in the first position.
How's it sound? About the same-bright and sparkly with a lot of depth on the bottom end. Sounds a lot like an AB165 with the bass and treble dimed and the switch on bright.
The builder used Tocos (Tokyo Cosmos) pots-very good stuff-but they're all linear. I may change them out for audio taper, at least the volume controls. All the connects from the pots to the board are shielded wire. I didn't see a nfb resistor but maybe it's there somewhere. I'm thinking that the first two preamp tubes share a common filter capacitor, because if they did not there would be four and there are only three e-lytics. Both preamp tubes have small electrolytics on both cathodes.
I don't know whether I'm qualified as an expert-I'm in the middle of doing a schematic layout and a bill of materials and when I'm done I may be an expert in which case if you find this drop me a line.
I am in a position to make some recommendations in order of importance.
Ditch the power receptacle and pilot light for something good.
Install proper standoffs to support the board and level it out.
Dump all the ceramic resistors except the power tube cathode resistor. They look stupid.
Revise the grounding to a star grounding common-it was strictly from hunger.
Replace the octal sockets with bakelite, Beltons, or military Amphenols
If you've got any questions about this amp, drop me a line.
If u have a schematic or layout that would be awesome. I just picked up one of these amps and all is there except the board. Any help would be great !
ReplyDeleteHello Robert, thanks for posting the Judybox amp info and other interesting stuff.
ReplyDeleteSome time ago I purchased a stripped out Judybox Revival chassis i.e. no circuit board. I received an info sheet about the transformers but no schematic.
I am not interested in recreating the complete original amp, but I would like to get the original power amp wired up so I can try some other preamps with it.
From the photos and info on your blogs, it appears to be a cathode bias power amp with a 250 ohm cathode resistor and no bypass capacitor. Can you confirm this?
I can’t quite make out the color coding on the dropping resistors. Can you tell me the values you are using?
Thanks and if you would like a copy of the transformer information sheet let me know and I can scan it for you. My email is pullshocks at yahoo dot com.
Mark.