Friday, April 08, 2011

Judybox Revival amp

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.


At 9:10 PM, Anonymous Anonymous said...

Hi, I just bought a Revival that has everything but the circuit board. I'm looking into putting a Ceriatone OTS board in it, and see if I can wire it up! I doubt I'll change the resistors, I prefer changing fuses to resistors. I do have the good sockets so those will go in. I also have a Tweed Deluxe type Xfmr from Weber that has a higher B+ for the 6L6. Of course then I might have to change out the 40 watt OT! I could run it with 6V6 and it would be fine. I just would rather have a higher B+ for 6L6.

Anyways, thanks for all the information!!


At 5:43 AM, Blogger Robert Luedeman, attorney at law said...

That must have been the one on Teddy Weber's for sale board. I've been posting more on
take a look you may enjoy it.

At 5:34 PM, Anonymous Anonymous said...

I can't seem to comment on your judybox blog, but if there is a way for me to get one of those boards.....It is all I am missing.


At 8:20 PM, Blogger Robert Luedeman, attorney at law said...

If you are the same CharlyG who posts on TGP, PM me there. I use the monicker Prairie Dawg. In fact you may have a PM from me. I can sure fix you up with a board. Or, you can email me direct at rwluedemanATmchsiDOTcom

At 12:01 PM, Blogger Duane Pfeiffer said...

Robert, I just won #7 on ebay. It is a revival. I'm hoping you have a schematic or know where to find one? Thanks for all the good info.


At 12:27 PM, Blogger Robert Luedeman semi retired attorney and amp mechanic said...

I've never seen a schematic for these. However, they are pretty simple stuff, no fancy stuff like channel switching or anything.

At 3:26 PM, Blogger Robert Luedeman semi retired attorney and amp mechanic said...

I was just thinking. If it doesn't work at all, you ought to be able to figure it out pretty easily by mapping the voltages. It is a pretty low voltage production, I only got about 360v plate voltages out of mine. I do have half a dozen more or less complete circuit boards should you need one. I've also got a spare power transformer. In order of probability I would check the speaker, and then start checking voltages and looking for open connections and the like. Remember it's cathode biased and very simple.

At 6:42 PM, Anonymous Anonymous said...

I see another on ebay without circuit board. BTW what's wrong with ceramic sockets?

At 6:00 AM, Blogger Robert Luedeman semi retired attorney and amp mechanic said...

I'm just the person you want to talk to then because I have 3 or 4 Judybox Revival circuit boards all assembled for cheap. I got them from a guy who went to the Judybox garage sale when they closed down so if you score the amp give me a holler. My issue with ceramic octal sockets is that they're very hard and inflexible and it is easy to break the guide pins on your tubes when removing them. I try and use Beltons when I can, and of course I use Amphenol military surplus on my own amps.


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