I had an idea for a gadget. You mount a socket on some kind of base, and you solder wires to the bottom, leading to pins which fit in the amp socket. You rig it up so one wire goes to your ammeter and then back to the amp, so you can measure the current while biasing. You put the tube in the new socket, and you plug everything in to the old socket and the ammeter.
It's 82K. I just checked the Mojo schematic. Of course, I have been known to put the wrong resistor in from time to time. And I'm not saying Mojo knows what it's doing. The schematic lists 47K for EL34s.
I could always jump another resistor in there to lower the resistance temporarily.
There are two parallel speaker jacks. Mojo has them hooked up as shown in the photo. There is a wire on the jack closest to the OT, going diagonally from one side to the other.
What's this for? If I leave it as shown, the other jack doesn't work. Does it do something when you use two speaker cables at once?
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The first is wired to short if nothing is plugged in. See how it breaks the connection to ground with a plug inserted?
umm.. hold on I can't see it while replying.. Ok, they think it's supposed to be wired like some other amps.. that is really silly, and forces you to (USE THIS JACK FIRST) use this jack first.
Here is how you do it. There is no series ground from the last jack on a JTM-45. I have never seen a novice complete a Mojo kit without raping someone else's schematic.
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Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Thanks for all that work. It seems like it works okay the way I have it wired now, which may be the way you do it. The amp isn't in front of me. I'm inclined to leave it alone unless flames shoot out of it.
Friend of mine is wringing the amp out in the garage. It seems like there is a lot of crispy high-frequency hum, which the presence control really brings out. It's more prominent on the bright channel. Wondering if anyone has any idea what the most likely cause is.
I stuck two big 10-ohm resistors between the PT and rectifier, as mentioned earlier. Is there any possibility these cause hum? I can yank them out. I just stuck them in there because of Merlin Blencowe's recommendation.
It turns out the tube was the problem. Apparently, JJ makes pretty bad 12AX7s. My friend brought a Chinese tube to replace it. Now the amp is amazing. Very little noise. Wonderful tone.
I screwed up one of the 1-ohm resistors, so we are putting off installing them until we get a new one.
More strange problems with the Mojo chassis. The caps extend half an inch below the bottom of the box. The cabinet is flat on the inside, so there is no place for the cap to go. How are you supposed to use this cabinet and this chassis together?
Our solution was to drill new mounting holes for the cap and move it up into the box. It fit perfectly, which makes you wonder why Mojo doesn't do it this way.
I had tried orienting the caps differently, but there was no orientation that made them fit in the chassis AND lined the clamps up with the existing holes.
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Glad you got 'er singing. A Metro JTM45 kit was my first build. It was sooo easy. Great instructions, great parts, great company. Also did a 100 watter. Too bad George stopped selling kits. My 45 is my quietest build to date.
Steve, are you saying this amp works and the door shuts now?
OMG ITSA MIRACLE! Congrats! Mojo takes all the fun out of not having lots of experience, and every tool ever made. They hate newbs with a hot passion. I know two guys with experience who still have 98% completed 18w amps that need to be built from someone else's good plan.
Mark likes them because he likes a puzzle that comes without a picture. lol
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
Oh well little things like that happen when building.