JTM 45 Build

[MikeT]

 just finished a JTM 45 kit from Mojo and there are issues. The b+ is near 500VDC at all b+ points with just the GZ34 in. Granted, the pre and power tubes draw current, but isn't this a bit high? When I put the pre's and power tubes in I got smoke from the middle of the board. I had them in before and got no smoke but very high b+ readings in the preamp. like 300 +. But this time when I got smoke I immediately shut it down and checked continuity fro the board to the tubes and the board to the pots. All were fine. I just don't know what to do next....Any ideas? Thanks guys....

[sluckey]

It's normal to have high B+ with no tubes in. Find out exactly which component smoked. Wiring error or wrong component values are the most likely suspects. Post some hi rez pics as well as your layout diagram.

[tubenit]

You can check for polarity on the cathode caps and see if something is amiss. If the cathode caps are reversed in polarity, they'll probably blow.  Check resistance and continuity on all the resistors in that area also.

With respect, Tubenit

[Ed_Chambley]

Here is a link where a group of guys built a Mojo JTM 45 step by step.

http://www.harmonycentral.com/t5/Bass-Forum/Jtm-45-build-instructions/td-p/21862924

It is really a good outline and should be able to help you with your problems.

[TubeGeek]

I am about 70% done on the same mojo jtm45 kit.  We could compare notes/voltages when I complete mine.

As for troubleshooting your build, I'd trace over every wire and connection and verify you have it wired correct.  It's tedious sometimes but also necessary.  Post a pic, we can have a look and let you know if we see anything obvious.

[dude]

TubeGeek is right, recheck everything. Most likely it's a misplaced wire. 

Start from the layout and trace every component one by one as if you were starting the built over. Check the wiring under the bd., make sure you have the wires from the B+ rail to the preamp plate resistors, use continuity setting on your meter.  You can use use a magic marker and place a dot over every re-checked wire and connection. This will help you find the problem, I know it's tedious work but it's easy to miss a mistake and marking the bd. at every connection that's rechecked will help tremendously, if the layout is proven.

Go slow, and look for a cap in the wrong way as tubenit and sluckey suggested, that would cause smoke. If you find one throw it away and use a new one even if it "looks" ok.  You can use a non-permanent marker.

al

[Tone Junkie]

TubeGeek is right, recheck everything. Most likely it's a misplaced wire. 

Start from the layout and trace every component one by one as if you were starting the built over. Check the wiring under the bd., make sure you have the wires from the B+ rail to the preamp plate resistors, use continuity setting on your meter.  You can use use a magic marker and place a dot over every re-checked wire and connection. This will help you find the problem, I know it's tedious work but it's easy to miss a mistake and marking the bd. at every connection that's rechecked will help tremendously, if the layout is proven.

Go slow, and look for a cap in the wrong way as tubenit and sluckey suggested, that would cause smoke. If you find one throw it away and use a new one even if it "looks" ok.  You can use a non-permanent marker.

al

This technic dude posted here works real well, I know it seems time consuming but it really helps in showing where your at. Even after a bunch of amps I still use this, I use a highlighter pen going down every wire and part. Know when Im done doing this the last 3 amps were good to go from the start.
Also build the light bulb limiter this will help in start up. I built one after I smoked my first power transformer a waste of $75.00 to force me to build a $10.00 tool.
Bill