JCM 600 wackiness...??

[acorkos]

Amp has new tubes. Play tested fine for a few minutes. Came back....none of the tubes are lighting up. I found some burned traces near C111 main power supply cap, and heat discoloration under rectifier bridge DB1. Cleaned up the traces, replaced DB1 and C111 + C112. Now the heaters of the preamp tubes light up only when the power tubes are not installed. Installing the power tubes...none of the power or preamp tubes light up. Swapped in different tubes....no difference. Where to look?



http://www.drtube.com/schematics/mar...jcm6-61-02.pdf

[The Dude]

The output tube filaments run directly off of the filament winding of the PT (AC) and the preamp tubes run off of DC rectified from the same winding. When you insert the power tubes is the AC filament voltage dropping? Maybe you have a shorted tube, although I would think F101 would blow if that were the case? I would hook my meter to W114 and W117 and see what happens to the AC voltage when you insert the output tubes. Also check that voltage directly on the rectifier to make sure it's getting there. Could be a solder/connection issue.

[acorkos]

4.4 vac at fuse f101 with no tubes installed. all tubes installed, 1.1 vac at fuse f101. preamp tube v1 has .65 vdc at pin 9 and pin 4/5

[acorkos]

I would hook my meter to W114 and W117 and see what happens to the AC voltage when you insert the output tubes.

7.3 vac across w114 and w117 w/o output tubes.
7.2 vac with output tubes

[The Dude]

Can you get to the pins of the rectifier (AC input) and make the same measurements? It should be the same electrical point as W114 and W117, but I'm wondering if there is a connection problem between or the fuse is not good and somehow adding resistance. You might also try installing the output tubes one at a time and see if it's only one of them causing the problem.