5E3 OT readings

[Chris Scott]

Hey all-

New build 5E3 problem here. Amp hums unacceptably with distortion at all volume levels, and hums even with both pres removed. After the usual checks, we find an almost 20v difference in pv readings (336/ 353), and a static test of output trans primaries (off ct) shows 682ohms and 319 ohms respectively, so off it goes back to MM for testing.

They say it's fine, but they're "sending me a new one anyway, just in case" (?!) It arrives, and sounds the same as the orig. unit. In the mean time, I subbed a generic 50wt 8ohm ot, and lo and behold, it's 95+% quieter hum-wise, sounds bitchen, and starts to clip where they all do, just like a regular deluxe (albeit with quite a bit more headroom, which was expected- the thing is as large as the pt!)

Are these readings WAY off normal like I suspect they are, or am I in error?


ps. it's NOT the output tubes, wiring, filters, power resistors, cathode resistor or cap, and as I said, hum was present w both preamp tubes removed as well.
TIA,
Chris

[tele_player]

Are you absolutely certain you have the correct color code for the MM OPT ?

Your results with a generic transformer suggest everything else in the amp is OK. So, it's defective OPT, or incorrectly wired OPT.

I notice that 682 is about 2x 319... like maybe you've got the wrong center tap?

I just checked on an unused MM FBFDR-O that I've had lying around for a few years - Deluxe Reverb OPT. Red is the center tap, red-blue is 130, red-brown is 115 ohms. Close enough for a guitar amp.

Assuming the primary is really a coil tapped somewhere near the center, take all three possible measurements. The highest should be the sum of the other two, and the highest measurement is the two ends, the other wire is the center tap.

[John Phillips]

+1 on what tele_player said - you almost certainly have the transformer connected wrong.

The center tap is the wire which reads 319 ohms (or close) to either of the others.

[Chris Scott]

My mom always used to say "...when all else fails, read the instructions"

What an idiot.

I've got the original data sheet, and it's right there!


What an idiot.


Thanks fellers.