Metropoulos Forum

I just got done with my Metro JMP 50 watt build. It has components from ValveStrom, transformers from Marstran (JTM 1202-118 PT, JMP 784-139 OT and a C1999 choke). I began testing yesterday. When the Power and Standby switches are off, and the amp is plugged in, I'm getting 119v-120v at the power switch. When I throw the Power switch on, the light comes on, I hear a slight hum and then the Mains fuse blows. I'm using a 3a slow blow fuse.

Any help is appreciated. At this point I'm at a standstill and would love to get the testing done and fire this baby up!

Thanks,

Steve Wagg


Here's the hyperlink to Photo Bucket:

http://s690.photobucket.com/albums/vv27 ... 50%20WATT/" onclick="window.open(this.href);return false;

Well, nothing jumps right out at me looking at the pics, except the grounding of the presence cap to the bass pot body? Don't think that would have you blowing fuses though.

The power tubes should be out until everything looks right.

I would try to troubleshoot by isolating things:
Disconnect bias wire (white?) from the rectifier.
Disconnect the two rectifier wires at the PT.

Assuming fuse DO NOT blow:
Confirm heater vac at the tube sockets.
Confirm the PT is putting out the expected vac at the points where the rectifier was connected.

Assuming fuse DO blow:
Perhaps the PT lugs are not what you think (the heaters have ARE right though!)
Disconnect PT connections from ground and ensure that the voltages are what you think they are at each PT lug.

Once you have confirmed the PT voltages, identified the issue, and regrounded the correct PT wires; reconnect the two rectifier wires to the PT (check voltages), and finally the bias wire to the rectifier (check voltage).

I'll give it a try. Thanks!

PROBLEM SOLVED. It turned out to be a bad bridge rectifier...

It seems to be the same problem as with the originals, which use this black bridge rectifier block: too low PIV (regular bridgerectifier, but only two diodes in use for 2- Way- rectification)
It's a very well known problem

In this case, the "traditional" uncorrect placement of the HT- fuse (after the choke!) can fry the Mains- transformer.
A way better solution is to use two 500mA SloBlo fuses between the HT- taps and the DPST standby- switch.
Or at least two additional Diodes (e.g. two 1N4007) in series with the bridge- rectifier!

Regards

Robert