Marshall Superlead Rectifier

[dpm309]

This 1974 100 watt Marshall Superlead has blown 2 power transformers after running for about 3 hours. The owner sent me this gutshot and my question is should this amp have a bridge rectifier? Instead of the bridge rectifier, there are 4 diodes (although they look like voltage regulators to me) that appear to be misaligned. The schematic I have shows a bridge rectifier.  This amp has undergone 2 Torres mods - line out and midboost. Don't know if the tech did anything to the rectifier in the power supply. Would this be a factor in the OT blowing? Also, if the diodes were misaligned, would the amp even work.

Thanks,

[92Volts]

I don't have a definite answer... but recent models use a full-wave (not bridge) rectifier which can be as few as 2 diodes: http://el34world.com/charts/Schematics/files/marshall/Marshall_plexi_superlead_reissue_50w_1987X.pdf

But in that example 2 are used in series for each "diode" to increase the effective voltage tolerance.

A schematic published in 1970 shows a bridge rectifier: http://el34world.com/charts/Schematics/files/marshall/Marshall_jmp_superlead_100w_1959.pdf

It's hard to tell from that picture but it looks like to the modern setup (2 diodes in series).

Blowing power transformers after 3 hours doesn't suggest a backwards/shorted rectifier. That would explode a diode, fuse, or transformer immediately.

Maybe if one diode is bad (open circuit) it's loading only one leg of the transformer. My second guess is it's just biased wrong and the tubes are drawing way too much current (if the transformer didn't die the tubes wouldn't last long, if that's true)

[2deaf]

That's a bridge rectifier.  Is the PT blowing or the OT?

[dpm309]

Thanks for the replies. I don't think it blew the OT since it ran for about 3 hours after the 1st PT blew. In the process of getting more information as to what the previous tech did.

[92Volts]

That's a bridge rectifier.  Is the PT blowing or the OT?

I took another look and agree. There are white diode outlines/markings on the PCB, they're visible at the end of the diode and look like a cathode stripe. Top-right diode actually seems to be missing its stripe but isn't (visibly) backwards either.

[2deaf]

Marshall made boards for years that could be wired for either bridge or conventional rectification.  There are some variations on the theme, but the basic idea is the same.  Once you are hip to what they were doing it makes it easy to determine which rectifier they are using with just a glance.  Also, bridge rectifiers tended to be used with 100W and conventional rectifiers with 50W.

I think the top-right diode is mounted with the stripe against the board so we can't see it.