Vintage Marshall Output transformer - wire color codes

[cap217]

I am trying to find answers on vintage marshall color codes for OT to make sure my amp/cab is matched.

I have a 72 100 watt that seems to be correct at 4 = black. 8 = yellow, 16 = green. Someone has resoldered the connections at some point but it appears to be right.

I have a 73 100 watt that seems off. On the inside it is wired just like the 72 but on the back, the selector switch numbers for 4 and 16 appear to be scratched off and swapped with new writing. So when you hook it up you get 4 = green, 8 = yellow, 16 = black. From everything I have found this is wrong but Im wondering if there is a reason someone would have done this? It was serviced by plexi palace years back and its hard to think this would be overlooked. Id like to be able to tell for sure.

I have a 74 50 watt that has the newer selector rotoray on the back and I assume its wired properly at 4 = yellow, 8 = green, 16 = grey. Is this correct and 100 w vs 50 w are different or is it the years that are different? Ive never seen a 74 100w wiring to know.

 

[william vogel]

Your 72 is correct and the 73 should be the same. The 50 watt is also correct and because it’s a Drake brand it’s different than the 100 watt Dagnall transformers

 

[2L man]

It is always best to measure OT. When it is not installed inputting mains voltage to the primary and measuring measuring the mains and all output voltages give very accurate winding ratios. Also to the CT there is half the mains voltage and tell that it is wired right. I use a scrab mains cable and tack solder it. If OT has wires I tape them to table and there won''t come shorts. This takes only few minutes.

When OT is installed spending few more minutes remowing rectifier tube and tack soldering pieces of wires from filament to anode, one on each tube side. Now OT input is usually less than 7VAC and outputs come few hundred mV but still this tell which output is what. If amp has a Standby Switch removing rectifier is not needed when it is kept Off but sometimes there is a resistor on Stby curcuit which "pre charge" electrolyts and then there come some some HV DC to OT CT but it does not screw winding ratio measuring, it just needs to be done carefully like most amp measurings.

 

[cap217]

I heard of people using an ohm reading off the taps. How? I tried this and I got readings that were so close to eachother for each one. And Im not sure how it should change.


For example on my 50w 74

set to 4 ohms
yellow = .4, green = .6, grey = .8

set to 8 ohms
yellow = .6, green =.5, grey = .7

set to 16 ohms
yellow = .9, green = .7, grey = .5


That seems promising but Im not convinced it means much.

 

[neikeel]

I suspect that the 73 was for someone not using the window selector just a banana type link. The window type is offset vs a simple u-shaped link.
50w is Orange common, yellow 4, green 8 and grey 16 (at least those are the original Drake colours)