Output Transformer for an old Fender Amp - Page 1
Forum section: electronics / repair

author: Kilroyonguitar
author_rank: Newbie
timestamp: February 26, 2025, 05:24:44 pm
content: I bought this beautiful old 1963 Fender amp knowing it needed a lot of work. It has an output transformer in it but all the wires are cut and I'm not sure if it's the original crappy one just not taken out or it's a replacement that just wasn't soldered in before the previous owner passed. I took a bunch of ohm readings but to be honest I don't have a clue what they're really telling me. See below. Any advice appreciated.125A6A – 606-3-19 Output Transformer readingsPrimary SideRed Brown BlueRed N/A 316.3 ohm 81.7Brown 316.3 ohm N/A 0.39KohmBlue 81.7 ohm 0.39Kohm N/AFrame 3.2 m ohm 3.2 m ohm 3.2 m ohmSecondaryGreen BlackGreen N/A 0.0 ohmBlack 0.0 ohm N/AFrame 3.2 m ohm 3.2 m ohmPrimary to SecondarySecondary Green Secondary BlackPrimary Red 12.2 ohm 12.2 ohmPrimary Brown 320.1 ohm 320.0 ohmPrimary Blue 81.2 ohm 81.3 ohm

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 26, 2025, 07:42:18 pm
content: From those readings the transformer is toast.Not only is it shorted from Primary to Secondary, the two halves of the primary winding are very different to each other indicating shorted turns.You should be able to tell which transformer it is supposed to have by the model number and date.What is it?It is possible 3rd parties make one that will do the job. (TAD, Mercury Magnetics etc)

author: Kilroyonguitar
author_rank: Newbie
timestamp: February 27, 2025, 10:46:30 am
content: Thanks for the quick reply. There is a few different places to get a replacement. I wonder what happened to cause such a failure.

author: CaptDon
author_rank: Super Contributor
timestamp: February 27, 2025, 01:59:05 pm
content: I can almost guarantee the most probable cause was either a bad output tube or the tube bias was set wrong causing the tubes to draw far to much idle current. There is so much improper information out there on the internet about adjusting tube bias and which tubes can substitute for others. And there are the idiots who simply keep putting larger and larger fuses in place when the amp keeps blowing them after 5 minutes or so. I see this type of meltdown often. The transformers are available but expensive!!! The biggest idiots appear to own Marshalls because I have replaced Marshall transformers about 10X more often than any other make and in every case the amp had wrong tubes / bad tubes / improperly adjusted bias / wrong fuse. The stupidest 'bias advice' I saw on the internet was "Adjust the bias until the plates begin to show an orange glow the turn the bias back until only a dull red color shows". That is hands down the dumbest advice I have ever read for guitar amplifier bias adjustments!!!

author: Audiorepair
author_rank: Frequent Contributor
timestamp: February 27, 2025, 08:03:16 pm
content: Many things could cause an OPT to go bad, including the OPT itself.The readings suggest that 12 ohms into the primary red winding, it somehow connected to one of the secondary windings.This could have been a spark between the two - the secondary is usually tied to ground one side, so the primary HT would have this path to ground.Maybe the insulation on the windings failed, or there was some sort of physical damage/corrosion.Or perhaps someone ran the amp without a speaker or intermittently connected - this will produce larger back emf voltages than the HT supplies themselves, way exceeding any parameters of the OPT design.This may then have overheated the primary windings causing some to short after melting the insulation, or more sparks.I doubt the amp will have sounded particularly great during all this, but then perhaps there was nobody except the Cat to observe it.Its a fun game designing such scenarios but in truth you will never know.