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Blues Jr. -- Output Transformer -6

Posted by Jim Black on 3/27/2015 to Mods
The last mod for the Blues Jr. is the Output Transformer. This mod is the key to getting that nice tight low end I am looking for. The new transformer has slightly more loudness than the stock transformer and no distortion in the bass end.
 
Every tube amp has at least two transformers. One for the power supply voltages and the other for the tube amp output. The output transformer is what drives the speakers. It is important to understand which one is which. Definitely don't want to remove the wrong one.
 

This photo shows the underside of the power transformer. This is the wrong one! The instructions included with the transform do a pretty good job describing the wire colors and positions. The instructions do not actually show a photo of the position of the old transformer.

This photo shows both transformers. The output transformer is the one that is hanging loose in the middle of the chassis. I had to cut the wire tie off a bundle of wires to free up the output transformer wires so they could be pulled through the chassis for removal. You have to do this to partially remove the PCB in an earlier step.
 
Billm and David Allen, the guys who designed this replacement, made sure to match wire colors and use the right connectors on the new transformer. This extra step is really worth the money and made this a clean replacement. The last step is to install the 100pF capacitor across R30 to prevent phase inverter oscillation. When I bought the amp, my buddy did mention that there was a squeal he heard from time to time that drove him crazy. I didn't know what he was talking about at the time; but, I am sure this is what he was referring to. This fix should take care of that problem.
 
The only quirk to this mod are the fender washers to raise the new transformer high enough to clear the chassis bushings. I put a thin coat of silicone between the washer and the chassis just to prevent potential rattling.
 
Next time, I have to find the right speaker to give me that bottom end...

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