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FDP Forum / Amp Mods, Repairs, and Projects / Blues Junior and Output Transformers

njdelyster

Hong Kong

Oct 20th, 2016 01:54 AM   Edit   Profile   Print Topic   Search Topic

Hi all!

Jumping into the Fender Forum for the first time with a question.

I recently picked up a Blues Jr 3 for a really great price. After reading up on how everyone loves the simple mods that are floating around I decided to start with replacing the stock output transformer.

I live in HK so unfortunately the TO20 Billm / Allen Amps option isn't possible (no overseas shipping). I checked in with David Allen who mentioned that a Heybeor / Hammond OT would be a suitable replacement and confirmed it with a couple others posts.

So I ordered the 1760F which Hammond says is the upgrade OT for a Blues Jr (http://www.hammondmfg.com/pdf/EDB1760F.pdf), dropped it into the amp, turned it on and then hit some issues.

After the amp came live a loud descending high pitched noise came through the speaker which eventually turned into intermittent mid range noise. Volume and Master (or any other pots) didn't change the volume/characteristic of the noise and I didn't get any guitar signal.

After double checking the cables were correct (same colour / same pins as stock OT) I swapped the new OT back to the stock and ... everything was fine. Amp played / sounded perfectly.

Is there something I'm missing with this OT? Is there another adjustment that I need to set the amp up for it? Is the OT possible a dud?

Does anyone have some thoughts that could shed light on the situation? I have a moderate amount of skill with mods (always been a tinkerer) and have build various pedals (some easy, some hard).

Thanks in advance!







pdf64

UK

Oct 20th, 2016 02:50 AM   Edit   Profile   Print Topic   Search Topic

My guess is that your replacement OT had a differing polarity relationship between the primary and secondary than the stock OT.
Unfortunately there isn't an industry standard for this.
Fitting an OT with the wrong polarity relationship is likely to affect the margin of stability of the amp, and, if there's a global feedback loop implemented, turn it from being negative to positive, possibly resulting in oscillation.
I suggest that you try the OT again but swap the wires from the primary winding to the plates around.

njdelyster

Hong Kong

Oct 20th, 2016 03:04 AM   Edit   Profile   Print Topic   Search Topic

Interesting - thanks pdf64!

I'll look into that and report back.

njdelyster

Hong Kong

Oct 22nd, 2016 10:05 PM   Edit   Profile   Print Topic   Search Topic

Got to revisit this yesterday. Thanks for the wisdom - issue sorted. Would have been much simpler if I had stopped to check the OT spec documents. Right there for the taking :)

With the new OT and the filter caps switched up - this is a sweet little amp!

FDP Forum / Amp Mods, Repairs, and Projects / Blues Junior and Output Transformers




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