Transformer hammond 291aex

Power Transformers
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1

I need help. Would you please explain the primary connections on this transformer. Hammond 291aex. to get 240v. i keep triping the clipsal on the main board to the house…..

2

Hello @mikecasey,

I’ll assume you meant 240 VAC on the input side, as the transformer is designed for about 350 VDC (after a tube rectifier).

Have you seen this image from the 291AEX data sheet?

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The general output wiring is

  • Yellow 5 VAC to the tube rectifier’s filament e.g. 5Y3
  • Green 6.3 VAC to the amplifier’s filament e.g., 6V6 and 12AX7
  • Red to the rectifier plates with red / yellow to ground

The input is dual voltage 120 or 240V

Here are the connections for a straight 120 or 240:

  • For 120 VAC connect the windings in parallel:

    • Hot (after your fuse): to blue to black
    • Neutral: brown to white

  • For 240 connect the winding in series:

    • Hot (after your fuse) Blue
    • Connect together brown and black together but do not connect them to anything else
    • Neutral: white

Note: It is possible to connect for 220 and 230. We will leave that conversation for another day as the connection matrix is considerably larger.

Recommendations

  • Add an input fuse to your amplifier. Based on a Fender Champ schematic 2A is appropriate. As a rule, no amplifier should trip the main house breaker (Clipsal by Schneider). Here is the link to a classic fuse holder.

  • Reach out to your community and see if you can find a repair technician who can serve as a mentor.

Happy soldering,

Aaron

3

I thought I would have to connect blue and black to get 240v and brown or white for the neutral . Is this wrong

4

Hello Mike,

Perhaps I misunderstand, are you attempting to:

A) Power the transformer from a 240 VAC source.

B) Obtain 240 VAC from the transformer.

As a side note, injection 240 VAC across blue and brown will exceed the 120 VAC winding’s voltage. The result is a magnetically saturated transformer that will draw considerable current with the potential to burn out a winding.

For 240 VAC input:

  • Hot to BLU
  • BLU/YEL insulated
  • BRN connected to BLK
  • WHT/BLK insulated
  • Neutral to WHT

Best wishes,

Aaron

5

Hi Mike, this advice comes from our transformer expert at Hammond.

”The proper connection for 240V is to jump Brown to Black and cap this connection. Then connect the 240V between Blue and White. The White/Black and the Blue/Yellow are not used. They should be capped and tucked away. He may have burned out the primary winding.”

If you have further questions you can email him at mmercer@hammfg.com, or give him a call at 716-630-7030 ext 2523.

6

Thankyou, yes that’s the way I have it wired now. But it’s not working. I like to learn by trial and error, I think this has been an expensive error. Thanks for all your help.