Why do I need two 15 ohm taps on my OT?

[harry]

Hi all,

I have a very old Linear Conchord output transformer. It has a single tap for 3.75 ohms, and two for 15 ohms.

Why do I need two taps for 15 ohms? Can they be paralleled to get 7.5 ohms? I didn't think you could do that.

I am confused as to why it would be wired like that. Can anyone offer any insight?

Thanks,

Harry

[olddawg]

Have you measured them? I would suspect a typo. It does happen.

[Steve Conner]

Some British made output transformers had two independent secondary windings (not taps) that could be wired in series for 15 ohms or in parallel for 3.75.

How many secondary wires do you have in total, and how many windings? Check with an ohm meter.

[harry]

Some British made output transformers had two independent secondary windings (not taps) that could be wired in series for 15 ohms or in parallel for 3.75.

How many secondary wires do you have in total, and how many windings? Check with an ohm meter.

Steve, that makes perfect sense... although there were actually two outputs! I took the amp apart a long time ago for spares, but I remember there were three connectors that went: 15 ohm, 3.75 ohm, 15 ohm. One wire to each 15 ohm tap, I seem to remember two wires going to the 3.75 ohm tap.

At any rate, I will check with an ohm meter as per yours and olddawg's suggestion. Thanks!

[harry]

Plus, if it does turn out to be two separate windings, what would the effects be of just using one winding and leaving the other floating?

[Chuck H]

You should be able to check if it's two windings by ohming the output leads. If there is any resistance between them then it's likely two different windings. Steve's question about the number of leads is important regarding how you can use individual secondary windings. If, for example, you have two windings but they share a common 0V lead, there is no way to connect them in series. You could still use them one, the other, both or parallel. The two windings present as parallel loads to the primary, but only if they're used. I don't know how the original amp was wired to operate so I can't say if you want to ideally use both windings or not to reflect the right impedance on the primary. Even then the speaker load can be changed to compensate. It would be a good idea to test with a low VAC and determine the ratios from primary to secondary.