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Old 18th November 2012, 02:40 AM   #1
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Default Parallel Output transformer question

Hi guys,

I've been reading various sources about the topic and, depending on the situation and configuration, there are many opinions.

So, in reference to this article on TubeCAD:

Big Amplifiers

I have a pair of A-470s, and have access to another. The last part of the article states to just parallel the primaries and secondaries and "voila", you get 2,150 ohm primary, and just slide rule down your taps i.e. 8,4,2 ohm taps. Sounds awesome and its what I want to do, but I have read that this is a recipe for disaster if the output transformers are even slightly different. These A-470s are not cheap, but I got a deal on them. Is this safe for PPP EL34?

The other option is to essentially build it as two identical amps with the same driver "ala David Bogen", and parallel just the secondaries leaving the same tap scenario.

Which is better/safer? Oh, and yes, I know the best is to buy a new 60-80W transformer

Thanks!

Blair
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Old 18th November 2012, 05:17 AM   #2
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What if you only own two Dynaco ST-70s? This time the desired reflected impedance is 2150 ohms and the solution is fairly straightforward. We start by placing two output transformers in parallel, both the primaries and secondaries. Next, we re-label the secondary taps; the 16-ohm tap becomes the 8-ohm tap; the 8-ohm tap, the 4-ohm tap; and the 4-ohm tap, the 2-ohm tap. Now each impedance load will see 1.414 times the voltage that a single ST-70 would deliver. (What I like about this arrangement is that none of the secondary is left flapping in the wind when an 8-ohm load is used.)

take note that the key here is that the 8 ohm speaker is now connected to the 16 ohm tap....

even with just one OPT, connecting an 8 ohm speaker to the 16ohm tap halves the primary reflected impedance....
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Old 18th November 2012, 11:30 AM   #3
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Hi,

Yes, I understand reflective loads. The question here is what is the best way to parallel output transformers safely without damaging them.

Thank you,

Blair
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Old 18th November 2012, 09:30 PM   #4
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just connect them as instructed, needless to say the primary phasing must be correct, connect the primary leads of one OPT to the corresponding leads of the other, usssually that will be the color coding, but best to verify after by injecting voltage, sat 6 volts at the secondary side and verify you got the primary phasing right...
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Old 19th November 2012, 12:36 PM   #5
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Thanks,

So, there is little or no concern about damaging the transformers if I do this?

Blair
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Old 19th November 2012, 09:47 PM   #6
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no problem....
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