Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nateflanigan on April 03, 2010, 10:49:20 am
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This is something that's been rolling around in my brain for a bit. How do you tell what output transformer is appropriate for a given amp?
For instance, I'm thinking of building something like a bassman or portaflex, it's easy enough to find a part sold as a bassman replacement OT, but if I wanted to save some cash and screw around a bit how would I know what to look for? Secondary impedance is straight forward but what about the other specs?
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You can find specs for common guitar amp OTs on hammonds web site.
For an unknown OT in a junk box, apply 1VAC sine wave on the secondary, measure the voltage on the primary. The voltage you measure will be the turns ratio. Square the turns ratio for the impedance ratio. Then multiply the impedance ratio by the secodary impedance to get the primary impedance.
E.g. 1AC applied to secondary, 8 ohm tap
31VAC measured at primary
Turns ratio is 31
31*31 = 961 - the impedance ratio
961 * 8ohms = 7688 ohms primary impedance.
This is an approximate, as impedance varies with frequency.
You might be able to see from this calculation that the transformer is just a ratio component. If we applied 4ohms to the secondary, the load on the primary would be half the 7688 = 3844
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Awesome thanks!
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If you're junk shop hunting, look for old amps that used the same output tubes you want to use, then test the OPT as ToN described. Often times the power transformer will work complimentary with the OT. Disconnect the load from the power supply and bring it up using a lamp limiter and measure the DC voltage. Plan to change all the electrolytic filter caps. I've built many a rockin Jalopy amp from old tube PA's, console stereos, organ amps etc.
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Thanks Butterylicious!