Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jalmeida on November 29, 2025, 03:29:42 pm
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Hello,
Most of the JMI Era Vox amps have a voltage selector that gets wired around. In the US we have 120-125v wall voltage and the Vox amps have 115v selections, but some are even closer to 110v for how the transformers are designed. Which is why many folks use brown boxes and the like to drop the wall voltage.
I had seen that some tech though have been instead wiring the 115v tap as the 0v and the 245v as the positive giving the primary ~130v which is closer to the actual wall voltage or lower. Any issues hardwiring this way? Is this safe?
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The risk is that the 0-115 primary section was wound with thicker wire than the rest of the primary, in order to accommodate the higher current level.
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The risk is that the 0-115 primary section was wound with thicker wire than the rest of the primary, in order to accommodate the higher current level.
That is what I figured was the case. But wasn’t 100% sure.
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Just spoke to a friend who winds transformers and 100% advised against it for the reason PDF64 suggests. Smaller wire and creating more heat. It will blow the transformer.
I read a post where a poster was advised by a well known and reputable amp builder who advised to use the 115v and 245v taps for US.
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Reducing the mains voltage seems the best option http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm
The heater voltage (with all valves fitted) is the metric to assess what mains voltage the transformer was intended for.