The Workbench

Universal power transformer voltage question


Tech question: I have an amp with a universal power transformer that is delivering higher-than-expected voltages. It has terminals for wall voltages of 115V, 145V, 160V, 205V and 225V. Even though my actual wall voltage is just over 120VAC the voltages when using the 115V terminal are far beyond the expected difference. I can unsolder and reconnect to the 145V terminal, but wondering what would I see on the 115V terminal if I just added a jump wire between the 115 and 145 terminal. (I'm thinking I'll still see 115, but can someone check me on this before I start experimenting this weekend?)

Edit: The voltages shown in this photo aren't correct for this transformer. See below.


Well the higher than spec voltages makes sense if you're plugging 120+ into a 115V transformer. 120 is 4% higher than 115 (and 125 is almost 9% higher), so I'd expect your secondary voltage to be similarly over. Or are you getting disproportionately higher on the secondary?

Regarding the jumper, I don't think so. Without knowing that transformer, I'd guess you're just shorting the windings between the 115 tap and the 145 tap. Without a potential between the two taps, I'd expect no current flowing between them.

Or maybe I'm misunderstanding the question?


You can buck a transformer with a separate winding but not the same winding. Don’t do it.

If the 145v primary winding doesn’t get you where you wanna be, just get a new PT. You can run zeners in series with the B+ center tap but that’s a PITA and you create a lot of heat. And that will melt all of the wax in your re-sleeved, rewaxed capacitors.


What is your filament voltage? Give a specific number.


Hi all - problem solved (or maybe "mystery solved".) This is a fawn Vox AC15, circa 1960. The front panel on the amp has settings for 115, 160, 205, 225 and 245VAC. But it turns out that the 115V setting is connected to a power transformer tap that's really 105 !! Tonight I found the photo shown here posted on a thread on a few months ago. (Although it's the same transformer, the one in my amp has no labels.) So apparently in 1960, Vox was only kidding about the 115 voltage!

Now my problem is, how do I lower my wall voltage to 105VAC? (I know, use a Variac - but any other ideas?)


Here's my amp, and the voltage selector switch. Also misleading on my amp is that the 220V and 245V settings are jumped in the back with a copper wire - both will connect the amp to the 245V terminal on the power transformer. Original I believe because a) it looks original and b) the transformer also does not have a 220V terminal.

I'm going to edit the photo in my first post in this thread since the voltages listed aren't correct for this transformer.

Billy - using the Variac to bring the filament voltages to 6.3VAC is one of the ways I confirmed that the amp wanted to be plugged into 105VAC.

Using the 145 tap with my 120 wall voltage results in voltages that are too low. The owner of the same amp on thegearpages suggested using a 2:1 step-up transformer to double my wall voltage and setting the Vox voltage selector to 245. I'm thinking that's a pretty good solution. (Anyone have experience or recommendations for step-up transformer?)


Hmmm... I wasn't able to edit or replace the photo in the first post of this thread. Just wanted to point out that the terminal voltages listed in the photo follow the voltages shown on the Vox AC15 control panel, but are not correct for the transformer they used.

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