Isolation Transformer Output

[g8wayg8r]

I've got an old Triad N-67A transformer and the output is 128V. That seems a tad high and out of range of the 115 +/-5% spec. This seems too high to use for radio work and I'm wondering if it would work better through a variac that shouldn't be sensitive to 128V input. Anyone have experience with isolation transformer and their output?

[Chuck Schwark]

You are measuring the output voltage under no-load conditions. Put a radio (within current spec of the transformer) on it and re-measure.

Chuck

[g8wayg8r]

I wired it to a Zenith 5-S-320 and the radio ran fine. I still get 128 volts and the output from the socket read 121 volts. Maybe I'm expecting too much since the radio really has no chance of loading the transformer.

The next question is, so what? 128 volts goes into radio transformer and the plate to plate voltage voltage supply rating for the 6X5GT is well above the stepped up voltage it should see. Will it shorten the life the the heater filament if the voltage runs at 6.6 rather than 6.3 volts? I don't know. That would be my biggest concern. How will the tube heaters hold up. If there is little or no harm for the few hours I would run the radio at 128 volts I wouldn't expect a serious problem.

Thoughts?

[codefox1]

use a bucking transformer to tame this. There's lots of info here on this forum and elsewhere on the web. You can also use a resistor but this is problematic, since they get real hot, and not always easy to find and install.

[radiomikee]

Use the priary as the secondary, and vica versa...now you have isolation, plus slight step down.

[g8wayg8r]

Thanks for both ideas. One is very simple but offers the least flexibility. Read about bucking transformers. Simple in concept but the wiring is a tad odd at first.

Wire the two transformers together to get two nodes. The transformers should have the same VA rating.
Node one is an isolation secondary, one bucking primary and one bucking secondary.
Node two is the other isolation secondary and the other bucking primary.

There's an unattached bucking secondary. This is connected to the radio.
Node two is connected to the radio.

If the voltage steps up (not down), just reverse th bucking secondaries. Got the 130V down to 115V using a 6.3-0-6.3 center tap transformer (with the CT unconnected).

Now to make a circuit without a variac that will allow me to step up voltage in reasonable increments when I power up a radio for the first time. Guess it wouldn't hurt to have an amp meter in line.