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Best way to reduce voltage from the power transformer?
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Return to top of page · Post #: 1 · Written at 8:59:25 AM on 15 November 2022.
	Robbbert Location: Hill Top, NSW Member since 18 September 2015 Member #: 1801 Postcount: 2116
I've got a troublesome radio here with excessively high voltages from the transformer. The voltages exceed the rating of the inbuilt 5Y3, and also of the filter caps. What is the best way to reduce these voltages, to stop them reaching the 5Y3? One 5Y3 already burnt up when I wasn't looking. I've thought of a few things but they all have their downsides. - Resistor divider network on the transformer wires? What values? - Underrunning the 5Y3 filament? This tube is quite capable of working even with the filament not glowing. - Only using one half of the transformer, so a halfwave rectifier? - Something else? Suggestions please.

Return to top of page · Post #: 2 · Written at 9:47:24 AM on 15 November 2022.
	Tallar Carl Location: Latham, ACT Member since 21 February 2015 Member #: 1705 Postcount: 2195
Whats the radio? Is the transformer original? Maybe a rewind is in order.

Return to top of page · Post #: 3 · Written at 9:52:12 AM on 15 November 2022.
	GTC Location: Sydney, NSW Member since 28 January 2011 Member #: 823 Postcount: 6803
What make and model radio? Are all voltages excessively high and by what percentages? Maybe the primary was designed for something less than 240 volts. If so, then I'd suggest a dropper resistor in the primary side. For instance, to drop the incoming AC to (say) 220 volts, R = E/I where E = 20 and I is whatever the primary current measures. Power = I x I x R. Alternatively, there could be a fault in the primary side of the transformer.

Return to top of page · Post #: 4 · Written at 12:07:05 PM on 15 November 2022.
	Robbbert Location: Hill Top, NSW Member since 18 September 2015 Member #: 1801 Postcount: 2116
It's my Tele-Verta that I've talked about a few times. The heater voltages are normal, and there's no other taps. The transformer looks original, although if it was replaced a long time ago there probably wouldn't be a way to know for certain. I haven't measured the voltages recently, but each side was (as I recall) about 440 volts per leg. If a rewind is the answer then that places it out of my hands. I've never rewound anything, and don't have the tools for it.

Return to top of page · Post #: 5 · Written at 1:06:17 PM on 15 November 2022.
	STC830 Location: NSW Member since 10 June 2010 Member #: 681 Postcount: 1307
The HRSA member who used to offer a transformer rewinding service has disappeared out of the Radio Waves classified section, so you are left with commercial options now.

Return to top of page · Post #: 6 · Written at 9:09:55 PM on 18 November 2022.
	Ian Robertson Location: Belrose, NSW Member since 31 December 2015 Member #: 1844 Postcount: 2527
The 5Y3 can take 1400 peak volts between the plates. If the output is too high, the best way to drop it is to add resistance between the 5Y3 filament and the 1st filter cap (which should have a 630 volt rating). Start with 47 ohms 10 watts. See what voltage you get when the rest of the valves warm up, it will start high. The resistor will get hot because it is absorbing the rectifier peaks.

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