Classic Hifi Care

I thought we had a topic on voltage bucking here on the forum in the past, but I can't find it.

What I am considering is installing a voltage bucking resistor ahead of the power transformer in the Imperial amplifier. Line voltage here the other day was 122.1 volts. The power transformer is rated for 105-125 VAC, but this is getting too close for comfort for me. If I recall the conversation correctly, I know the wattage draw on the device, 200 watts. I'd like to buck, or drop, the line voltage to at least 117 VAC, 115 VAC would be better. So let's say I want to drop 6 VAC from the line. What value resistor do I need? I saw this done to a Zenith radio one time, but didn't get the particulars.

I made up a voltage-bucking transformer to reduce the supply voltage to my GE E-86 radio. I probably asked about it in one iteration of this site or another. I found several web sites for choosing transformer specs and wiring up the windings properly.

Chris Campbell

I don't want a hum-bucking transformer. I want to install a resistor in the AC line to the power transformer. I saw a Zenith radio with this setup, but never got the particulars. I want to set this up so I don't have to mess with anything extra when I run a stereo.

The transformer is not a hum-bucker. It's a voltage-bucker, working as an autotransformer to reduce voltage without acting like an electric heater. Big power resistors make lots of heat that must be dissipated.

Here's a useful article, which corrects my comment by distinguishing between an autotransformer and a bucking transformer configuration. Read the introductory comments too, about preserving older equipment.

https://sound-au.com/articles/buck-xfmr.htm

Your parts box probably has a suitable old used power transformer. These don't have to be huge, just big enough for the current demands of the unit.

A search on "bucking transformer" shows lots of other articles and threads on AudioKarma, etc.

Chris Campbell

I meant voltage bucking, not hum. It's been one of those days....

Greg,

The easiest thing to do is measure the current being drawn at the main leads to the power supply of the unit.

This assumes a fully restored radio/phono/whatever.

I find you can't just do the math even if you know the "rating". The manufacturer may have indicated "200 Watts", but what was that based on? 120 volts? 117 volts? 115 volts? Etc. etc.

Also, once you "restore", you end up changing characteristics slightly with modern parts. Also, age comes into play on devices you did not replace.

In other words, measure the current after the rebuild is complete, using the power source you will be using (if you have a small workshop in the backyard, you might not have the same voltage levels then you would in the HOUSE in your living room where the device will reside.

Watch the current as you play it at different volumes and base the measurement on what the device will be typically used for. Be warned not to put the resistor in line with the whole shootin' match. In other words, you want to drop the voltage to the one or more amp's transformers(s), not to the turn tables that will be pulling current from the same 120 feed lines. In fact, with multiple amps, you'd want multiple resistors for the dropping of voltage to multiple transformers.

Once you know the real world current and real world voltage. Then you can figure out a voltage drop resistor.

I used a MIL-SPEC aluminum finned 50 watt power resistor mounted to the chassis to drop the voltage to the power transformer in my RCA Victor 17K console. RCA transformers were notorious for running "warm" due to cheap parts sourcing.

I have 120 volts at my outlets. With the resistor in place, the primary of the power transformer only "sees" about 111 volts.

Thanks, Doc! That is exactly the information I was looking for. I was thinking about bucking maybe 6 volts, which would put line voltage down to 115-116 VAC. How low could I drop the line voltage and not interfere with the phono motor operation?

Okay Doc, I have 242 watts draw from the line at 121.2 VAC line voltage. This includes amp, tuner, MPX and changer. What size resistor do I need to buck at least 6 volts from the line. I wouldn't be offended with bucking 10 volts, but the resistor to do that might be too big to hide under the chassis.

Resistors achieve the desired result by using up part of the current via heat. Heat you pay for every time it is in use, especially with the Wattage demand of that big console.

An old filament transformer, wired to reduce the output voltage does it without the heat. In my experience, they are usually smaller than a comparable resistor.

DG