Magnavox stereo, voltage bucking devices

[electra225]

I have been considering some kind of voltage-lowering device so power transformers will run cooler. I recently saw a Zenith radio chassis with a resistor on one side of the PT primary. Wonder if something like that would work in a big tube stereo?

[TC Chris]

For my GE radio that had the output tube running slightly pink, I used a voltage-bucking transformer. No heat problem or wattage issue as with a resistor. Do a google search for circuits and values or I can dig out the plans I used.

With a bucking transformer of sufficient size, you can use on on multiple devices by unplugging one and plugging in another. Mine is built into a box with standard outlets on it.

Chris Campbell

[electra225]

I wanted to avoid the expense of a bucking transformer. The resistor I saw in the Zenith radio was finned and it was mounted to the metal chassis for heat dissipation. I'm open to any ideas. The transformers in Magnavox stereos are rated for 105 to 125 volts AC. Considering that they are 60 years old, one is hesitant to ignore how hot they operate. Thanks, Chris.

[Firedome]

Also there's some pretty cheap Chinese variacs out there these days, can't vouch for quality, but believe they'd have to pass UL or such?

[hermitcrab]

Today's line voltages are a problem, most stuff built in the 50's and 60's were meant to operate in the 105 to 120 range , as I stated in another thread I put a small variac in line with the line controlled AC motor on a R2R to cut back on the voltage because tapes played on it were on the verge of sounding like alvin and the chipmunks. I have had people tell me that motors cannot vary as they are controlled by the 60 cycle frequency...to a certain extent they are but it is a rough speed usually around 3600 rpm... but variations in line voltage can and does effect the overall speed . I have a VM R2R that says operate on 105 - 115 ... I ended up putting a 18 ohm 10 watt resistor in the line and mounted it to a convenient heatsink . it keeps the voltage right around 115 - 120 with no problems . it will all depend on how much wattage your set draws to determine what size and wattage a resistor you need to use and what voltage you want to keep it around.

[electra225]

I guess that's what I need. The mathematical forumla for figuring how big a resistor it would take to buck voltage in a stereo that draws up to 600 watts. I know in the CG, I could put individual resistors in the various chassis, so that wouldn't be a problem. With the single-amp-chassis models, where the amp chassis powers up the rest of the console, that might take a pretty large resistor to drop enough voltage, with them drawing between 200 and 350 watts.

[hermitcrab]

does the back of the set say it draws 600 watts? that equates out to 5 amps at 120 volts.... seem excessive for a home stereo ... but I guess with all the tubes it could... energy concerns did not exist back then...

[Motorola minion]

Other methods to reduce heat are raising chassis off flat bottom of cabinet to aid convection airflow and replace tube rectifiers like 5U4, 5Y3, 6CA4 and 3DG4 with diodes and suitable series dropping resistor. Each 5U4 uses 15 watts just for the heater alone!

To drop voltage, put two 2.7 ohm 10 watt resistors in series of a big trans for my Magnavox with a 9300 series amp. I like the idea of buck transformers but not the fact I need one with a 3 amp secondary to do it properly.

That 62 Symphony is about average at 200 watts with a tuner and record player added. 2 amps across 5.4 ohms drops 10 volts! Line voltage runs 123-125 here and that makes me nervous about using 60 year old power transformers. I normally place a CL-90 surge limiter in series with line and a 2-3 amp fast-acting fuse. A 2 amp slow blow would do the job just as well, not opening on inrush current of trans.

[electra225]

Do I understand when you say to add a 2.7 ohm, 10 watt resistor to each leg of the primary of the PT? Or two 2.7 ohm, 10 watt resistors in series on one leg of the PT? Why two resistors in series? Why not one 5.4 ohm, 20 watt resistor by itself? And, what math is used to come up with the value of the resistors required to drop 10 volts? I could separate the chassis on the CG and use resistors in each chassis, instead of trying to drop 10 volts when it draws 650 watts. Mine actually draws about 450 watts without the remote being on. I don't intend to question you, or be argumentative, I'm just trying to make sure I understand, since I'm interested in trying something like this.

[Firedome]

Knowing MM personally as well as I do, when he speaks, I listen... he'll know the appropriate mod if there is one!

[electra225]

I'm not questioning his knowledge. I'm just questioning my understanding. I want to make sure I have it right.

[Firedome]

Understand, it's always wise to make sure, and to know what you know or don't know... in my case the latter is far greater than the former. I've learned a great deal about audio from MM over the last few years, and he's my vintage color TV guru
as well!

[electra225]

If we were to separate the various power supplies in the Concert Grand, we could put resistors in each amp chassis, then in the power supply chassis for the tuner that would get the MPX, tuner and phono motor. I could use the Kill-A-Watt to get the total wattage, then put the Kill-A-Watt on each amp chassis. Then the balance would be for the tuner, MPX, and phono motor. I'd like to see 112-115 volts if that's possible. Another consideration is how close to the PT the rectifier is mounted. Maybe some kind of heat shield might help.

[19&41]

In selecting a higher wattage resistor, I have used and suggest a mountable resistor like the Dale. you can mount it on a flat aluminum plate sink with a little thermal heat sink compound and set the sink up so it can be mounted in the equipment space.

[electra225]

That Dale resistor looks a lot like the one I saw in the Zenith radio I mentioned. That could be mounted under the chassis, couldn't it? Where might someone acquire such a device?

[19&41]

I'd look on Ebay. Figure what value you want and search it.

[19&41]

Here's another source. Some are a bit pricey. You'll want the aluminum bodied type.

https://www.surplussales.com/Resistors/ ... d_list.htm

[electra225]

I really like that idea. I may get a few of different values and experiment a bit. Thanks, Rex.

[TC Chris]

That Dale resistor looks a lot like the one I saw in the Zenith radio I mentioned. That could be mounted under the chassis, couldn't it? Where might someone acquire such a device?

My preference would be to mount it outboard, where it wouldn't shed its heat in the innards of the device, cooking other components. Yeah, it might look non-original outboard, but at least it would ventilate effectively and into a space where nothing wold be roasted. Heat is a big enemy of our equipment.

Chris Campbell