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Reducing high line voltage - Diverted - 02-15-2014

Hi all,

I'm trying to stabilize a 41-285. Its a 65W radio meant to operate at 115V. My house AC is 122.
Opinons from experts on whether I should let this slide, or insert a bucking transformer or finned resistor?
I was thinking resistor; would a 10 ohm 10W finned do it? I did some rough calculations and I may have done it wrong; I came up with pretty much that rating.

Also, where would I put it along the hot line? I have already fused the hot AC line prior to the line filter caps. Thanks!

Ted

RE: Reducing high line voltage - Alan Douglas - 02-15-2014

Will you be running it a lot? Is it going to someone who is not a radio technician?

RE: Reducing high line voltage - Diverted - 02-15-2014

Yes, and yes.
It's going to my mom, who had one when she was a kid and loves to listen to the Red Sox on AM.

RE: Reducing high line voltage - morzh - 02-15-2014

Try to look into a bucking transformer.

RE: Reducing high line voltage - Diverted - 02-15-2014

Thanks. I was up in the air, you tilted the balance toward the BT.

RE: Reducing high line voltage - tom.seeger - 02-15-2014

I would not bother. You are well within the"normal" line voltage range.
I read far too much about people worrying about line voltage. After all do we worry about our fridge or expensive flat screen TVs blowing up? I have dozens of old radios on my high line voltage and never lost a single one.
That's a beautiful model by the way.
Tom

RE: Reducing high line voltage - Raleigh - 02-15-2014

My 42-255 with 8 tubes pulled about 500 mA at 123 VAC. I used 15 ohms to drop it to 115 VAC.

With 9 tubes and 122 VAC, 10 ohms might be about right for the 41-285.

I put the resistor in the hot side of the line between the fuse and the switch. In the schematic the switch is after the line filters capacitors. I moved the switch to before the line filters. So the hot side of the line now goes first to fuse, then resistor, then switch, then line cap, then transformer.

RE: Reducing high line voltage - morzh - 02-15-2014

60 Watts at 115V makes for roughly 0.5A. 10V dropout will dissipate 5V max, which will require a 10-15W resistor. It will not get overly hot but 5W is still a bit of dissipation.
It is safe both ways, the transformer probably a bit more expensive but is a cleaner way.

Also the dropout with resistor at least in theory will vary with power, so at low volume it is less. By how much I do not know. Maybe not by much as 3 to 5 watts is not a lot compared with 50 to 60 watts overall.

RE: Reducing high line voltage - Raleigh - 02-15-2014

Would there be enough room inside the chassis for a transformer?

I decided to go with 25 W after checking the derating curve of the resistors. I don't know how warm it gets in the chassis after the radio gets good and warm, but the price and the size difference seemed insignificant to go with 25 W.

With a thin, even layer of thermal compound, both the side of the chassis and the body of the resistor get just a bit warm to the touch.

Most of the AC power in the radio goes to heating the filaments and the panel bulbs and in rectifying the B+. I doubt the B+ DC current varies much with volume, so the only variation would be the AC signal across the output transformer. I'm not sure how much variation that is with volume.

Just for fun I measured the AC input current on my 41-280, which doesn't have a dropping resistor in it yet. After about 10 minutes warmup, the AC input current was .402 amps regardless of volume.

RE: Reducing high line voltage - Eliot Ness - 02-15-2014

For about $20 you can get a Kill-A-Watt meter that comes in pretty handy for checking stuff like this. I find it to be quite useful at times:

http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU/ref=sr_1_1?ie=UTF8&qid=1392493248&sr=8-1&keywords=kill+watt+meter

Do a search and you might find them even cheaper.

RE: Reducing high line voltage - morzh - 02-15-2014

yes this is a handy thingy Nd also allows you to see yor power factor or to calculate it. As I remember it from using it once at work.

RE: Reducing high line voltage - Diverted - 02-15-2014

Thanks all. And Raleigh, you're right. It's a beautiful set, I love it. My mom is going to flip out when she sees it. It's a part of her childhood and she's going to love it.

Ted