Heater Voltage Dropping Resistor Issues

[David Root]

My little 4xEL91 amp is running 2.2A on a 2.5A nominal 6.3V supply. This includes the rectifier, a 6X4. Voltage is 6.75 to 6.8V.

So I calculated (6.8-6.3)/2(2.2)= approx. 0.1 ohm per side dropping resistor. Pd=2.2 squared*0.1=0.48W, so two 2W resistors is correct.

Well, I put in a pair of 0.1 ohm 2W resistors on the 6X4, from pins 2 & 5 (not connected to the tube) to the heater connections, pins 3 & 4 respectively. And relocated the winding leads to pins 2 & 5.

To my surprise the voltage dropped a full volt to 5.8V! So I pulled out one resistor, got just under 6.3V. But the remaining resistor is as hot as a pistol.

Resistors are color coded properly and big enough to be 2W.

When I measure them with my DMM they measure 0.4 ohms each. Deduct 0.2 ohms residual resistance of the probes, I get 0.2 ohms for each resistor.

Did I screw up the calculations or are these resistors no good? I know they're Chinese product.

[Phil_S]

...When I measure them with my DMM they measure 0.4 ohms each. Deduct 0.2 ohms residual resistance of the probes, I get 0.2 ohms for each resistor.

That is likely exactly your problem. You need a total of 0.2, not 0.2 * 2. Your calculations are perfect. With one 0.2, you have 6.3V.

You don't need to fiddle with a resistor on both legs of the supply. One resistor is fine. The load across that resistor is about 1.1W (that's .5V * 2.2A), so you'd think 2W is enough. I think, in this situation, you want 4x derating. Remember, the flow on that circuit never lets up. Use a 5W sandblock and allow chassis contact for extra heat sinking and to be sure the heat isn't a problem.

Really, though, I think you are chasing something you don't need to chase. There is nothing about 6.8V that should be a problem. +/- 10% seems to be the standard. You aren't close to it.

[sluckey]

What color are the resistors? Should be Brown/Black/Silver.

I would not be concerned with voltage at 6.75 to 6.8.

[David Root]

Thanx guys. Color code is brown black silver. 4th band is gold, so 5%.
But my DMM (Fluke 187) says they are 0.2 ohms each, not 0.1 ohms. If that be so, then they are mislabeled. Or my DMM needs calibrating.

You're right, I would be happier with a 5W resistor. And two .05 ohms too. Maybe I'm being a bit anal, but that's just the closet engineer in me.

[jaysg]

I would not be concerned with voltage at 6.75 to 6.8.

Same here. I suppose a large Schottky diode would do as well or better.

[sluckey]

Use the REL Delta button on your 187. Short your probe tips together and push REL to 'zero' the reading. Then measure your resistors. This is usually a good idea whenever you need to measure low ohm values.

[Ken Moon]

David,

I have some extra Dale NS-5 series 0.1 ohm 1% 5 watt resistors - I could pop a couple in a padded envelope and drop them by the Post Office tomorrow.

No charge - I like to help the members here when I can

Just PM me with your snail mail address and you should get them early next week.

Cheers, Ken

[David Root]

Thank you Steve. I must read the manual one day!

Ken, thank you! That is most kind of you. I'll PM you.

I finally realized I don't need 2 x .05 ohm as I have already balanced the heater circuit with a pair of matched 100 ohm resistors to ground. Duh!

[Garthhog]

I wouldn't rely on a standard Multimeter to measure a resistance that low. You need a Milliohm Meter.

[David Root]

I have an ESR meter for caps but it didn't want to work for me on this application.

[Phil_S]

I wouldn't rely on a standard Multimeter to measure a resistance that low. You need a Milliohm Meter.

If you have a good meter, you might get by with putting a larger value in series with the sub 1 ohm resistor, maybe 5 ohms or so, to get beyond the low reading problem, but not so high as to allow the bigger one to dwarf the little one. Read the 5 ohm and then place the "target" in series to get the difference.

[David Root]

Thanx Phil. I just did that. My DMM is a Fluke 187.

The meter probes crossed reads 0.17 ohm, I put an old (1977) Dale CP-5 0.5 ohm on it, got 0.67 ohm, so the Dale is exactly 0.50.

Then I put one of the 0.1 ohm suspect resistors in series with it and got 0.92 ohm, so the "0.1" is actually 0.25.

I checked another one, it came out at 0.30 ohm, so they are junk. They look like metal oxide type.

I guess the lesson here is don't buy 0.1 ohm resistors that aren't 1% wirewounds and Made in USA!