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Big a$$ resistor useable?

Zolton

Zolton

Well-Known Member
I'm messing with a well used Harman Kardon 430 and getting ready to set the bias. The technical manual says to use a 50 watt 8 ohm resistor as a dummy load at the speaker terminals. I came across this large adjustable resistor (rheostat?). It's an Ohmite Model R made in Skokie Ill. We all have this sort of thing just sitting around don't we? :yes:
It's adjustable from zero up to 25 ohms and is rated for 4.47 Amps and 600 volts max.
Can I use this as my dummy load?
 

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Only thing that would concern me is that its wire wound, so lots of inductance. Don't rightly know if that will affect things or not.
 
The power rating @ 8 ohms will be much lower than the rating @ 25 ohms.

Just so you know.
 
gadget73 said:
Only thing that would concern me is that its wire wound, so lots of inductance. Don't rightly know if that will affect things or not.
Click to expand...

It makes the load lighter at high audio frequencies, which results in overly optimistic power output measurements and artificially low distortion measurements if that's what you're up to. It's okay for the OP's purposes.
 
tally

It looks like one of the gadgets from Dr. Frankenstein's lab.:D

Parts Express sells a dummy load 8 ohm 100 watt resistor for such an application. I figure if I already have something that'll work...why buy another.

So far I've got one "yes" and two "maybes" Any other opinions?

Edit; make that two yes votes
 
Oh yeah like the old old plant switchgear that had rheostats - but some were as big as a steering wheel. That brand/manufacturer name seems familiar with the smaller ones, but it's been some years.
 
I use those PE resistors, they're non-inductive so are nicer in the long run. The ones I got were VERY close to 8 ohms, measured on a nice 5-1/2 digit HP meter with the math function used to zero out the leads. I was too lazy to hook up the 4-wire mode.
 
if just setting idle current i cant really see it being a problem as long as its set at minimum vol and no input . if checking the power output then observe its temperature .
 
Zolton said:
Parts Express sells a dummy load 8 ohm 100 watt resistor for such an application. I figure if I already have something that'll work...why buy another.
Click to expand...

Those resistors are claimed to be non-inductive, so they're more suitable for amplifier performance measurements, but claimed resistance tolerance is only 5%. Amplifier manufacturers usually specify 1% tolerance for this application. Not that it matters in the present context, but if a body had access to a good four-wire ohmeter or Z bridge, the Parts Express resistors could easily be trimmed to better than 1%. Just don't add inductive resistors when you do it! It's hard to say how successful this scheme would be -- perhaps the reason for the 5% spec is that resistance isn't stable over temperature.
 
I got these loads fairly warm (like 20 watts) and they were still very close. Maybe I just got lucky for once!!
 
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