Slide resistor

[Bushbaby]

Hi All,
I want to use this to put a dummy load across the speaker output transformer to protect it whilst Im testing my Bronco amp. I've realised even though it looks easy enough I've know idea have to connect/adjust it so that I can put at 3.2ohm resistance across the transformer? It indicates that its 15ohm and rated at 4.05amps
Can anyone offer some help please?
Many thanks

Rob
   
   

[pwdrive]

That's a beauty Rob, should be no problem, with nothing else connected to the variable resistor connect one lead of an ohmeter to any end and the other ohmeter lead to the slider, then adjust the slider to show the required resistance you need on the ohmeter, make sure when you connect it to the output of your amp that all connections are good (including the slider contact on the resistance wire) as pushing an amp at volume without an output transformer load can cause damage to the transformer and possibly other components.

Lawrence.

[Radio Fixer]

Well, dead accurate it doesn't have to be! If its clearly marked 15 Ohms you could just adjust the middle slider part so that its 1/5th + a tad from either end. Lets say the LH for example. Then take a wire from the LH end and one from the slider and there you are.

If you are going to measure power out performance then more accuracy maybe needed. So for that a meter capable of reading low value Ohms would be required and of course only specialised ones can do that.

It is a bit wieldy, and seeing as you are local to me I'm sure I can fine you something of more appropriate size if you wish to call round to Thorpe Bay.

Gary

[pwdrive]

An Avo 8 will measure 3 ohms no problem as will an ordinary DMM, you should still measure for continuity anyways for the reason I outlined in my previous post.

Lawrence.

[Radio Fixer]

An AVO 8 wont do 3.2 Ohms but anywhere near 3 will do the job as you say Lawrence. Your post must have been an instant before mine.

Gary

[pwdrive]

The Avo I have has graduations in .5 ohm increments on the divide by 100 range, like you say good enough, 3.2 ohms can be guestimated from the 3 ohm or 3.5 ohm marks.
I sense a tune coming on from Robs guitar amp soon...Chuck Berry please...

Just a thought for Rob, the resistance of the speaker and the speakers quoted impedance will be two different values, as a rough guide the resistance value will be roughly 20% lower than the impedance value because the impedance value is always higher than the resistance value.

Lawrence

[Radio Fixer]

I haven't used an AVO for years Lawrence ... I seem to remember fathing around with connections and dodgy zeroing pots. Digi is much easier.

G

[pwdrive]

I'm the opposite Gary, I have always used an AVO, I only bought a DMM (cheapo) a couple of years or so ago.

Lawrence.

[Ed_Dinning]

Hi Rob, set slider to mid position by eye and use this as one lead to speaker. Link both ends of track together. This is the other lead to the speaker.
You now have a 3.75 ohm high wattage resistor (1/2 of 15 is 7.5; 2, 7.5 ohm resistors in parallel give 3.75 ohm, close enough for most purposes)


Ed

[John M0GLN]

I don't know what Rob's amp' specification is, but I always thought that speakers were nominally 3.75, 7.5 and 15 ohm anyway, the first two are then rounded up to 4 & 8.

John