Questions on using a speaker load to bias an amp...

[kingsxman]

someone recommended using a 100 watt resistor instead of my speaker cab when I bias my amp. I've got a Marshall jcm 800. I bought a 100 watt 15 ohm resistor. (big ceramic tube). Since this is 15 ohm instead of 16 (they didnt make 16 ohm) will this negatively affect my current or plate voltages? I dont think so but thought I'd better check.

I assume I can just take a speaker cable, put a 1/4 jack on one end and solder the resistor to the other end of the cable and plug that into the amps speaker output. Correct?

I'm also assuming through this whole thing that you DO in fact need SOMETHING plugged into your speaker output when you bias.

 

[John Phillips]

Do you mean 'idle' biasing (by checking current with the amp running but no signal) or by using a scope and looking for crossover distortion?

If you're just doing it by current, the resistor is fine and will make absolutely no difference. Just connect it to a speaker cable like you thought.

If you're doing it by scope, some people would actually prefer the resistor - it gives a linear load on the amp, rather than the non-linear load of a speaker which is quite inductive. That's kind of the "theoretical" way though - I always prefer to use a real speaker (or more usually, a good dummy load since I don't like the sound of a test signal at high volume), since that's what the amp will see in reality.

 

[kingsxman]

Thanks John. I am planning on idle current. I dont have a scope and am not sure HOW to test via scope. Isnt the resistor and a "dummy load" considered the same thing?

 

[John Phillips]

The resistor is a form of dummy load, yes. Some others are more sophisticated though - ie they match the impedance curve of a real speaker more closely than a simple resistor does.

In fact, you don't even really need any load if you're going to bias the amp at idle with no signal going through it - it's safe to run an amp with no load as long as it isn't producing any power.

BUT... that relies on the amp being stable and not self-oscillating with no load - a few do though. I got caught out like this by a Vox AC50 not long ago... I should have known better! I ran it with no load while I was working on it, and even though there was no input signal the OT started to make an audible whistling noise, which means the amp was developing fairly high power. Luckily no damage...

So in fact, the resistor is a very good safety measure. You may not be able to tell when an amp is in danger of oscillation, or even actually doing it.


(edited for emphasis on when it isn't safe to run an amp with no load. Knuckles rapped by Todd )

 

[kingsxman]

Thanks John. I'm trying it now without a load and getting some goofy readings. If you can, check out my other post.

 

[Wakarusa]

For my $0.02 unless you've been working on amps since birth like Mr. Phillips, it's best to always use some kind of load whenever power tubes are installed and the amp is off standby. Always.

A big resistor works fine for this purpose. Most techs whip something up just for this purpose (ferinstance, mine is a collection of power resistors, couple of impedence selectors, 1/4" phone jack, all mounted on a big aluminum plate)