Slave Output & Load Resistor

[dr.indiflidual]

The 20/20 power amp manual says if you "do not want to listen to the live speaker outputs, connect a load resistor with a rating of at least 40 watts and with a 4 or 8 Ohm impedance rating" when using the slave output.

I understand why this is required, but what exactly should be used?

I've seen a couple of DIY load resistor articles on the net, but not much else.

Is there something I can buy that does what I need for this?

I've read about the THD Hot Plate, but this seems to be more of a 'soak' to turn down the volume of the cab. I think I want something much simpler (and cheaper).

Any ideas?

 

[t0aj15]

The 20/20 power amp manual says if you "do not want to listen to the live speaker outputs, connect a load resistor with a rating of at least 40 watts and with a 4 or 8 Ohm impedance rating" when using the slave output.

I understand why this is required, but what exactly should be used?

I've seen a couple of DIY load resistor articles on the net, but not much else.

Is there something I can buy that does what I need for this?

I've read about the THD Hot Plate, but this seems to be more of a 'soak' to turn down the volume of the cab. I think I want something much simpler (and cheaper).

Any ideas?

I use a Hotplate (it has a load setting) for that with my mono amps, but your right, two Hotplates would be rather expensive to use on a stereo amp. I'm surprised no manufacturs have jumped on this.

 

[nocluejimbo]

https://weberspeakerscom.secure.powweb.com/weber/stmass.htm

 

[ylo]

You need exactly what Mesa says -- a resistor. You can buy one here:

http://www.electronicplus.com/content/ProductPage.asp?maincat=RE&subcat=REJ

 

[ylo]

Sorry, for a stereo amp, make that TWO resistors.

 

[fatboy135]

You EVER need to load a tube amp, ever, ever, ever. I will give you the answer but the mainly thing is that you ever remember to ever load your tube amp, ever, ever :wink:
A tube amp has a very low output impedance that is prepared to drive the load of your cabinet. The 20/20 slave, uses the el84 to warm the signal, even the slave output is made by a voltage divider of 10, that means that the slave output is 1/10 of the total voltage that drives the cabinet.
Well as power tubes are working you need to load with a cabinet because you need that the power generated be consumption on the cabinet load, if you don´t load your power tubes then the 20 watts per channel that generates the 20/20 will need to consumption and the only way to consumption them is dropping all the voltage and current just into the power tubes, so then the power that must to be driven the cabinet are only overheating the power tubes, damaging them in a couple of minutes.
Maybe you need some electronics knowledge to understand my poor arguments but rememeber EVER load your tube amp

 

[dr.indiflidual]

You need exactly what Mesa says -- a resistor.

thanks for your reply ylo

So for the two outputs I just need to wire up a jack plug with a resistor and use that instead of a cab? Is it that simple?

Does it need a heat sink? Or anything else?

Do you know if I can buy something like this ready made?

 

[ylo]

Hello dr.indiflidual

The main thing is to have a load that has the correct impedance, more or less, in this case 4 - 8 ohms, across the output where the amp expects to see a speaker.

If you run the amp at high enough volume, the resistors will get hot (note "at least 40 watts"), so you should mount the resistors on something non-flammable where they can get some air circulation. Some hot plates use fans, but if you do not put the resistors in a box, you probably do not need a fan.

Two hot plates (THD, PowerBrake, Scholtz etc.) will cost alot more than just buying a couple of $14 ceramic or wirewound resistors, but will save you some time and effort. The best hot plates include inductive loads to more closely approximate the dynamic response of a speaker, but in your case you do not need this.

I have successfully used lightbulbs in a pinch. You have to measure the resistance and wire several together to get close to 4 ohms. You could wire these in parallel and series with ceramic sockets. Note that if a bulb burns out you could toast your amp, so ceramic resistors are much safer.

I like the precision wirewounds shown here: http://www.resistor-manufacturers.com/precision.htm

They are gold in color and have built-in cooling fins. You can find them at electronic / industrial parts stores and on the web.

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[fatboy135]

I recommend to you to load with a 16 ohm dummy load, with 16 ohm the output current is less and then the dummy load will not overheating