8 ohm amp into 30 ohm dummy load,does it hurt ?

[modmod]

i am totally beginner and found out this thing at other forum :



it's a 30 ohm dummy + ss poweramp attenuator, i'm just wondering if it's safe to use this with my deluxe reverb which is 8 ohm , is it kind of "under-load" ?
Also if anyone got the similar project for DIY ? I'd like to diy a dummy load + line out for computer recording.
thanks in advance !

[Paul Perry (Frostwave)]

As a general rule, tube amps don't like being run into a higher than intended impedance load , but solid state amps can usually handle it.

[Connoisseur of Distortion]

i was under the impression that a tube amp could accept a higher than intended load.

i thought the danger was in under-loading it...

[d95err]

it's a 30 ohm dummy + ss poweramp attenuator, i'm just wondering if it's safe to use this with my deluxe reverb which is 8 ohm

The dummy load resistor should have the same resistance as the impedance of the amp. (I.e. 8 ohm resistor for an 8 ohm amp). Pick the closest resistance value you can find. 30 ohm for an 8 ohm amp output is way off and could probably damage the output transformer.

The important part is the power rating of the resistor. It needs to be able to handle more than the power output of your amp (guitar amp power is rated when the power amp starts clipping, you would usually run it a lot harder, especially when using a dummy load). You also need a big margin for safety. I would a resistor that can handle at least twice the power rating of the amp (i.e. 100W resistor for a 50W amp).

The power rating of power resistors depend on how you mount them. I'm using a resistor just like the one in the picture for an amp I'm building. It is rated for 50W, but only if you mount it tight to the chassis. By itself, it can only handle 20W. If you mount the dummy load inside the chassis, the temperature will go up a lot, and the resistor will be able to handle even less power. I mounted mine on top of the chassis to let it have plenty of air to "breathe".

[Paul Perry (Frostwave)]

i was under the impression that a tube amp could accept a higher than intended load.
i thought the danger was in under-loading it...

One possible source of confusion is what is a 'higher' load.
As the impedance (ohmage) increases in numbers, the actual "loading" of the amp decreases (less power is being sucked from it). This may actually be bad for the tube amp, because it could lead to higher than normal peak voltages on the tubes.
I'm no tube guru though, so I hope the experts here will chime in.

Short circuiting the output of a solid state amp, or running it at too low an output impedance, can be sudden death (if it does not have adequate built-in protection). As anyone who has shorted out the speaker wires at the back of a cheap hi fi knows...

Momentarily shorting a tube amp output is usually OK (though I don't recommend it).

[modmod]

I know few guys are using it,one of them is Doyle Bramhall II,so i wonder it sohlud be save for 8 ohm tube amp ,and i'd like to find a similar project for DIY. thanks guys