resistive load to tests?

[lowell]

is it ok to run a tube amp into a resistor to ground while testing the amp on the workbench? It can get pretty annoying listening to that oscillator. if this is ok what value/voltage rating for the resistor?

[Albert Kreuzer]

Hi Lowell,

yes, perfectly ok.
Same value as the appropriate speaker(s), e.g. 2, 4, 8 or 16 Ohms.
Make sure the wattage is at least twice that of the amp and provide some kind of cooling (heat sink, fan, or both). This thing will get hot.

Cheers,
Albert

[olddawg]

I'm not quite sure about this. I worked professionally on SS amps for years. The load did not matter in most cases as long as it was not below the minimum rated load for the amp so you would not draw to much current from the output transistors.

In a tube amp you have an output transformer. This interacts with the inductive load of the speaker. One of the reasons power soak attenuators are massive and expensive is because they have to be made with an inductive load in order not to destroy the output transformer of the amp when it is cranked. I would be very careful using a non-inductive load on a tube amp. I know there are bench tricks using light bulbs and such. At one time I had a commercially made bench inductive load bank with big wire wound resistors designed for 250 watts max.

Maybe someone like Enzo with years of tube amp bench experience could comment on this. I would like to know for sure myself.

[CarlZ]

Absolutely perfectly safe and actually recommended! It's called a dummy or dead load, and you really really dont want to run a tube amp without some sort of load on the output. The easiest way to make these is to get one of those little metal project boxes from hammond then add in a 4, 8 and 16 ohm wire wound resistors of 1% tolerance to their own jacks. You just plug the speaker jack into its matching load and youre golden! Use either Dale or Arcol wirewounds of around double the rating of the amp and you're good to go. They'll run you about 10 bucks a piece from Mouser depending on the rating, so they're really a minimal investment if you're doing service work.

Carl Z

[Enzo]

Using the Dales and Arcols, I find it cheaper sometimes to use several 50 watt ones instead of one 250 watt resistor. I have a bunch of the 250 watt Dales, and I'd hate to have to replace them now. I'd be stuck eating hot dogs for dinner for weeks.

ABsolutely nothing wrong with resistive loads for tests. The reason power attenuators like to add inductance is to more closely mimic the load of a speaker so the reaulting low power output SOUNDS more like the amp into a speaker at full power.

WHat destroys the output transformer when cranked is the LACK of a load.

Will the resistor exactly mimic a speaker? No. But for troubleshooting and bench tests it works just fine. The differences between a resistor and a speaker would be things you'd have to hear anyway - can't measure tone on an oscilloscope.

I suppose it is possible the amp might respond to a lot of inductance in some electrcially significant way, but I have yet to see it matter in a tube guitar amp. Frankly I'd be more worried about some SS amp being unhappy.

[jrfrond]

DEFINITELY necessary to load a tube amp, unless you like the smell of arcing output transformers!

The 250W Dale loads are pretty universal. We use those and 1000W Milwaukee loads for real big stuff.

There have been a few occasions in my tech career when non-inductive loading would not allow particular problems to manifest themselves. Not really enough to worry about, but I do keep a simulated inductive speaker load around for such occasions.

[lowell]

ok thanks guys... how about ss? ss amps do not need a load right? is it bad to have a LARGER load than specified?

[Albert Kreuzer]

ok thanks guys... how about ss? ss amps do not need a load right? is it bad to have a LARGER load than specified?

1. Right.
2. No. Anything between open and the minimal load will work.

Cheers,
Albert

[Enzo]

Larger is a confusing word here. SS amps don't want to see a lower impedance load than specified. It is a larger load in the sense that the amp works harder, but it is a lower impedance that does so.

So, agreeing with Albert, right, SS amps need no load, and do not put a lower impedance load on an SS amp than its rated for. ANy higher impadance up to an open is fine.

[lowell]

great thanks all.