Amplifier testing/repair and dummy loads

[Mick Duprez]

Hi All,

I have watched some videos where the presenter didn't use a dummy load for signal through the circuit. I can't remember correctly but I think it was a valve amp and the volume and gain was set 'just on', he did mention dummy loads but noted it was ok how it was setup for that purpose.

Is this setup only suitable for valve amp's or can this be used for solid state/mixed technology amp's?

As a safe alternative, if I build say just a 100 watt dummy load (for each channel) would that be ok to use with low gain/volume just for signal tracing? I'm thinking it would be ok as I'm not pushing too much 'power' around with low settings or will I need more power to test the signal path correctly?

The amp's I'm working on are usually around 450 rms watts per channel and some much greater but I'd like a simple setup just for signal tracing/testing for a complete audio path, not testing for clipping etc. I do intend on building a large dummy load at some point using something like water heaters in a drum but that's a bit cumbersome at the work bench for preliminary testing etc.
Are there any other alternative for this type of testing to narrow down to the problem area?

Thanks,
Mick

[donlisms]

Solid state amps are perfectly happy to run with no load.  Fire 'em up and take all the measurements you like.  It's tube amps that have trouble. 

[Mick Duprez]

Solid state amps are perfectly happy to run with no load.  Fire 'em up and take all the measurements you like.  It's tube amps that have trouble. 

Thanks for confirming that, it will make things a lot easier

cheers,
Mick

[TimFox]

The hazard with unloaded tube amps is not from the tubes themselves, but from the output transformer coupling the tubes to the load.
Without secondary load, the load presented to the tube plates is the magnetizing inductance of that transformer, which is a very high impedance.
Very few solid-state audio amplifiers use output transformers.

[Mick Duprez]

The hazard with unloaded tube amps is not from the tubes themselves, but from the output transformer coupling the tubes to the load.
Without secondary load, the load presented to the tube plates is the magnetizing inductance of that transformer, which is a very high impedance.
Very few solid-state audio amplifiers use output transformers.

Thanks for that extra information TimFox, that makes sense if I understand correctly:
If I have speakers then the load is removed from the output transformer reducing the impedance, that is, it gives the constant build up of current somewhere to go whereas with solid state, the current is simply reduced (due to things like feedback etc) as it has no where to go?

cheers.

[Mick Duprez]

I found this whitepaper from Peavey which helps explain this in much more detail:
https://peavey.com/PDFs/Chapter7.pdf

Thanks All for your help!