Tube Amp and Speaker Load Impedance

JakeAC5253 · Post by **JakeAC5253** » 19 Apr 2012, 01:35

I have a strange question. How would a tube amp, assume 100w cranked stadium rock kind, react to seeing an 80Ω load impedance while set to 8Ω? Basically here is why I am asking... I want to build a passive dissipative load box for use in the studio. The load will be 8Ω and I want to implement a plan B safeguard for if the single resistor would fail, that it would not be a catastrophic failure, just enough to give the operator enough time to turn off the amp without any permanent damage. So I could use one 8Ω resistor to carry the brunt of the job and another 80Ω resistor in parallel with it. I would get roughly 7.3Ω with both resistors, and 80Ω in "failure" mode.

The sad part is that I know that I could use two 16Ω resistors in parallel performing roughly the same job, the live mode would be closer to ideal and the failure mode wouldn't be as far into danger, but well... that's just too simple. I'll probably end up doing that anyway, but actually I just want to see what the answer would be for the first one. And if anyone knows of a good reliable way to indicate to the operator when the device has gone into failure mode, I would like to hear it.

defaced · Post by **defaced** » 22 Apr 2012, 14:20

Pretty standard safeguard. Often it's 120R or 220R, 5W if often used, but you'd be best to run the numbers to see what you should use. The way I understand it works is if the primary load fails, reflected primary impedance goes through the roof, so output goes to near-zero, but flyback voltage isn't *as* bad as a full open secondary, so you have a better chance of saving the OT.

If you were interested in using a microcontroller, there are other ways of achieving your goal. RG Keen has suggested this approach on the Music Electronics Forum a couple of different times, though I don't think anyone has actually done it.