Postby KevinOConnor » 18 Jan 2011, 12:04
Hi Guys
There is a myth that operating a tube amp without a load will damage it - and this myth is incorrect. As we have detailed in our books, the only time an open load is an issue is when the load becomes open while the amp is driving it. This is called a "discontinuous load".
At the point of disconnection under drive conditions, the amplifier output can spike, just as the mains can spike when industries go off-line. The amp output - just like the power distributor's generator - has been delivering high current at a given voltage, trying to regulate the voltage output to a specific value. Disconnection of the load causes an irregularity in the regulation for a partial cycle or two, and the output voltage is suddenly unregulated.
If you disconnect the speaker and feed a signal into the amp, you can drive the amp to clipping, etc, to see how the amp behaves. This is a normal part of bench testing for new amp production and for repairs. You can run the amp like this forever without any issue. It gives a good indication that the amp is working properly, but the real test is to add a bench load and see how it behaves under load.
As was pointed out earlier in this thread, the headphone jack is fitted with a 100R resistor to protect the headphone. This is very common and is standard practice in hi-fi and MI and is perfectly safe to the amp. When you plug in the phones, the speaker is disconnected and the amp sees 100R plus the headphone resistance - which is insignificant compared to the 100R. The amp is now driving 100R instead of, say, 8R. Does the amp care? No.
Speaker impedances are "nominal". An 8R driver is only 8R over a portion of its frequency response. At resonance, driver resistance can soar to 80-100R. Impedance also rises in the treble range to many times the nominal value. This has been the case since dynamic drivers were first built prior to the age of electronics. Again, the amp is designed to deal with such loads because that is the nature of dynamic drivers.
Open loads are no problem to the amp. Varying impedances are no problem to the amp. The only things of issue are discontinuous loads as above, and shorted output. The latter will really cook an output stage.
The hard-wired connection of an amp to the speaker in a combo amp is a given. Fender dropped its speaker jacks back in the 1980s. It is s a simple matter to add such jacks, but they will void the amp's warranty. Having such jacks certainly makes experimenting with home-brew and store-bought attenuators easier.
In my experience, the Marshall OTs that failed did so because of bias failures in the output stage. These bias failures were most often caused by tube defects - poor quality EL-34s,and tubes that had been mishandled.
Have fun
Kevin O'Connor