@cavscout19d I appreciate you sharing the detailed explanation of the ‘how and why’ tube amps need to see a load upon power-up. I found out the hard way when a blew up a beautiful old Crosby radio because I didn’t know the speaker needed to be wired up. I accept @gs5556 correction concerning ‘ideal’ theory vs. actual implementation. I would be very curious to learn how Cary builds their tube amps so that speakers don’t need to be connected. I did run across a video recently in which a tube amplifier incorporated a very special RCA jack that I think was shorted until the RCA plug was inserted. It was in a video in which XrayTonyB explained how ‘multiplex FM’ worked. The video featured an H. H. Scott FM de-multiplexer that de-coded the FM Stereo signal so that a mono FM tuner could feed a stereo amplifier, but I digress.
Quality answer to dangers of Tube amps running under no load
Messing with my VAC tube mono 160's, wound up surfin' tube related stuff and came across a guitar techs answer to a blown Marshall. Thought it was cool, figured I'd share:
"The answer lies in impedance and its sibling inductance, which affect AC signals like those that pass through your amp in use. The power tubes generate AC current, which works against the impedance offered by your output transformer and speaker together to create voltage and thereby power. Basically, current (I) times impedance (Z) equals voltage (V).
Now, transformers don't really have an impedance on their own. They multiply (in a fixed ratio) the speaker ohms a few thousand times to a level that's ideal for your tubes, which is a few thousand ohms. 8 ohm speaker = 8000 ohms to the tubes, for example. The tubes work against that impedance to make AC voltage, which can normally peak at around 800V or more.
If you have the speaker unplugged, the load is now nearly infinite ohms. That gets multiplied by the transformer to an even higher number of ohms (hundreds of thousands or even millions) that the tubes are trying to work against. The result of this is that instead of making hundreds of volts, the tubes now try to make thousands of even tens of thousands of volts. That much voltage can destroy the insulation in your output transformer, and can cause tubes to arc and self destruct. Some tube amps, but not all, use shorting jacks on their speaker jacks to help protect against this but plugging in a cable with nothing on the other end can bypass this feature and still cause damage".
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I get a lot of amps back that customers powered up forgetting to hook up speakers or a load. Usually due to excitement. Depending on the topology, the damage can be different. In this situation, the cathode resistor goes up in flames. |
Solid state amps can also do this. My Sunfire Signature sub is now not working because it powered up without a speaker connected. The amp had been separated from the speaker cabinet.
My "Universal Switcher" unit (for use of comparing any two components/speaker systems) has provisions to switch in 261 ohm, 30 watt resistors for use with tube/whatever gear so that even when the relays are open (milliseconds) during switching operations, there is a load on the amp. |
@gs5556 , that is a good idea. It's kind of like the (unwritten?) mechanic's rule that if you are installing a bolt and a nut and the maintenance manual does not actually spell out directionality, to install it with the head of the bolt pointed up so that if the nut was to back off the bolt wouldn't immediately fall out. |
Well. - I have no output transformer. So does this still apply? - Four months ago, I powered up my left monobloc. There was a bright flash and a loud pop. It blew the 15” diaphragm in my Altec 604Cs. A tube had apparently arced at powering up. Now, I disconnect my speakers before I turn the amps on, then reconnect them as soon as they start warming up. - the tubes were all NOS, so maybe it was just that tube blowing but I’m afraid to leave the speakers connected at startup Thoughts? |
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