noelren Posted March 11, 2008 Share Posted March 11, 2008 I have a couple of Valve Jr heads which I use live 1-3 times weekly. I have powered up the heads numerous times without a load (due to forgetting to plug the cab in) and it often takes a few moments or even minutes before I realize what I've done. I know it's bad for the amp to do this and my question is what part or parts of the amp is this hardest on? And what kinds of problems could be caused... sound degradation... , shortened component life span... , fire hazard!?! Thanx guys, Chris in Baton Rouge. Link to comment Share on other sites More sharing options...
cGil Posted March 11, 2008 Share Posted March 11, 2008 Fire hazard? You mean as your output transformer goes into thermal overload and melts insulation, thereby causing shorts which cause more heat, which melts more insulation, which causes more heat... and winds up like a mini Chernobyl meltdown? Could be. Most of the time when one forgets the speaker, one is not actually using the amp... like playing. It's when you plug your guitar in and nothing happens that you notice something wrong, and which point you flip the amp off really quick. Fortunately, as long as there's no signal input, there'll be no damage to the output transformer. The Mesa guys clued me into this trick a while back regarding using only one channel of my Mesa 20/20. They don't recommend doing that with other amps, though. However, if you ever forget the speaker and in a panic you plug it in without turning off the amp first, your OT will definitely be instant quick fried toast. Probably doesn't do the tube or the speaker much good, either. So anyway, you got lucky. Now don't do that again! Gil... Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 12, 2008 Share Posted March 12, 2008 I've done that a bunch of times. Never hurt my amp. Won't do it again. Link to comment Share on other sites More sharing options...
TWANG Posted March 12, 2008 Share Posted March 12, 2008 my repeat mistake has always been to power it up and forget to put the tubes back in first. someone reveresed theirs once.. that's a definate no no! Link to comment Share on other sites More sharing options...
layboomo Posted March 13, 2008 Share Posted March 13, 2008 Here's the deal...SS and Tube amps are the exact opposite with regard to output loads. While a SS amp will run all day long with no load...a short (even a quick pluging in or unplugging) will fry the outputs. A tube amp will be far more forgiving of shorted outputs and impedance missmatches,but a "no load" situation with signal for any length of time is a Big No No! Link to comment Share on other sites More sharing options...
cGil Posted March 13, 2008 Share Posted March 13, 2008 Unless you're a tech, in which case sometimes you get lucky. Sometimes not. I left an HO amp I was tweaking last night on all night to bake the caps, but forgot to plug in my bench load. I just discovered it. The OT was still nice and cool, so I turned it off, plugged everything in properly and fired it up. No sweat! However, a month ago I did the same thing but only for a few seconds. The difference was, I plugged the speaker in while the amp was on and volume was up. Scratch one brand new Hammond 125ESE! Toasted that sucker in a millisecond! Gil... Link to comment Share on other sites More sharing options...
layboomo Posted March 13, 2008 Share Posted March 13, 2008 Unless you're a tech' date=' in which case sometimes you get lucky. Sometimes not. I left an HO amp I was tweaking last night on all night to bake the caps, but forgot to plug in my bench load. I just discovered it. The OT was still nice and cool, so I turned it off, plugged everything in properly and fired it up. No sweat! However, a month ago I did the same thing but only for a few seconds. The difference was, I plugged the speaker in while the amp was on and volume was up. Scratch one brand new Hammond 125ESE! Toasted that sucker in a millisecond! Gil...[/quote'] Yup I believe it.....you would think looking at the Hammond ESE that it would survive but NOOOOOOOOOOO! It's funny(or not actually) but your natural instinct is to quickly plug the load in which is the worst thing you can do! Link to comment Share on other sites More sharing options...
Dave Posted March 23, 2008 Share Posted March 23, 2008 I'm not sure what kills these things in the real world, but theory suggests that without a load, there is no wattage produced, thus no heat and no strain. Power=voltage x amperage (ohms law (P=EI) for wattage. Most tube amps are designed with a load resistor wired to the output when the speaker is disconnected. The load resistor prevents damage if you unplug the speaker accidentally. Provided that there is no input signal, no power should be produced. Since the load on a tube amp is inductive, X sub L = 2piFL (Inductive Reactance = 2 * Pi * Frequency in hertz * inductive load in henrys). Somebody do the math using 60hz for the freq and the inductance of the output transformer when no speaker load is connected. I'm betting that the load will be negligible since the secondary of the output transformer can't produce a load on the primary when no connected to a speaker load. Remember that the inductance of the transformer changes with frequency and that we are not providing a drive signal from the preamp. I could see where driving the power amp with no load wouldn't be a good thing. If your amp doesn't have a load resisitor, the OT would be left with no balanced output to absorb the load. That could be a messy situtation. With no signal, there should be no corresponding power produced on the plate, thus no instantaneous self destruct. With a signal, tubes can create some out of range output voltages that can arc and destroy the sockets and go up in smoke. The moral here is never drive the amp with the speaker disconnected. If you don't hear sound, shut it down immediately and check the connections. Link to comment Share on other sites More sharing options...
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