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Can You Operate a Vacuum Tube Amp Safely Without a Load? |
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By S. Lafferty, 12/13/09, Rev. 2 |
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It is often said that you should never operate a vacuum tube (VT) amplifier without a load. I have sometimes wondered about how true that is but until recently had not really delved into it. A friend and I discussed the matter in a series of email messages and came up with the rules and reasons given below. You may be surprised at the results. It is often not necessary to load a VT amp. Of course, you can always be safe by testing with a proper load resistance. But there are times when that isn't convenient, so it's good to know the real limitations on safe operation. Knowledge of the details is as important to avoid damage as it is to increase flexibility. Once, when my friend and I were doing A/B testing, comparing a VT amp with a solid state (SS) amp, we found ourselves with a switch which didn't provide a load for the off-duty amp. Both amps were being driven, continuously. Though we could easily have worked-out the limitations this placed on operation, this was before we had really sat down to do that formally. We rationalized that this was a very stable VT amp and we wouldn't be pushing things very hard. As the testing went on, we decided to push the SS amp to clipping, and then without really thinking about it, wanted to compare that to the VT amp clipping. We were going back and forth, when he suddenly exclaimed and lunged to cut the power to his EL34-based VT amp. He had noticed that the screen grids were glowing brightly! Fortunately, the EL34 output tubes were not actually damaged, though, from all the howling and moaning, his nerves weren't so lucky :) We had violated one of the rules of operating without a load, given below, by driving this ultralinear amp into clipping. I hope that they can help you avoid similar issues. Rules of Operating Vacuum Tube Amplifiers Without a Load 1. These rules only allow amps with negative feedback (NFB) to be operated without a load. Guitar amps and hifi amps which have little or no NFB are not considered here. The reason for this is that the NFB constrains the amp to a fixed value of gain. The gain of non-NFB amps may rise greatly under no-load conditions. 2. Only amps which are stable without a load can be operated that way. Most NFB amps are less stable without a load. Some might actually oscillate. It would be very rare for a properly working commercial amplifier to do that, though. 3. Triode amps have no risk of damage. That is because the principal risk in unloaded conditions is excessive screen grid dissipation. Triodes do not have a screen grid and the screen grid of pentodes connected as triodes will not have excessive dissipation if the plate dissipation is in spec. 4. All amps which are not driven into clipping have no risk of damage. As long as a pentode amp isn't driven into clipping, its screen grids will not draw excessive current. 5. Ultralinear amps should not be driven into clipping in unloaded conditions, because the screen swings typically only 43% of the plate. That leaves the screen voltage at a high enough value to cause damage, when the plate bottoms-out. 6. Pentode amps do risk damage, when driven into clipping. Screen dissipation could rise high enough to do damage, in continuous operation while clipping. This is because the plate voltage pulls low and the screen remains steady, diverting a large amount of current to the screen. In conclusion, we see that triode amps and any stable NFB amp not driven into clipping, can be operated safely without a load. This narrows the scope of the old rule of thumb saying that all VT amps must be operated with a load. Comments are welcome. Please see the About button on the home page for the email address. |
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