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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 done 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. All amps which are not driven into clipping have no risk of damage. As long as a pentode or ultralinear amp isn’t driven into clipping, its screen grids will not draw excessive current. Similarly, amps not driven into severe clipping will not suffer inductive spikes which are otherwise possible.
4. 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.
5. 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.
6. I found in the lab that unloaded triode amps driven into severe clipping by lower frequency square waves do risk damage from inductive spikes. There may be other such cases involving clipping.
The bottom line: You can operate safely, any stable, NFB, vacuum tube amp without a load, as long as you don’t drive it into clipping.
One useful result: There is no need to connect a load to a normal vacuum tube amp if you just want to adjust bias.
Comments are welcome and you can email me directly at the address given here.
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Reader Comments
| Posted by
Steve L. |
January 19, 2023 - 10:44 am | |
Hi Tristan, Thank you for your kind comments. I'm happy that this put your mind at ease about changing amp connections when on-but-idle. Yes, it can be disturbing to hear music from an output transformer (OPT) with the speaker disconnected but it doesn't necessarily indicate undue stress. This sound is ascribed to magnetostriction, which is the change in physical dimensions which a ferromagnetic material (such as the OPT core) undergoes when a magnetic field is applied to it. The effect of the contractions and expansions is to create small acoustic waves in the air. It's always there to some extent while music is playing but you don't normally notice it because the speakers are on.
Incidentally, it turned out that it's not so easy to generate those feared inductive spikes as one might think. In my lab tests, I'd almost given up on seeing any spikes until I realized that low frequencies could cause them more effectively than higher ones. The transformer inductance needs time to build up energy in the magnetic field. Short cycles don't give it time to do that as much. I had been testing an unloaded triode amp with overdriven square waves around 1000Hz and hadn't seen any spikes. But they did show up when I tried low frequencies.
However, notice that a bass guitarist who's amp is way overdriven could meet the criteria if the speaker voice coil opens while playing. I've had a report from a musician who experienced a blown voice coil damaging the OPT in that scenario. Back in the hifi world, though, that would be extremely rare indeed! |
| Posted by
Tristan H. |
January 18, 2023 - 07:48 pm | |
This is a fantastic practical write-up, thank you. I've got an amp-switch that doesn't connect my tube amp (EL34 Push/Pull) to a load when it's switched off of it, and I've often wondered if it was okay to swap it when the amp was on, but idle, or as the amp was cooling down (takes about 10 seconds for the HV caps to clear). I've read a lot of FUD about this configuration, but nothing actually useful until now. Especially considering there's little to no danger when there's no signal into the amp, I'm fairly confident in my switch as long as I don't turn the volume up all the way.
I did do that once to this amp. I was wondering why there was no music, and cranked the pre all the way up. I heard the music alright—as arcing in the transformer. So, don't do that. But otherwise, it's probably fine. My amp survived even that 5 seconds of fire no worse for the wear. |
| Posted by
Steve L. |
June 27, 2019 - 05:48 am | |
| Hi Craig, Glad that this was helpful. Thank you for your comments and best wishes for the restoration. |
| Posted by
Craig T. |
June 27, 2019 - 01:07 am | |
| That is timely information. I'm just now working on restoring a Universal Audio power amp and the question arose yet again. There is more to it than just core loss and hysteresis currents. |
| Posted by
Steve L. |
June 17, 2019 - 06:34 pm | |
Hi Robert, There is some concern about operating tube amps without a load IF they have little or no negative feedback (NFB). That's because the voltage gain of the output stage could increase greatly which would risk clipping. That clipping could threaten the screen grids of tetrode and pentode output tubes because plate current is diverted to the screen when the plate voltage drops too low.
With stable NFB tube amps however, the overall gain of the amplifier is about the same, with or without a load. Hence there is no additional risk of clipping without a load. I guess some would imagine that the voltage on the primary of the unloaded transformer might somehow spike or rise in some uncontrolled manner. So long as the amp isn't driven into severe clipping, this won't happen because the NFB keeps the output voltage tightly constrained. Since the primary and secondary voltages of an output transformer are tightly coupled, control over output voltage also insures the primary voltage is well-controlled. Thus, there is no risk of an outlandishly high primary voltage.
I suppose some might argue that, without a load on the amplifier, the power supply voltage would rise and that could lead to overvoltage. But this is not much different from the amp operating at idle and the supply voltage rise is modest. The extra voltage swing will be well within transformer limits. Dave's comment from 2010 quite rightly suggests that effects on the output transformer should be considered and the article has been amended to prohibit clipping without a load, as a result. Many thanks for raising this interesting topic and for giving me the opportunity to address it. |
| Posted by
Robert K |
May 18, 2019 - 03:26 pm | |
| Echoing what David said, the output transformer has always been cited as the reason in conversations where someone has explained to me why not to run a tube amp without a load. The idea, AIUI, is that no-load operation at moderate to high signal levels can cause arcing between adjacent turns in the OT windings because the load on the secondary is needed to keep the instantaneous voltage in the primary below rated levels. |
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