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. 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.
The bottom line: Triode amps and any stable NFB amp not driven into clipping, can be operated safely without a load.
Said another way: The only cases in which operating without a load causes a problem are pentode and ultralinear amps driven into clipping. (Amps without NFB also.) This narrows the scope of the old rule of thumb saying that all VT amps must be operated with a load.
Comments are welcome and you can email me directly at the address given here.
March 14, 2013 - 01:57 pm|
|Hi Tom, Thank you for your kind comments. Since bias is adjusted with the amp at idle, whether or not a load is attached does not matter. That is a great example of a case in which the rules can help. Doing a quick bias tweak on the bench need not involve attaching the loads. |
March 14, 2013 - 01:33 pm|
Very interesting information! I have fallen into the category of one who lives by dummy loads! So, having made the investment in several sets, I'll probably just keep using them. But, I'll take some solace in knowing that should I ever fail to remember (and old age can do that to ya), or if I ever find a broken switch (ha ha), I'll at least recall your comments and not have too huge a panic attack. I am curious about one thing; does the 'lack of load' have any affect on bias draw, such that either 'with' or 'without' might affect proper adjustment when in actual use? Many thanks for the articles and website! Great Stuff! Tom D.
February 14, 2010 - 08:20 am|
|Hi David, Thanks for the kind words! Will give the transformer questions some thought... |
February 14, 2010 - 07:35 am|
|Nice write up! These are great rules from the perspective of safety for the output tubes. As a suggestion, it would be nice to also include any relevant rules from the perspective of safety for the transformer as well! |
February 13, 2010 - 08:09 pm|
|Please feel free to add any comments or questions you might have. No sign-up is needed. |
Add your comments here...