Hi, If you are looking at this forum then you are likely to be both using tubes in projects and doing some diy builds.The subject of how to do a first power up (or power on if you rather) comes up from time to time. There are many ways this can be done. I will tell about how I do it as I have numerous occasions during the developmental process for the projects I post. My way is not the only way, but it works quite well for me. This method was developed over several years to do two things. First to make sure that safety is paramount. Getting fried by a project is not a good thing.
Second to minimize false indications that a project has problems on start up when it really doesn't. A typical thought process is to test each portion of a project separately. Fine, but tests of power supplies in tube gear can damage some components if there is no load and at the minimum give false readings. As a third, but not primary focus, the procedure has so far prevented any serious damage to the projects. For sure I have cooked a few small resistors, but major and / or costly components never.
1. First, most importantly check the wiring. Verify that the correct parts and wires go to the right tube pins. Check the drawing or schematic against the build.
2. Turn the thing upside down and shake it to remove any loose wires, parts or solder.
3. If it uses tubes, put them in place. This is not intuitive and seems like the wrong thing to do. The reason for doing this is that some voltages, particularly the B+ ones on the various tubes will not be correct if there is no load. Worse, you may exceed the ratings on the filter caps if there is no load as they will charge up to the peak voltage (and not the normal operating voltage). If you think about this a bit.... tubes are quite rugged and usually can handle severe operating conditions for a while. They are not usually in danger from faults during an initial start up and voltage check.
4. Attach at least one meter and really as many as you can to key points in the circuit. I use the B+ going to the first filter and the B+ to the last filter as a minimum.
5. Be sure to fuse the equipment. I use slow blow fuses in the AC mains side of the transformer. Usually 3-4 times the expected current level. I use 4-5 amp ones in the KT88 mono blocks (they draw just over 1 amp in operation). This is because there is a very large initial surge on power on. The filters in the B+ chain are a virtual short circuit when you use solid state rectifiers, plus the cold heaters have a far lower resistance than when they are fully warmed up. I find this size fuse to be about right. If you have a serious short or failure it will blow in fairly short time. Remember tubes are really rugged. Too small or the wrong type of fuse will lead you to believe there is a fault when there is none.
6. Now this is important.
Do not plug in the project. First turn on the power switch if it has one. Then if you use IEC power cords like I do (those are the ones that you see on desk top PCs). Attach the end that is going to the chassis first. Now,
do not touch any part of the project (in case there is a serious fault and the chassis gets energized). Now plug the other end of the power cord into the AC mains. The reason for this procedure is to insure that you do not become part of a potentially energized circuit with potentially serious consequences. Some folks will use a Variac to power up the first time, I do not (even though I have one) as it really didn't protect the project all that much (still had to see if the fuse blows....) and didn't add to the personal safety issue.
7. If the fuse blows, you have a fault and need to find it.
Do not touch the chassis, pull the power cord out at the mains end. If the fuse does not blow proceed to the next step.
8. Check the meter readings. If OK proceed. If not shut down by pulling the plug - still not touching the chassis.
9. At this point I like to get a meter and check for AC voltage between the chassis and a known AC ground. Just in case.
10. Now you can start to verify other circuit values, like heater voltages and such. If something is way off, shut down and hunt for the problem.
11. The next step is to make any adjustments that are required like setting voltage regulators for the exact values wanted. Virtually any circuit - even those with large power tubes will not be harmed during the short interval needed to do the previous checks.
12. Now you can hook up inputs and outputs to see if the project works.
13. Something not usually critical, but that can be important if the project is a power amplifier is to put a load on the output connections. A fixed resistor is fine. I use 40 watt 8 ohm ones for this. The reason is that some amplifiers will be unstable and oscillate without a load. Not generally harmful, but it can throw off the voltage readings. (BTW, Oddwatts are stable with out loads). Another similar hint that sometimes matters is to short the inputs to an amplifier so it will not see any input signal (noise or hum) on start up and cause the voltage readings to be off.
I hope this procedure will be of some value to you. If someone has others let's hear about them. Like I said at the start, there are several methods to do this.
Good listening
Bruce
Howdy, A great tip on the exchanged outlets. I do have devoted exchanged outlets, yet figured the average diyer would not. I likewise utilize an APC H10 AC mains control conditioner before the switches
Login
Register
FAQ
