DIY Audio Projects Forum

How to Power Up DIY Tube Projects for the First Time
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Author:  gofar99 [ 22 Jan 2012, 14:47 ]
Post subject:  How to Power Up DIY Tube Projects for the First Time

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. :blush: 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

Author:  Les [ 23 Jan 2012, 01:09 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Good tips Bruce! :thumbsup:

Some are probably not thought of by some DIYers. The only thing different that I do is that I have a dedicated switched AC outlet for my tests. I plug the project in, then flip the outlet's switch.

I think one of the best things you can do for yourself when you set up your work bench is to incorporate 1 or 2 switched outlets - 2 is better as you can plug some test instruments in to one and power them all on/off with a single switch, and then one dedicated for testing projects.

Author:  Gio [ 24 Jan 2012, 21:27 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Great tips Bruce - thanks for sharing these. I have made this a sticky thread. Please PM me if you need to edit or update the first post. Another alternative to make a permanent page on the main site with the info.


Author:  gofar99 [ 24 Jan 2012, 21:45 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Hi, A good tip on the switched outlets. I do have dedicated switched outlets, but figured the typical diyer would not. I also use an APC H10 AC mains power conditioner in front of the switches. For things that I feel might be hazardous (like stuff sent me as it doesn't work) I have a pair of GFCI outlets as well. I have been bit by the electrons :blush: in the past (long time ago) and do not wish to repeat the event.

Good listening

Author:  Geek [ 25 Jan 2012, 06:25 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Might I suggest goggles?

Silly me one time had wired in a 47uF, 450V cap in backwards. The 5U4 warmed up and *sssssssssssssssssBANG!*

Hot electrolyte everywhere! :blush:

Author:  gofar99 [ 26 Jan 2012, 21:45 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Goggles. Another good tip. :thumbsup: I have prescription safety glasses.

Good listening

Author:  alexg [ 27 Jan 2012, 23:25 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

A friend uses a Variac to slowly power up his DIY tube projects.

Me, I don my helmet and hope for the best! (am joking of course).

Author:  alexg [ 27 Jan 2012, 23:27 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Serious now, don't use your best speaker to test your amp, I use a pair old 6" car speakers.

Author:  Bill in AZ [ 13 Feb 2012, 14:03 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Good tips guys. Thanks for sharing. I want to get a pair of the Oddblocks in the next month or so and because I hate hunting for "that last part" or two, I will but the kits. Might do a custom chassis..

Author:  Tube Tinkerer [ 20 Feb 2012, 19:30 ]
Post subject:  Re: How to Power Up DIY Tube Projects for the First Time

Thanks for sharing... I also use an old car speaker as a test load for projects. Wouldn't want to damage a good speaker finding out a project doesn't work. Great idea with the switched outlet. I always double check my work, but if I'm powering up a vintage amp or something that includes some older wiring and components I like to bring it out to the barn and use a several foot extension chord. I place the unit in an open space, stand back and hope for the best. If the fuse doesn't blow and there's no smoke after a couple minutes I figure it's safe to bring inside and test on the kitchen table.

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