DIY Audio Projects Forum

Hi.

Too true.
Even old hands like us can overlook such miner details that can be fatal.

Same thing happened to me when I opened up for the first time a disposable camera to 'explore' what makes it flashes, The sparks caused by 1KV-4KV HV discharged from the HV flash charge cap stunted me!

c-J

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"If the facts don't fit the theory, change the facts" quoted Albert Einstein.

Ok. I came across this picture on the forum the other day and it made my blood run cold!
Now I realize that everybody prototypes in their own way. But this is EXTREMELY dangerous and certainly no way to handle high voltages. Looking at this picture, I wasn't quite sure where to start so I thought I'd start at the beginning.

This circuit appears to be connected directly to the mains without the benefit a a switch, fuse, nor any type of protection. So I thought I'd start there. Mains voltage is lethal. It will kill you if you give it the chance. So don't. To that end, here is my first recommendation. Develop a simple interface to allow you access to mains voltage that provides some protection. Here is my simple solution.
This simple box provides a sure connection to the main plug, integral grounding protection, a power switch, a live indicator light, surge protection, fused over current protection, and sure rigid connection for power transformer leads. And the best part is it cost me less than $10 USD and about 1-1/2 hours to put the whole thing together. And here is the simple schematic.
If you are going to be working with straight mains voltage you need something like this.

Here are some other simple rules to follow when working with high voltages:
1) Every component should be rigidly attached to something (a screw terminal, Fahnestock clip, etc.)
2) Avoid alligator clip leads. These pull off and can represent high voltage snakes just waiting to bite.
3) Keep it neat! A tangled mess of high voltage wire is a SERIOUS accident just waiting to happen.
4) Think, Check, and Think, Check again before throwing the power switch.
5) Never work with high voltage when you are tired, distracted, upset, intoxicated, or in any way impaired.

Ok. I'll get off my soapbox now.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

matt, i saw the same thing and it concerned me greatly as well. your points 1-5 sum up pretty much how to not get dead.
your little box there is a very very good idea. the only thing i'd add to it is a safety switch - prior to starting on this journey i went and spent a small fortune on a very expensive power board with safety switch - so if i do get anything a little wrong, hopefully that will let me know about it before i get dead.

marks idea of testing with one hand only at any given time is also a win. if you're using both hands, your're doing it wrong

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LUCAS - god of darkness.

I'll add my .02 cents.

I use a variac for the first power up. I start at 0 volts and go to 10V. I can usually find mistakes at low voltage. If you use a seperate transformer for your heaters you can then use the variac on your B+.

A lot of input in to this thread has focused on safety - and rightly so. But for beginners (and not so beginners) this little tid-bit is really important (and generally applies to both tube and solid state amplifiers):

While in your designing phase you may consider variables like power output, THD, linearity, NFB or no, it's easy to forget the most important part of the whole project - the power supply. It may not seem intuitive to think about the power supply for anything beyond correct voltage and current handling, but where do you think the amplification comes from? The tubes, transistors, or chips? NAH! The components of the amplifier draw the current (and voltage) from the power supply to feed the output transformer, or more correctly, to vary the current through the output transformer (or in the case of SS or OTL, the speaker). As such if the power supply is poorly designed, your amplifier will produce somewhat less than desirable results. My guess is that 90% (if not more) of the audio quality issues in an amplifier relate to bad power schemes, poor filtering, and poor or improper grounding. So, now that you have designed that awesome KT88 amplifier, have you designed an equally awesome power supply?

Here's a good book that focuses on the linear power supply design:
"Designing Power Supply for Tube Amplifiers" by Merlin Blencowe
It's aimed at the individual who has a good understanding of basic electronics and is useful for those who are more advanced. He describes power factors of transformers and determining impedance, power filtering, power regulation and shows the related algebra in an intuitive way.


BTW: Is that case in to which you're going to stuff that awesome little amp large enough? You don't want to cramp your amplifier's style, give it plenty of room! Not only will your amplifier appreciate it (and it really will), but you'll be thankful later if you need to do some repair / modification.

Let's build something!
Cheers!

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The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
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LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Quick beginner's question about diode rectification:

The 1n400x style of diodes; how close to the peak repetitive reverse voltage do you get before going to the next size up? Case in point: I bought a pack of assorted diodes (and it didn't have any 1n4007!) and I want to do full wave rectification of 240 Vrms. Is the 1n4004 [Vrrm=400 and Vr(rms)=280] large enough? (Amp draw is around ~0.1 A)

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Thanks,
Ross in Wisconsin

@ Suncalc:

I really like your safety power box. I think it would be a handy thing to have, but could you tell use the values of you MOV and fuse?

Thanks,

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grym

Jameson's PopPop

Rule of thumb: 2x rating - that is, the reverse rating should be twice the forward being rectified.
When considering a diode, you always consider peak reverse voltage above all else. Obviously, if the AC being rectified exceeds the peak reverse of the diode, it will not long be a rectifier. That peak rating is the "break-over" point, that is, the point where it will conduct. In your case, I would not use any smaller than the 1N4006 (uF4006).
At 240 VRMS, peak would be 339.36VPeak. The 1N400x (uF400x) series diode are rated for 1A continuous forward DC draw.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Use any standard MOV recommended for your power type (105, 110, 220/240). The purpose of the MOV is to absorb any surge from the power line. For the fuse; NEVER larger than the circuit breaker/fuse in your breaker/fuse box. Recommend using a value that is not more than 50% larger than the expected draw for your project. Remember that the purpose of the fuse is NOT to protect the device (or you), but to protect the house/shop wiring.

_________________
The key to a successful build is to keep the smoke IN the circuit.
-Les

We cannot solve our problems with the same thinking we used when we created them. - Albert Einstien
_________________________________
LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board

Aloha,

I'm new here so please bear with me.

Is there a glossary of terms and abbreviations available?
Can tubes be installed in the horizontal position?
With all of the grounding issues I read about is there any advantage to running a separate earth ground out of the chassis?

I'm learning a lot reading through the various threads but I'm needing to steepen my learning curve.

Thank you

Ed