DIY Audio Projects Forum

Hello and welcome to DIY Audio Projects.

I'm making this thread in response to the steady stream of beginners in tube audio (like myself) that end up on this forum in the hopes that it will help more people experience the great fun in DIY tube audio without getting injured/frustrated beyond sanity and experience tube technology without getting ruined. In this thread we will present advice relevant when starting out on your tube-career, advice useful for a safe and successful project.

Once again welcome and I hope you have a good time here and many hours of good building and listening.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.

I will start out by presenting some info on electrical safety, unlike transistor-circuits tube-circuits normally have a lot higher voltages, these voltages are in some cases lethal and even non lethal voltages can still hurt a lot or lead to dangerous or otherwise bad situations. To avoid injury, damage or death there are some precautions and things useful to have in mind:

1: Always voltage test before you do anything.

2: Always use tools tested for the voltage. I suggest using a multimeter rated at a minimum of category III (3) which is rated and tested for up to 1000V

3: Have an insulated workplace, i.e. stand on a plastic or rubber carpet, avoid grounded objects such as chassis, pipes, power-sockets and so on. This prevents current from flowing through your body and especially your torso/heart.

4: Large capacitors can hold a charge for a long period of time which means a circuit can be live even when it is disconnected from the mains/power source, discharge them by shorting them with a resistor for a few seconds, I use a 10 Ohm 10W wire-wound resistor.

5: Polar electrolytic capacitors can explode when connected the wrong way or used at voltages above their rating (exceeding maximum ratings will destroy all capacitors regardless of type), when electrolytic capacitors explode they send hot electrolytic fluid flying, this can give bad burns, stains and it stinks! Make sure you cover or at least point it away from your person when testing it. I suggest using a transparent plastic shield when testing.

6: If possible use a power socket that is connected to a ground-fault-interrupter. (http://en.wikipedia.org/wiki/Residual-current_device)

7: Make sure your amp-chassis is grounded if it is made out of a conductive material. Always test this by measuring resistance between several points on the chassis to the AC intake ground-connector, 5 Ohm to mains ground is an absolute max.

I'm sure there are more good tips and suggestions to this so I will add as we go along.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.

All sound advice, probably a bit scary, but then this stuff can kill.

However, valves can work with high tension (B+) voltages far lower than will kill. I'm proposing to publish a circuit soon which uses a 24V supply for both heaters (in serial) and HT. Perhaps others have examples of "safe" valve circuits for new experimenters. There are examples from the early days with HT as low as 6V.

When I build PSU always I connect between +B and ground res. 100 kohm/1w to recharge the caps for 2-3 min.
after switch off main power. When ampl. is ready to work, I disconnect resistor.

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Hi End is back proportional to the number of composite parts!
Projects: OTL 6AS7 Gen, Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, PP 6005 Gen. Ellectric , headphone ampl. OTL Loftin White 6AS7 RCA....SE E84L& E80CC Siemens&Tel-n.
http://azazello-sound.blogspot.com/

That's why I said normally. There are two hybrid schematics running on 12V that are great to start with but if you build a PSU for them your still handling 110-230V. It might be a bit scary but the point is electricity is dangerous, treat it with respect. Don't let this discourage you though, just be careful and you'll be fine.

I will post some reading material on design and construction later this evening.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.

Now, I didn't make this thread just to scare people with what can go wrong, I will now present some reading-material for design and construction.

First of all I would suggest finding yourself a textbook on the physics behind electricity and a textbook on electronic components, this will be a firm point from which to throw yourself into the DIY wilderness. The first one is more important to you if you are aiming to one day design an amplifier yourself, whilst the knowledge of basic electronic components is completely essential in order to build a good amp regardless if you design it yourself or use someone else's design. After having acquired this knowledge you have good foundation on which to proceed forward from.

Now I will present a few books and pages that are good to read, there is also an archive of links and book suggestions on the main site (http://diyaudioprojects.com/Technical/) which contains some useful info and links, there is also another part on the main site with a list of some books http://diyaudioprojects.com/diy-audio/diy-audio-books.htm which is well worth looking at.

First of all I personally recommend both the titles by Mr. Morgan Jones, "Valve Amplifiers - Third Edition" and "Building Valve Amplifiers", the first one is about the theory of valve-amplifiers and the second is about the practical part of actually building one. The theory can be a bit heavy reading at times but is worth it and well, it's good to read what you should do in practice.

A guide basic design and a good and simple introduction to load-lines can be found at http://www.freewebs.com/valvewizard1/se.html. This one is a bit more reader friendly than Mr. Jones but is very thin, I suggest reading this one alongside Mr. Jones book, after all, there is never one answer.

Bruce Rozenblit who designed the successful Odd Watt amplifier has written a book called "Audio Reality" that is available at his sight (http://www.transcendentsound.com/audioreality.htm), I have not read it myself but it has been suggested by others in an earlier thread.

Another option is the US navy's NEETS modules made available by http://www.hnsa.org/doc/index.htm#train, here is a direct link to the pdf-documents http://hnsa.org/doc/neets/. NEETS is short for "Navy Electricity and Electronics Training Series" and contains a wealth of information about all types of electronics and electricity, This is possibly a good option for learning the basic stuff too if you don't want to buy a textbook but in my opinion the best option is to read both.

These are some examples, more books and links are very welcome.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.

For all beginners i recommend a cheap universal car rubber-mat ( workplace ), like illustrated below. It's also good to keep electronic components on the mat, it simply avoids them rolling down to the floor. ( great thread 91lieb ...thanx ).

One thing I would like to add about power.

While working on high voltages required by tube amps, don't forget amperage is the real killer. It only takes milliampers to kill or cause serious injury.

So while 350+ voltage is very dangerous, don't get too complacent working with 120-220v either. It can hurt you just as bad.

Just my 2 cents

Brian

Correct, about 30mA at 50Hz through the torso will kill a healthy person.

/Ebbe

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"I've done this a thousand times befo..." on not voltage testing before getting to work.
"I'm gonna take my time on the next amp....." on First tube project.

I'd like to second the suggestion about putting a resistor from B+ to ground to bleed the voltage off the caps. However, I'd recommend leaving it there. It's easy to open an amp that hasn't been used in days and forget that there's still danger lurking. There's no reason for it IMHO. A 100K load on B+ isn't going to affect the amp. At 500 volts it would draw only 5 mA. Use a 1 meg if the current draw of the 100 k is too much for you to cope with.

Sorry, didn't mean to get too technical, just watch out for charged capcitors.long after power has been removed and the amp has been unplugged. They can shock the hell out of you. I have a small puncture hole in my thumb where it touched a charged cap laying on the carpet yesterday. I had removed it from the circuit carefully while still charged but forgot and picked it up with my hand. Dumb.

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Stephen