[V-DiV]

Hi All,

I'm interested in building a tube or hybrid amp to pair with my K701s. All of the projects that run the tubes at high voltage warn against building one of these until getting experience working with high voltage. But the HV amps are also supposed to sound better than the low voltage tube hybrids (Stacker II and Bijou >> Millet Max and SOHA II).

So, how does one get this experience? Or alternatively, what is a good source of information on HV safety procedures?

I am a careful person, plenty of respect for electricity. I have done some electrical work around my house (re-wired my kitchen and the odd outlet/switch/light). The trick there is to make sure the breaker is off and test the circuit before working on it. I have an AC filter/transformer circuit on a breadboard right now and when I plug it in to check voltage, etc. I'm very careful to not touch anything.

Of course a mistake at 300 volts may hurt a lot worse than a mistake at 110 volts. So what additional safety precautions are required to work with a HV tube amp? Don't touch anything except with insulated probes when plugged in. Bleed the capacitors with a resistor after powering down and before working on the circuit. What else??? Make sure to have a fuse in line so body can't draw too much current???

Thanks,
Vic

 

[fran]

All of those are good ideas. Just to also be aware that there is a lot more danger in 300V DC compared to 300V AC.

Other things - remember safety all the time, don't work at it when you're tired, only use one hand - keep the other behind your back. If say one hand is leaning on the chassis and the other touches a live part, the DC will flow through one arm across your chest and to earth through the other arm. This can stop your heart. Not good.

Other than that I would say you seem to have the right attitude. books like Morgan Jones valve amplifiers are well worth the investment.

Fran

 

[pabbi1]

Here are a few:

diyAudio Forums - Safety Practices, General and Ultra-High Voltage
Electrical Safety
Safe Meter Use

All that aside, I really dig the Millett Max - it sounds really, really nice, and is fun to boot.

 

[Uncle Erik]

Good advice so far. The biggest thing is to be super careful whenever the thing is plugged in. Not so dangerous otherwise, though be sure to discharge the caps.

Practically, I put heatshrink spaghetti on *every* lead for *every* component. I also run a star ground with insulated green wires. These take longer, but give you an extra margin of safety. If your probe slips or you accidentally touch an insulated wire, no big deal. Same with running a ground bar or copper plate for ground. Electrically those work well, but they are usually unprotected and a slip can curl your hair. I think you're better off insulating everything you can in there. Also tends to keep you from making accidental shorts, so troubleshooting is easier.

You might want to invest in a Variac and an isolation transformer. The Variac works like a big dimmer switch and lets you troubleshoot at much lower levels. You don't need full power to check everything. Also lets you bring up an amp slowly the first time in case - sometimes they'll start smoking or do something or other unpleasant the first time they get a taste of AC. The isolation transformer will trip if it detects a fault to ground, as well. Not always effective on the other side of the power transformer, but you can never be too careful.

For books, be sure to check the ones by Morgan Jones and Bruce Rozenblit. They're excellent.

 

[V-DiV]

Thanks for the advice.

Fran and Uncle Erik thanks also for the book suggestions. I've looked them up on Barnes & Noble and will buy one or two of them. How much does an appropriate Variac cost? I worked in a magnetics lab in graduate school and we had a Variac. It seems like a pricey piece of gear for low/moderate budget do-it-yourself-ing.

Pabbi, thanks for the links. All the cool pics of the Millett Max are what originally got me interested in tubes. Then the rumblings about the SOHA II (though the prototype pics don't look as cool as the Max). I wonder how it will sound compared to the Max? But what I've read about the Bijou and Stacker II makes them sound a good bit better and I may well decide I want to build one of them despite the great care required to work safely with the high voltage.

Regards,
Vic

 

[Uncle Erik]

I got my Variac at a Hamfest for $30. It's from the 1940s, but works fine. It just needed a new cord and rubber feet. You should be able to find a used one for $25-$50 or so. They're stable components and generally used by people who know what they are, so I think used ones are safe. I don't think that's much to pay for the margin of safety you get. If you slip up when there's only 100V going through the amp, you'll be OK.

As for the books, I think the only way to get Rozenblit's "Audio Reality" is through his Transcendent Sound website as a download or CD-ROM. His site is worth checking out - he has nice kits and there's a discussion forum.

Also forgot to mention one of the best safety tools: someone else. When you're doing full power tests, invite one of your buddies over, a neighbor, family member, whoever. Order a pizza as a bribe. Tell them what you're up to and keep something like a wooden bat or broomstick nearby. They can use that to separate you from the amp if something goes wrong. You don't want to be alone when you make contact with HV.

 

[fran]

It is always a worry of mine that I would be outside in the workshop at night and the missus in bed asleep and that she would come looking for me in the morning and find me stuck to an amp!!!

So I always run any tests when someone is around. BTW, if you don't get a variac a good standby is to use a lightbulb in series with the AC. It acts as a resistor and initially is bright but then dims soon. Another thing: it would be no harm to put a fuse on the DC side of the system just until the amp is running OK. That and the bleeder resistors are important. I used 1M to ground on my last project and when I turned it off it took close to a minute for the voltage to run down to <100VDC (from 375)

If you are going to buy some books buy the 2 morgan jones ones - well well well worth it!!! Don't forget the tube section of diyaudio.com as well.