Biasing an amp yourself: How dangerous is it really?

[emperor_black]

I'm thinking of getting one of these bias testing tools from the bay,

Link

Once I wear the rubber gloves, insulate the tip of my screwdriver, wear beach sandals and sit on a wooden chair, can I just try and bias the amp and not risk getting electrocuted if the screwdriver touches the wrong lead?

I'm planning on biasing my carvin ts100 cause it needs power tubes real bad. Its over two years old and holding on. I could swear, I hear the volume go up and down now and then even if I'm standing in the same place and we're playing real loud. The preamp tubes are brand new which I replaced in june.

How dangerous is it really? How many of you have actually biased an amp that does not have bias test points like a Randall amp does? The carvin has bias pots but they're buried between at least 5 leads.

[UFORocks]

I think the major concern is stored energy in the capacitors, which I believe you can discharge by turning the amp "ON" with the power unplugged. Leave it "on" and unplugged for an hour before cracking it open and you should be good.

Electricity is always going to look for the quickest route to ground. If that route happens to be you then you're fcked. of course with the amp discharged (not sure that's even the correct term) there's nothing to worry about.

-Steve

Edit: note that Im not an electrician

[emperor_black]

Yeah, I've heard about that tip steve, thanks for the reminder.

[Jackson-Charvel]

How do the capacitors stay discharged when you bias the amp as you need to have the amp on when biasing tubes?

[UFORocks]

How do the capacitors stay discharged when you bias the amp as you need to have the amp on when biasing tubes?

Oh, I didnt know that it had to be powered on. They woudn't stay discharged then.

[emperor_black]

How do the capacitors stay discharged when you bias the amp as you need to have the amp on when biasing tubes?

You're right. The discharging was while changing the tubes. Not for biasing them. Double And while I have got some attention on this thread, the ebay biasing tool has the 8 pin base on which a power tube sits so it can be biased. However, the thing with the carvin is that the back plate gives just enough room for the last two power tubes to sit. I dont think there is any extra space between the top of the power tube and the back plate. In that case, can I bias the first two tubes using the ebay tool and then turn the pot to approx the same position for the last two without actually seating them over the base and biasing them?

[guitarsjb]

I've heard that if you replace the tubes with the exact same ones, rated the same, you don't have to bias the amp. *disclaimer* that's just what I've heard, the person (s) saying that might be idiots.

[Spivonious]

If it's not completely obvious how to bias the amp, I would definitely take it to someone who knows what he's doing. There is a LOT of electricity in there and even a little can kill you.

I would make sure it's discharged, open it up, and make sure I can see exactly where to put the leads. If not, take it to a shop.

[emperor_black]

I've heard that if you replace the tubes with the exact same ones, rated the same, you don't have to bias the amp. *disclaimer* that's just what I've heard, the person (s) saying that might be idiots.

I would have LOVED to do that since I like the tone of the amp already. However, the darn tubes are blank. there is no marking whatsoever on them. Anyone know what brand of tubes have no markings on them? I guess I could ask Carvin. :idea:

[marcus]

I would have LOVED to do that since I like the tone of the amp already. However, the darn tubes are blank. there is no marking whatsoever on them. Anyone know what brand of tubes have no markings on them? I guess I could ask Carvin. :idea:

Variation between tubes by the same manufacturer is so great that just knowing just the brand of tubes is no help in this process. If you had tubes that were rated by their electrical characteristics, you could replace them with tubes of the same ratings. I believe groove tubes does this, gives each tube a rating number. You just replace the tubes with new tubes with the same rating, and you don't have to rebias. It sounds like you can't get away with this, though.

[Rsmacker]

It's not just dangerous, it's FUCKING DANGEROUS, even if you know what you are doing. If you aren't familiar with everything on a particular amp, you could find yourself accidentally shorting out very High Voltage components. It's not like a 240v AC tickle, which is bad enough and startles you, no, a DC shock is a different beast, painful, it will actually scorch your insides. Short out a cap with a screwdriver if you don't believe me, see how big the spark is that jumps out. It will hurt you!

And an RCD won't offer any protection, like with AC, when you discharge a cap, you get the lot whilst you are shorting it. Watch professionals when they poke around in an amplifier, many will automatically stick their left hand straight into their back pocket before they touch anything - that way there's no danger they will accidentally short anything between two hands ie. straight across their heart. It's all too easy to do.

So,if you don't know what you are doing, don't fuck with it, really. It's not too bad if there are bias points on the back, but going inside an amp is not a good idea.

Of course, feel free to ignore me and come back and tell me how wrong I am!

[Racerx2k]

Rs is absolutely right. We're talking upward of around 500 volts in these things, guys. Nothing to screw with, lightly. As part of my job, I've had to watch video presentations on electrical safety. some of the videos are not re-creations, but actual REAL stuff, and it's not exactly pretty, especially some of the vids pertaining to arc flash safety. While I HAVE biased my amp out, it was something I took very serious, and was extra careful, but I think it gets a little more scary, the more of them you do, cause most people begin to get rather complacent after a while.

[phill_up]

emperor_black you only live once, and there is nothing to biasing an amp. give Bob a call at Eurotubes "clicky", tell him what you have and he will tell you how to do it. as far as draining the capacitors you would use a bleed resistor but why mess with that part of it? all you want to do is change the tubes, and bias the amp. don't TOUCH or concern yourself with any other part of the amp. get some DeoxIT and clean the tube sockets also or you will more than likely have problems.

If you have never worked with electricity and feel nervous about it then take it to a shop

[Bengal65]

I bias my amps all the time and check plate voltage as well and do general maintenance. I'm no electronics expert, but have learned how. Safety first. Yes, most filter caps store 480VDC. They must be discharged. You can turn your amp on, play and kill the power at the plug or power strip, while leaving your amp switch and standby on. Play till there is no more sound coming out of the amp. Let the amp sit awhile.

Also, you need to know if your amp has a pot for adjusting the bias. These are much easier, than the old switch, resistors method.

I use a cap discharge kit as well. (bottom of page) http://www.studiosoundelectronics.com/ampkits.htm

For biasing and checking plate voltage I use an Amp-Head adapter-MPD. He makes a single, but the dual is here: Does 90% of tubes. If you have EL84's, you need a 9 pin adapter setup. But with EL-34's, 6L6's, KT-88's etc. the 8 pin covers it all. With a flip of the switch you can measure your Mv bias and plate voltage.
http://www.amp-head.com/product_info...products_id=31

If you just want Mv, and don't want to bother with plate voltage, you can get an adapter cheap. Or, you can get one from Eurotubes for about $11. If you already have a digital multimeter your halfway there.

While doing, the tube installation and biasing you don't need to discharge. Just keep your hands away from the filter caps and chassis to be safe.

Just remember, the hand in the pocket rule. "One hand in your pocket, one hand works on the amp".

Check out Bob at Eurotubes for some of his biasing video's. www.eurotubes.com

[emperor_black]

Cool! Thanks Phill_up and Bengal65. That's what I wanted to hear I am no electrician myself, but everybody in my family is. I have worked a lot around electricity and know the dangers.

The hand in pocket is something new, but I have heard other similar precautions. Bengal, if I were to wear thick plastic gloves, it should act as a very good inductor right? I know I cannot expect it to work on a 50,000V electric line, for example, but 500V, I think it should do. I will watch those videos.