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Hello everyone, I have been reading more and more about the VJ as I wait for mine to get here, and to be perfectly honest I have gotten quite scared about the voltage and the caps. How do you guys drain yours, I read that holding a resistor (with a clamp or something) with 2 lengths of wire and touching the positive and negative. Is that common practice? I have heard every now and then the the V3, which i'm sure I will be getting, comes with something that drains the caps. Also as long as I have a good meter I should be ok?

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Yah.. I used the alligator clip/resistor to bleed caps for a long time..

then I just used my meter and found, even on the V2, that the caps drained every time.

I'd use the meter and not sweat it!

Hand in pocket is a good rule.. so the juice, if you ever get it, doesn't pass over the heart and kill you.

 

The smaller caps are no problem at all.

Nor the resistors, switches, etc.. just check with your meter and go to work.

 

stick a small V cap into an extension cord.. then run the cord away from you several feet. get behind cover, and plug in the cord.

Even a little guitar cap will pop! and you'll see a burn on the cement .. not in the house!..

so you can get a really good idea of what happens with hundreds of Volts.

 

the bleeder resistor has worked in mine every single time.

 

TWANG

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Sincer version 2 there has been a resistor in the power supply to bleed off voltage from the power supply capacitors when the amp is turned off.

 

All you need to do is unplug the amp from the wall and let it sit for 15 minutes or so. You will want to have a digital multimeter to check the circuit as you do not want to depend on a $.50 part to spare your life.

 

The tricky part comes when you rebias the amp. The amp has to be powered up and running and you have to make measurements on the live circuit. If you follow the basic safety rule of keeping one hand in your pocket or behind your back, you may get shocked, but it is unlikely to kill you. This is not a license to be reckless, though. Read through these forums, join and read through the Sewatt forums. Ask questions if you need to. That's what these forums are for.

 

I've answered this question many times and I don't get tired of it. It is important to me that beginners understand what is involved and how not to get killed; that is worth a few minutes of my time.

 

tung

 

 

Hello everyone' date=' I have been reading more and more about the VJ as I wait for mine to get here, and to be perfectly honest I have gotten quite scared about the voltage and the caps. How do you guys drain yours, I read that holding a resistor (with a clamp or something) with 2 lengths of wire and touching the positive and negative. Is that common practice? I have heard every now and then the the V3, which i'm sure I will be getting, comes with something that drains the caps. Also as long as I have a good meter I should be ok?[/quote']
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Well you need certain voltages at certain places.

The VJ runs an EL84 output tube and a 12ax7 input tube. each require a range of voltages.

What happens is in stock trim; the voltage within the VJ is a little high to support a longer life of the tubes (and of course to make it sound good).

So we have to measurement with a multi-meter to know where we are, then we change selective components (typically called a resistor) to adjust the voltages inside the amp.

Increasing the value size of a resistor lowers the voltage and reducing the size increases the voltage.

(we of course change these parts with the power off and checked for no voltage with a meter before removing the board)

 

It would be best for you to head to the FAQ post (VJ 101 post at the top), find the Ver3 schematics (since your amp is just being delivered, I’ll assume its new, then a Ver3 you’ll have). Then compare what you read to the schematic, that way you’ll get an understanding of what is being changed and why.

There’s no 1,2,3 type manual so print the schematic and read.

(note: some of the faq references www.sewatt.com, you'll need to create yourself a free account there before seeing the notes on that web site)

Doug.

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Other way around. You need to drain the caps before you touch anything with your fingers. Since this is a single ended, cathode biased, el84 amp, you won't ever need to rebias the amp for tube swaps; but you do need to tweak the bias and rail voltages to get them within normal operating limits if you want the tubes to sound their best and last a while.

 

Rebiasing the power tube is necessary because the stock bias is too hot, and the tube is already under excessively high plate voltage conditions thanks to the fact that many people live where the AC wall power is higher than normal. That's a double whammy and causes these amps to chew through power tubes real quick.

 

Rebiasing the preamp tubes is done just for fun and the tones you can get.

 

Gil...

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If I understand right... tube bias puts the tube's base voltage (i.e. against amplitude = 0) in the middle of the power curve, such that amplitude = 1 doesn't overdrive the tube and amplitude = -1 doesn't deliver a flat signal out? (Yeah tubes apparently underdrive)

 

That's roughly how I had it explained to me. But I don't trust the guy that told me this and I don't know enough to figure out if the power curve theory makes sense (normally I've got enough data to reverse engineer anything I get explained to me so I can verify it on pure logic; I can't tell if this makes sense or not).

 

Anyway, I can't pin down where tube life comes into play here. ?_? Running the tube hotter should wear it more, running it on low bias I would assume would make it last longer but it would sound bad too so...?

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Haha yeah im really lost on that one too. But anyways just one more question, could someone just post a picture of a power cap for me please? It would be really helpful, just so I know exactly what to look for, as I can read a schematic but you know what they say, a picture is worth a thousand words :-)

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But anyways just one more question' date=' could someone just post a picture of a power cap for me please?[/quote']

 

Power caps are large electrolytic capacitors. There's 3 of them and they look like cylinders. The other caps are tiny things the size of a pencil eraser or less.

 

ExtraFilterCaps.jpg

 

This mod adds extra filtering to the power input stage to remove some of the wall hum. Those big 450V electrolytic caps are the originals.

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Yeah you can touch anything but electronics. If you want to play with electronics, make sure your other hand isn't grounded, and take a meter across the caps. When they read 0 volts they should be safe but eh. Err on the side of caution (don't complete a ground path through your heart).

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My hands begin to sweat when I read these postings. If in doubt' date=' take your amp to a qualified tech to have the work done.

I've been servicing tube amps for about 30 years. Rather than mod a new amp, why not buy one off the shelf that you like.[/quote']

 

Really, this is good advice, take it to a qualified tech if you are unsure about anything! Careful even touching a chassis, if it's at a different potential than say, a ground on a workbench you can still get shocked. Don't trust internal bleeder resistors either. Make a bleeder resistor from a 1K ohm 10 watt power resistor with alligator clips on each end, put it across the filter caps, measure voltage with a multimeter and LEAVE the bleeder on until you are done. Electrolytic caps have what is called soakage and can recharge themselves over time. Also remember there is a positive and negative lead to electrolytic caps.

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My original electronics training was tubes with a little introduction to SS devices. I learned about ICs and transistors on my own.

 

As I remember, bias was a DC voltage applied to the plate and cathode to bring the range of the tube somewhere in the center of its conductivity so that the positive and negative components of the sine wave signal wouldn't drive the tube into distortion when a signal was applied. Think Clean class A amplification. Of course, a hot signal would drive the tube into clipped distortion equally in the positive and negative directions. It's been too many years since tube theory but that's what I remember.

 

Caps can hurt you, especially if you have one arm grounded when the other touches a charged cap. I always clip a test lead to the ground point on the chassis and touch the leads of the cap momentarily with the other to bleed off the charge. If you keep one hand in your pocket, the damage comes when you touch and react by jerking your hand across something that slices you open. The shock is limited to the hand, it's the reaction that hurts you. If you have a hand on a grounded point or a point with a difference of EMF potential, the current flows from your hand, across your chest, to the other hand, thus passing a charge through your heart. In an audio amplifier circuit, caps are usually dissipated through bleed-off resistors after a few hours unplugged. It pays to go through the motions, and dissipate the charge, anyway.

 

I was taking a 600v reading in my microwave one day, while it was running, to check a component. My arm touched the case, a finger on the other hand touched a HV point and ZAPPPP. MY chest contracted, everything went a little dark, and I broke out in a cold sweat for a couple of minutes. Don't let it happen to you. I was lucky. BTW, I fixed the microwave, but the experience left me with a renewed reverence for high voltage components.

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