Capacitor Discharge and Safety.

[Cochise Montana]

Aloha.

I am slowly dipping my toes into the pool that is amp repair and building. I have a V4 Ampeg and I live in a rural area. My first experience with the only tech in the area was not so good. So I have slowly started gaining knowledge about my amp and tube amps in general. I read Tom Mitchell's book which was a great refresher on electronics but less impressive in the repair section. I also have watched Gerald Weber's DVD.

I have a long way to go, but I had question at this point in my knowledge hunt....

Capacitor discharge: I have read/seen a few different ways to accomplish this.

One is to use a resistor drain to ground touching each positive side of each capacitor. I constructed a resistor drain with a 1K ohm 10 watt resistor between two alligator clips and heat shrink wrapped.

The second which was promoted by Gerald Weber is to connect an alligator clip to Pin 1 on the preamp tube closest to output (he later says any preamp tube will due) and turn the power and standby switches on.

The first method makes sense to me, I am having trouble trusting the second method but that seems like an easier way to go.

Does anyone have an opinion on these methods?
What is your typical routine for capacitor discharge?
Do you confirm voltages in the caps after discharge?
Do you have other ways to confirm that it is safe to proceed with work inside the amp? Another measurement or sign?

I am a safety freak of sorts. I know this amp repair/build process has risks, I wanted to get as much knowledge as possible. The cap issue right now is my biggest concern. I want to have this down solid before venturing forward.

I apologize for such a long post on what for many of you may be a very basic part of the process. Thanks in advance for any advice.

[selloutrr]

IMHO if you have no experience working on amplifiers the AMPEG V4 is not the amp to get your feet wet on.

The voltage in that amp will kill you - no second chance.

The boards are prone to fall apart as you work on them. the copper traces peal off the paper board when over heated or tugged on.

They are difficult to get to all the parts.

I service a few dozen of these amps regularly, I've been inside 1500+ amps and I still get uncomfortable working on the V4 line.

To answer your question. the voltage stick is the safest but you always test voltage before you assume any amplifier is safe to work on. you hook up an alligator clip to keep the amp drained. Capacitors can repower and discharge even after being drained.
never work with an amplifier plugged into the wall.


Your amp if it has not been modified is a fixed bias. you will need to change it to a variable fixed bias or replace the bias resistor. Make sure you are very skilled at soldering in these amps! Get in and get off as quick as you can to prevent damage to the board traces.

fliptops sells ampeg parts

this is a nice read
http://www.ampegv4.com/

If you aren't comfortable workingon your V4 PM I'll help you find a tech or you can send it to me and I'll repair it.

[Bob S]

My routine :--
Unplug / disconnect from mains
Turn on - check nothing lights up.
Turn off Standby.
Connect insulated croc clips with 100k from B+ to ground.
Check with voltmeter - All B nodes.
Check again.
Check again.
NEVER PUT BOTH HANDS INSIDE THE CHASSIS - trust me on this.
Never hurry.
Never THINK it's ok to go in - always check.
And check again.
It is potentially lethal.
At the very least it can realy hurt.
Lots of us know this...

[Cochise Montana]

Thanks for the replies. Selloutrr I sent you a PM.

After looking at other amp rigs on videos, my V4 definitely seems a little tighter and more cramped then some others. When G. Weber said, "Let's do the caps now" and then flipped an amp over and there they were all tucked nice and neat in a protected area, I was like, "Hey, how come mine isn't like that?" I could do that cap job, this V4, mmm maybe I'll heed the advice here.

Another question though, do you hook up alligators to each individual cap and keep it grounded while you work? Or do you do the first pin on a preamp socket trick?

[Structo]

Aloha.

One is to use a resistor drain to ground touching each positive side of each capacitor. I constructed a resistor drain with a 1K ohm 10 watt resistor between two alligator clips and heat shrink wrapped.

Howdy and welcome.

You have to understand that tube amps can kill you.

So with that out of the way, DON'T GET YOUR FEET WET!

Just kidding....

Your 1K ohm resistor may be a bit small in value and could spark when you connect it, probably scaring the crap out of you.

A 100K will take a little longer but shouldn't arc when connected.

Take a test lead and cut it in the middle, solder in a 100K 2 watt resistor. (2 watt simply because it is bigger and more durable)
Put some heat shrink tubing over it so you can't touch it.

Clip the ground lead first to a bare spot on the chassis (ground).

Now clip the other end to the + end of a large cap in the power supply area.
Usually the biggest cap is the one that supplies the power tube plates.
In theory this should drain all of them but keep a volt meter around to double check everything before touching anything.

[img:509:466]https://lh5.googleusercontent.com/-6iMI ... 20Tool.JPG[/img]

Set meter to DC volts, clip black lead to chassis and red lead to positive + end of cap to measure for voltage.
Try to drain them down to below 10v.

[Cochise Montana]

Damn, I waited a week and a half to get those 1k ohm resistors. Well patience is a virtue and I'll get a few of these 100k ohms for the future should I delve into this.

Lol, nice avatar btw.

[rp]

It was mentioned by I'll stress it again, make sure the stby is on when you drain the caps. I know this cause early in my tinkering days I took a 100mf on a marshal, one hand on the chassis the other on the cap on the other side of the off stby. I'm sure I'll do other stupid things but I'm never going to do that twice.

I've also made it a habit to always take the ac plug and stick were I can see it right next to the amp, and every time I go in I look for it no matter how sure I am. If it's an amp I'm going to be in a lot I put a red zip tie on the plug so I don't mix it up with other plugs lying around.

I also use yellow or red jumpers to drain as I've been known to forget them when powering up. I don't usually use a resistor on the jumper I just connect to a plate resistor on V1. I'll then check then with a meter but if I'm in a hurry I'll just spark them with a screwdriver as a last check before reaching in.

[overtone]

I use a yellow bleed-lead too - always easy to see if I hung it back on the wall before restarting the amp.
Thanks rp for the red cable tie tip for the plug, that is a major blind spot in my case.
Good quality test leads, as Structo mentioned, have the advantage of lying comfortably and are less likely to get in your way, or fall off.
There are some really good high voltage test lead systems from companies like Hirschmann or MultiContact.
Best, tony

[Ian444]

Nearly all amps I've worked on have bleeder resistors whose sole function in life is to drain the caps. But you never dive into an amp without measuring the caps, or having a schematic, and working out where the high voltages may be, sometimes the bleeders fail.

[selloutrr]

Nearly all amps I've worked on have bleeder resistors whose sole function in life is to drain the caps. But you never dive into an amp without measuring the caps, or having a schematic, and working out where the high voltages may be, sometimes the bleeders fail.

not every version of the V4 Series used bleeder resistors. They are not present on the first version or the transition between non master and master volume.

it would be nice if it was law just for safety that every amp had to have them.