recapped my ampeg V4...

[methodofcontrol]

..before i fire it up, is there anything i need to know?
do new caps need to be brought up on a variac, i've heard different
answers to this question. either way...it seems safer to bring it
up on a variac so i can measure voltages and make sure i did everything
correctly before i fry something!
which brings me to my next question, i'm pretty sure i know the answer...but i'd like to double check.....
if i bring the variac up to say, 60vac....would my amps' voltages be HALF of what they would be at 120vac? (as 60v is HALF of 120v)
or would my amps' voltages be 60v LESS than what they should be? (as 60v is 60v less than 120v)
thanks! i can't wait to see if i finaly solved the mystery of my V4 making weird noises!

[Rob Mercure]

Hey MOC,

If the filters are brand new units that haven't been on the shelf long it probably doesn't matter - I usually just fire an amp up after recapping and I've never had any problem. Butttttttttt it's extremely easy to put a 100W light bulb in series with the amp when firing it up to not only ease the filters up to voltage but to ensure that you haven't accidentally dropped a glob of solder between a couple of the board traces (guilty) or had one roll under the board and short to ground (guilty) or drip off a can cap terminal and touch ground (ouch, guilty again, but your honor, it was 30 years ago).

Or you can use the variac - but using a light bulb limiter is quicker - if you use the variac monitor the B+ rail as you crank up the voltage to see if anything funky happens. Such as the voltages climbs to a certain point but doesn't climb higher, etc., anything that might give you a clue if you've been guilty of the above also. When recapping a large amp with filters in several locations it's easier to have committed one of these "sins" than in a Fender's "doghousse."

The voltages at the secondary of your amp's PT should be proportional to the voltage input - 1/4 in - 1/4 out, etc. I don't know what sort of variac you've got but if it's got a calibrated dial of 0-100 this is not voltage but percentage of full voltage. That is, if the variac dial is on "50" the voltage will be 60VAC (this is the way my both my 3 amp and my 18 amp variac are "calibrated). I don't know if you've got more than one meter but I like monitoring both the output of the variac and the output of whatever power supply I'm powering at the same time - I've added "tip" jacks to my variac just for this purpose. That way if the PS's output seems to "lag" the voltage increase from the variac it indicates something's funky (after a short initial filter charging time that is). Years ago I was going to build in a voltage meter on my variac but now that I can pick up a DMM at Harbor Freight for less than $5 (amazing, ain't it) I just use the cheap meter to monito the variac and the Fluke for where I need more precision and reliability.


Hope this helps

Rob

[stokes]

It is a good idea to use the variac or light bulb limiter even if you know the caps were made yesterday.During manufacture the caps are not pre-formed or tested at high voltage so the oxide coating hasnt been formed on the foil,that is the purpose of "forming" the caps,not to just rejuvenate an old dried cap.You may get away with not doing it and a lot of people do,but I dont think the cap will be as effective or last as long as they would if they were properly formed,thats the way I was taught to do it,and it is just a good precaution,as well as for the reasons Rob pointed out.If you have the variac,or limiter,there really is no reason not to do it.I tend to feel the variac is more effective,if it werent I would think the light bulb limiter would have replaced the variac for this purpose since it is so much quicker.When bringing the volts up on the variac I usually start at around 40 or 50 and raise it 10 units every hour or so.I monitor the volts on the caps,since you should do this with no tubes and the dc volts will rise above the voltage ratings of the cap,so dont expect to get the variac up to 100 on the dial.

[methodofcontrol]

hey stokes...
when you say "start at 50 and raise it 10 units every hour or so"... do you mean "50" as in "50vac", or do you mean "50" as in "50% of wall voltage" like rob was saying?
the variac i'm borrowing from a friend starts at 0vac and goes up to 140vac, it's labeled in volts not percentages.

[Enzo]

A transformer is the electrical equivalent of a set of gears. WHatever comes out is a ratio of whar went in. Ratio is determined by the gears.

A transformer is a set of turns rations. The transformer has no particular voltage of its own. If I have a 12v transformer - 120v in and 12v out - I can just as easily connect 12v to its secondary and expect 120v at the primary wires.

If you run the primary of an amplifier at 60v, then yes, all the secondary voltages of the transformer will be half what they should. SO think in terms of percentages.


Had a guy bring me something wherein he had replaced all the e-caps. Now it blew fuses. Upon extracting the board, I pointed out to him that the excess lead wire on each cap should be trimmed off, not left there to contact the chassis.

I am not a cap former. I am likely to bring a fresh cap job up on a variac just to watch the current in case of trouble. Most anything involving power circuits gets brought up on a variac first. But I don't worry about the caps. I have been soldering over 50 years, and have not formed a cap in that time. I have never had a new cap fail on me in that time. I cannot recall ever having a repair come back with a new cap dead unless it was something like a rectifier shorting AC through it. In short, I see no evidence of initial failure for lack of forming, nor have I seen evidence of unreliability. Caps work, that is pretty much my experience.

When I was learning, I learned that the dielectric was "formed" and that gave the cap its polarity, but that was part of making the cap, not something technicians did. In fact I never heard of cap forming as a procedure until I got serious into guitar amps. And then it was the people who labored oveer every detail who were into it.

Forming the cap certainly won't harm it. The case against it is, well, your repair takes an extra day and requires your ongoing attention to the forming.

How did we decide it takes an hour at each 10 volt level? How about a half hour? How about 10 seconds?

I have the dial on my variac in volts. 0-100 is more math than I want to do.

[Enzo]

MOC, just how much difference is there between 50% of 120v and 50v? ABout 10v. It doesn't matter. STart someplace in teh middle and advance it some every now and then until it is up to 120v or whatever the target might be.

[stokes]

Unless the cap has a manufacture date on it,and the Sprague I am looking at has none,you have no idea how long it has been sitting on some shelf.Which side of the "forming" debate you are on depends on what and from who you learned.I learned a lot from a guy who had nothing to do with guitar amps,so I dont think the practice has its basis in guitar amps.
"The case against it is, well, your repair takes an extra day and requires your ongoing attention to the forming".If time is a big concern why not put the unit on a light bulb current limiter to check for problems?I have a jig I connect to a tube matchers power supply to form multiple caps at one time,when I buy a supply of caps they get formed before they go into the drawer.It doesnt require an extraordinary amount of attention,just plug it in and come back and raise it in an hour,or a half hour or 10 minutes,my variac is connected to a volt/wattmeter so problems are easy to see.When I get to about 80vac input I start checking the dc on the caps so I dont go over the caps dc rating.Is all of this absolutely necessary?According to some people no,but I learned otherwise,so that is how I have always done it,and like I said earlier,I have no way of knowing how long the cap has been sitting in somebodies stock so it is mainly a precaution, I suppose.