Re-cap first time fire up?

[nosaj]

Here's the setup in case anyone is interested in a basic one.

The Dim-bulb Radio Tester
jason

[52 Bill]

I never used a light bulb limiter until I worked in a shop where I wasn't the person taking in units for repairs. We have one bolted to the top of the bench and now nearly everything that comes in is plugged into it before anything else is done.

The limiter can quickly determine if something will power up safely and can be helpful as a diagnostic tool in some cases. It is not perfect, as in some power amps the limiting will cause the amp to not fully power up and it will seem to be drawing an excessive amount of current. So, no it will not replace the information that you can and should be getting from test equipment, but it can be a helpful tool to have on the bench.

[Enzo]

When we say use a bulb limiter or a variac, it is expected that the variac will have a current meter. No, I have never seen a variac with a settable current limit, but that is what my eyes looking at the current meter are for. As I turn up the variac, if the amp meter starts to trend upward sharply, I back off. The amp may have never seen over 20vAC.

I agree with the above sentiments about less experienced techs and the bulb.

[Ronsonic]

I bring up most repairs on the bulb, and why not.

It's quick, I'm so used to looking at it and the analog voltmeter next to it I can recognize common problems.

After something like a cap job it's nice to see that everything is right before hitting the gas pedal.

[km6xz]

Well, if someone is happy with ambiguous information that if fine but a metered variac will tell useful data. As Enzo said, with a hand on the knob and eye on the current meter you can tell important information on trend, actual value(after working on similar units you will know right away if something is amiss even if the current is not excessive but if it deviates from other units from the past) and rise character. In my pre-diagnosis basic tests measure the same tests for every unit and it is logged. For example with a history of 100 of a particular model, the idle current might be 1.2 amps from the mains with a narrow range of deviation from that norm, you learn something important if the one you are about to work on is 0.9 amps. The final tests after repair value gets logged also so it adds to the accumulated database of working units.
If someone wants to have a current limit switch on their variac, it is not that hard. There used to be current sensing breakers where were adjustable. They worked like the old mechanical voltage regulator for cars. Surplus or junk stores with a lot of old electronics might have one. But a better way I think would how I built mine, an opamp sensing the drop across a precision shunt will trigger a relay to open the circuit. The value for tripping is set by a pot representing a range from 1 to 10 amps.

Faster and more accurate diagnosis is the key to profitable and successful repairs. Any tool that increases the meaningful information and decreases confusion inducing ambiguous information helps reach that goal.

I see a lot of false priorities in the hobby repair arena, no one questions spending $80 on a questionable NOS tube but resists spending that for a permanent tool to make diagnosis more effective on all repairs. Getting into a habit of not assuming but measuring all the basics as a base foundation from which to start to build a set of observations that lead to consistently good conclusions is time well spent. Creating a form, mine is on my computer feeding a database, where the same measurements are made and recorded for each amp is a big help, it forces one to take measurements in a methodical way before making any assumptions. Valuable clues are missed if assumptions are made too early, and the figures on paper/screen makes a difference.

[R.G.]

Just as there is a cost for ignorance, there is a cost for knowledge. The trick is to buy the right amount of either one to suit your particular needs.

This forum includes a few very good professionals, a number of journeyman and apprentice professionals, some very good non-professionals, and a lot of beginner and very amateur amplifier enthusiasts. For a professional who sees many amplifiers a day, detailed statistics is probably as you say - a key to making a living servicing amplifiers. But there is a cost to accumulating and carrying that knowledge. Someone who builds or repairs only one amplifier a month - or a year - has quite different needs. They will *never* see 10 of the same model of amplifier, let alone 100.

Priorities are only false or true within a context.

[Rhodesplyr]

Another useful way to bring an amp slowly up to power with a variac--if it has a tube rectifier--is to wire up an octal plug with two diodes to use in place of the tube. Yes, you have to monitor the B+ voltage to make sure it doesn't go too high, but that's relatively easy. You can monitor the current with a meter and/or install a lower-than-normal value fuse to blow in the event of a sudden failure. It's very difficult to achieve the same control of voltage ramp-up with a rectifier tube.

I've primarily used this method with amps when no one has any idea when they were last turned on. Depending on the way the amp is wired, you can also use this method, with no tubes installed, to check for leaky coupling capacitors without risking tube damage, but you have to understand the circuit well enough to know that you're doing no damage.

[gbono]

My $.02 - km6xz's comments are spot on. Even an entry level hobbyist can learn from a detailed database/note book. How do you get accurate data - good equipment and technique (following the principles scientific method also helps)? You might not see the same project over and over again but you will always learn something from your current one.

[big_teee]

My $.02 - km6xz's comments are spot on. Even an entry level hobbyist can learn from a detailed database/note book. How do you get accurate data - good equipment and technique (following the principles scientific method also helps)? You might not see the same project over and over again but you will always learn something from your current one.

My O2 Cents is RG is Spot on.
This Forum has all different Levels of Techs and builders.
There is no way a Sometime Amp Builder like a lot of us will be able to Have a large DataBase.
That is why we all check in to this forum to share Our Combined Knowledge.
I always make a Tube Voltage Chart when I build something, and will be glad to share that.
Hopefully this won't turn into a Forum where you have to be a highly Skilled Trained Professional to check in!
Later,
B_T

[cminor9]

Not a pro, just a successful hobbyist here: I don't have a limiter, or a variac, or anything of the sort. I have learned to be very very careful. Check any components you can check before installing them, and then double and triple check that they are installed correctly. After it's built or reassembled, then take a break or go to bed and check again later. I learned that it is several orders of magnitude faster and easier to test a resistor, for example, *before* you solder it in than it is to find a bad resistor in a completed guitar amp.

Also, follow the steps for first power up: Paul Ruby Amplifiers

I wouldn't say that someone without all that fancy stuff isn't serious, it's just that it may be cost prohibitive for a hobbyist. If I had the money for it, or did this work every day, then it'd be different. But building my own amps and performing the occasional fix for a friend doesn't justify the cost of a lot of the fancy equipment the pros have.

tl;dr an ounce of prevention is worth a pound of cure if you can't afford a lot of tech gear

[km6xz]

I never said a hobbyist working on their own gear should have the proper equipment, the whole point of this thread was someone rebuilding an amp for a 3rd party. That implies some level of responsibility and minimum level of competence owed to the owner. About 1/2 the people who responded do this for a living, working on other people's equipment. That was the context the comments were directed, in keeping with the original posts. When someone wants to set their own broken leg that is fine, but when doing it for a trusting client is a whole different level of minimum professionalism and equipment, like an X-Ray machine.

[gbono]

My data base consists of lots of $.99 comp notebooks from the mart - not excatly skilled trained professional but it gets the job done. I can use my database when our combined knowledge database has server issues

[R.G.]

Another useful way to bring an amp slowly up to power with a variac--if it has a tube rectifier--is to wire up an octal plug with two diodes to use in place of the tube. Yes, you have to monitor the B+ voltage to make sure it doesn't go too high, but that's relatively easy. You can monitor the current with a meter and/or install a lower-than-normal value fuse to blow in the event of a sudden failure. It's very difficult to achieve the same control of voltage ramp-up with a rectifier tube.

I've primarily used this method with amps when no one has any idea when they were last turned on. Depending on the way the amp is wired, you can also use this method, with no tubes installed, to check for leaky coupling capacitors without risking tube damage, but you have to understand the circuit well enough to know that you're doing no damage.

That's a handy dodge. Also you can install a 100K or so resistor in series with those two diodes. With no tubes in the amp, this turns into both a capacitor leakage tester and a capacitor re-former rig for the electros. If the voltage across the 100K is more than about 10% of the total voltage, some thing is way too leaky.

The whole point of watching current, either in a variac meter, light bulb limiter or adjustable meter is to keep an overcurrent from becoming a damaged amp again. I trust things that react faster than my hand darting to the variac knob or switch.