Re-cap first time fire up?

[MikeH]

The more I work on amps, the more I'm trying to get into the 'right' way to do things.

So I have this silverface bandmaster that I just did a re-cap on, as well as replacing the screen grid resistors (they looked kind of cooked; one was fine, the other was reading double what it should have been). In the past after a power section recap, I generally would just start the amp up on a load-limiter, and if all was well there, fire it up, and let the amp sit for a while, double check the bias and then rock out.

Question is, should I be using a variac to bring the voltage up slowly? Do you guys treat a re-cap start up the same way you'd start up a brand new amp that was just built? ie - Check continuity on the transformer taps, start without tubes on limiter, take voltages, set bias to maximum coldness, add tubes, fire up agian on limiter, check voltages, fire up without limiter, rebias, check voltages, let sit, bias again, etc etc?

[Jazz P Bass]

Sounds like a plan to me.
The only "if: for the variac is a tube diode.
I guess you could test it all alone, no other tubes installed.
It will start to function as the variac voltage is raised.

[big_teee]

I use my Dim Bulb tester on any Major Amp Mods or changes, just To make sure nothing is shorted.
After I see the bulb go bright, then Dim, Then I fire it up normal.
B_T

[MikeH]

Sounds like a plan to me.
The only "if: for the variac is a tube diode.
I guess you could test it all alone, no other tubes installed.
It will start to function as the variac voltage is raised.

So - firing up at lower voltage, say 50-60% is bad when using a tube rectifier? Is that what you mean?

[nosaj]

You can't use a variac very effectively with a tube rectifier to bring the voltage up slowly. The reason is that the rectifier doesn't function until the AC voltage is pretty high. And by the time the rectifier sees enough voltage to start to conduct, it will be passing a lot more voltage than you'd want to have applied to your caps, etc. downstream.

You won't hurt your tube rectifier by trying to bring the voltage up slowly on a variac, but the components downstream will not get the benefit of the slow increase in voltage.

1.Remove all tubes from the amp. Make a note of where each tube belongs, so you can later replace it in the correct socket.
2.Place a 25-watt or 40-watt bulb in the dim-bulb tester and plug the amp into the tester.
3.A good transformer will cause the lamp to glow dimly after a few moments. If the lamp glows brightly, you have a short circuit; the transformer should then be disconnected and checked.
4.If the transformer is OK, put in the rectifier tube, put a 100-watt bulb in the dim-bulb tester, and try again. If the rectifier tube lights up and the lamp glows brightly, you have a short-circuit in the filter capacitor of the power supply.
continue on

jason

[Enzo]

If you do a bunch of work, firing it up on a limiter at first to see if any glaring wiring errors arise is fine with me.

As to coddling new caps, I never bother. In well over 50 years of working on tube circuits, I have never had a cap fail for lack of conditioning or "forming." Not one. And I guarantee that Fender, MArshall, Peavey, Vox, AMpeg, or any other amp maker over the last 50-70 years has not manufactured an amp and then slowly brought it up on a variac to form the caps. It surely won;t hurt it to do so, but it is also not necessary.

[jrfrond]

Bring it up on a variac or on a current limiter to check for current draw. If all is good, just fire it up. Manufacturers don't baby the caps. You don't need to either. No one does.

[nosaj]

With a tube rectifier shouldn't it be a dim bulb test versus slamming it one it starts rectifing? The dim bulb would automatically back the current off if something was miswired, versus manually having to do it with a variac. I'm just talking about saving parts not forming .

jason

Bring it up on a variac or on a current limiter to check for current draw. If all is good, just fire it up. Manufacturers don't baby the caps. You don't need to either. No one does.

[km6xz]

As John mentioned above, the variac is for testing for abnormalities, not "forming" caps. The oxide is already formed and will just be a dead or shorted no-good part if it isn't already at its rated capacitance, which is a function of oxide layer thickness and total area of foil.
Bringing up the mains slowly with tube rectifiers is debated but it is simple, indirectly heated cathodes are safe to bring up slowly, there will be no conduction until the cathode rises in temperature however. Directly heated filaments on the other hand should not be raise slowly, the oxide of the heater can be stripped if anode voltage is present when the bare heater is coming up to emission temperture. In most application for high power directly heated cathodes, high voltage is turned on only after the heater is up to temperature, which can be as little as .5 second for big high current rectifier tubes.
No particular anode voltage needs to be reached before the rectifier conducts, it will work with 1 volt across it if the cathode is boiling off electrons. Part of every conduction cycle is as little as slightly positive since it is fed with a sine function source.
Variacs with analog meters for voltage and current are one of the most useful tools in a shop, a bench is not complete unless it has one. They are expensive to buy new but very cheap and simple to make so there is no excuse for not having one. To be useful it must have at least an analog current meter, so you can see trends develop very quickly, which you can't with digital meters. Digital meters are great for low cost precision so you want those also, but analog hi-z meters such as a VTVM can tell you things about the period or time domain that digitals can't. Old Heath, Eico, RCA VTVMs have an important place on a service bench, even if you can't afford a HP410C.
I have never used a current limiter, never saw a need for one but reading the posts it seems to be popular nowadays.

[nosaj]

I think with a variac raising the current slowly here's an analogy.. you have a dam that is not full, you start letting water in it once it's full you get a full rush of water over it now if it bust a hole and starts drawing water faster you have to rush to close the flood gates feeding the dam.
with a dim bulb or current limiter once it starts drawing more current the light bulb takes the excess and prevents it from hitting the system.

I haven't seen a variac that you can set the amps on it to not let it pass more than the set current.

I just think the dim bulb tester protects your mistakes better than a variac because it's automatic versus having to turn the variac down.

jason

[km6xz]

Watching the trend of current and knowing the typical values tells you quite a bit more than a light bulb. If there was a sudden avalanche of current draw at a low voltage, that is informative also and not particularly destructive. Even a shorted output section or filter which would pull a lot of current, would pull a modest and informative amount of current at low voltage. Besides, that was the breaker is for on the variac.
I am not saying avoid using it, I have just never seen a case where it was needed in 50 years of dealing with repairs. It is better than nothing if one has no test instruments but my thinking is that if there are no test instruments, the person is not very serious about the craft and probably will cause more damage than solve, because without data or evidence, all is a guess.

[big_teee]

Watching the trend of current and knowing the typical values tells you quite a bit more than a light bulb. If there was a sudden avalanche of current draw at a low voltage, that is informative also and not particularly destructive. Even a shorted output section or filter which would pull a lot of current, would pull a modest and informative amount of current at low voltage. Besides, that was the breaker is for on the variac.
I am not saying avoid using it, I have just never seen a case where it was needed in 50 years of dealing with repairs. It is better than nothing if one has no test instruments but my thinking is that if there are no test instruments, the person is not very serious about the craft and probably will cause more damage than solve, because without data or evidence, all is a guess.

Apparntly you real Techs don't need the dim bulb tester, but the novice Amp builder on this site does.
I have seen countless Threads where if they had used a dim bulb tester they would have seen they had trouble, and stopped.
Read a lot of posts where they spent lots of time fixing what was torn up initially.
If nothing else you can keep from blowing those expensive fuses.
That is strictly IMO.
Don't run your amp on it long term, but just a quick plugin can be a good idea for us Novice builders.
B_T

[nosaj]

To go to the radio forums and the old timers definately recommend them. We all make mistakes and this is just a safety belt for a entry level tinkerer to a pro. Sometimes home built tools can surpass what you can buy after all where we are is from people who liked to tinker.....Edison, Marconi, Tesla, Fender and they made their own stuff.

jason

If you haven't needed one for 50 years don't be the old dog check one out maybe 5 dollars in parts. You can always dress it up to fit your lab image. You can't have trends without trying something several times, this is for first time powerups and changes to your amp, or whatever your working on.

[MikeH]

While I've never actually had a problem that a current limiter has caught, I definitely don't think I'm immune to such errors, so I always use the limiter.

I've made mine a compact version, using a halogen bulb instead of a big incandescent type. I've also put a voltmeter on it, so when use it in conjunction with a variac, I can see where the voltage is at, as opposed to trusting the dial on the variac. Also a switch to by pass the limiting bulb, so it's basically as if it's not there with the limiter off.

[nosaj]

I saw a similar setup on the antiques radio forum using a 500watt halogen. I have found mine extremely useful in fixing other people's repairs.

jason