Light Bulb Limiter, not Variac for 1st start up.

[Willabe]

I'm seeing more and more people posting about powering up a new build with a Variac in stead of a light bulb limiter (LBL). I don't know why, where and when this change happened. 

But it's much better to use a LBL to check for shorts caused by wiring it up wrong.

Used correctly, with the right sized bulb, it will provide better protection. And it's less money than a Variac.

It will give you a very clear warning by the bulb glowing brightly and will not stress any parts anywhere near what can/will happen when using a variac.

With the LBL, bulb stays bright, you shut down the power. And because it limits the current, it keeps parts from over heating. With the variac, you don't know there's a short until things start to over heat or do over heat. 

[uki]

LBL are very cheap and easy to build.

[AlNewman]

I agree, I don't understand using a variac as a safety measure when firing up an amp.  I really don't see the advantage of using a variac in a tube amp at all.

[AlNewman]

I guess setting wall voltage at a certain point.

[zircontweezer]

Why use a variac on a tube amp? one reason is charging up old electrolytic caps if the amp hasn't been used for a long long time, but yes a light bulb limiter would be my go to if I was testing a new amp build.

[mresistor]

Willabe   thanks for explainingg the whys in more detail about using a LBL at startup.  I also have no idea who/what/or where is promoting the use only a variac at startup of a new build. I also certrainly do not use a variac to fire up an amp first time that is brought to me with a serious problem ether. 

[shooter]

Step 1 before you even power up is to ohm the 1st "filter" past the rectifier to ground.   anything << 50k ohms indicates there might be a problem anything >>150k ohms, then add the LBL and power.

[scstill]

RobRobinette's startup calls for a variac (with LBL) in step 5 (https://robrobinette.com/Tube_Amp_Startup.htm) to "form" new power supply electrolytic caps. I have always done this. Is it necessary?

[Willabe]

RobRobinette's startup calls for a variac (with LBL) in step 5 (https://robrobinette.com/Tube_Amp_Startup.htm) to "form" new power supply electrolytic caps. I have always done this. Is it necessary?

I'd say no, you don't need to do that. It's a little confusing on Rob Rob's web site, but,

Robinette says he uses the variac to form the new electrolytic caps in the amp. He uses the LBL to protect from shorts. (Gerald Weber, Kendrick amps/books, said the same for forming new e-caps.)

Which if their new(er) caps you don't need to do that. And the LBL does slow down the surge dcv current so they don't get with full dcv if the amp has a SS rectifier. He says to use a 15w bulb to slow down the dcv to help slowly form the e-caps without a variac. It might help with older caps, but you should be using new e-caps anyway because of the electrolyte in the cap drying out. 

And e-cap makers form the e-caps at the factory before they ship them. And e-caps are always re-forming when voltage is applied.

To me, I don't want to use both a variac and a LBL at the same time for 1st power up. I wouldn't want a direct short to be hidden by slowly bringing up the ac wall voltage until it's high enough to get the LBL to light brightly. Yes, the LBL will be limiting the dc current, but it's still a direct short having dcv applied to the amp longer than if it was just the LBL. No thank you. 

Just use the LBL by itself, 1st with no tubes, them if that passes, put in the tubes and test again.

And his LBL picture shows the type of box/cover + the plug in light bulb socket adapter I was talking about. He just doesn't have the optional limit/full power switch. And he didn't use the deep box.

(Above from this thread; https://el34world.com/Forum/index.php?topic=32422.msg359408#msg359408)

[Willabe]

RobRobinette's startup calls for a variac (with LBL) in step 5 (https://robrobinette.com/Tube_Amp_Startup.htm) to "form" new power supply electrolytic caps. I have always done this. Is it necessary?

From R.G. Keen on e-caps, forming them, shelf life, cap life....;

http://www.geofex.com/tubeampfaq/tube_amp.htm#Section14

[acheld]

Short answer, no.   Assuming you buy caps from a supplier with good turnaround such as Mouser, Digi-Key, Newark -- and I assume there are others out there.   

[Willabe]

I built this small e-cap forming circuit and tried forming caps with it. 

It has a meter reading R in the caps ground lead connection to read the caps leakage. It works great. I saw how the leakage current went down as the cap formed. It went down to almost nothing, but it took 8, 10 hours to get it all the way down.

I don't know if the leakage would stay as low low after time. Or, if it even matters. You want low leakage, but some leakage is ok and normal.

Seems to me the more dcv is applied to the cap, the better the e-cap stays formed.

But you'd have to measure the same cap for leakage that was formed just in an amp from it's power supply. After being on for a few hours it might be the same or very close.

I'd use this below instead of using a variac to form new caps on a new build. Just form the caps up before you put them in. 

https://www.electrojumble.org.uk/reforming.htm

[Willabe]

Short answer, no. Assuming you buy caps from a supplier with good turnaround such as Mouser, Digi-Key, Newark -- and I assume there are others out there.

Yeah, Keen says cap makers design their caps for a 5 year shelf life. And if their new caps he says just turn the amp, new build on.

So, large parts dealers should sell their stock of e-caps before 5 years.

I think Gerald Weber said it was only a year before the electrolytic paste crystalized and could not be brought back. Seems like kind of a short shelf life to me for new modern e-caps.    

[shooter]

re-formed a bunch of large computer grade caps from the 80's n 90's 2 exploded on contact, the rest worked well

I don't know if the leakage would stay as low low after time. Or, if it even matters. You want low leakage, but some leakage is ok and normal.

my initial "short test" mentioned earlier give a pretty good indication of "leakage"; the "faster" the cap charges from measuring ohms the more likely the cap is "good", or low leakage.

[scstill]

So for startup build just connect it to the LBL and turn it on? Is it suggested to start with all tubes in place?
Or as Rob link first no tubes, then add rectifier, then add preamp tubes, then add speaker and Power tubes?
When and for what purpose do you then use the variac?

[shooter]

Is it suggested to start with all tubes in place?

I NEVER startup a fixed bias with PA tubes until I can confirm bias voltage at the tube side of the sockets, with "full power applied" (no LBL, variac...)


and ohm meter and schematic will find MANY errors long before LBL, AC is ever supplied.
1  see my "short test mentioned earlier
2 ohm tube side plate pin to B+ tap for that tube
3 repeat cathode pin to ground
do 2 & 3 for every tube besides PA
4 dummy plug with wires into input jack, ohm signal wire to "V1A" grid pin, ohm ground wire to chassis
these checks take 20 minutes, saves days of rabbit hunting.
 

[Willabe]

So for startup build just connect it to the LBL and turn it on? Is it suggested to start with all tubes in place? Or as Rob link first no tubes, then add rectifier, then add preamp tubes, then add speaker and Power tubes?

Yes, just plug the amp into the LBL, no variac, no tubes, no rectifier tube, if amp has 1.

This tells you that all the PT secondaries, and whats wired up to them, on/off switch, fuse, pilot light, heaters, 5acv rectifier tube socket, and -bias supply, if amp has 1, have no short. If SS rectifier, this also tells you power supply, B+ filter cap string should be good, no shorts. 

If the amp passes that test, then add the rectifier tube, if it's got one. This tells you power supply, B+ filter cap string should be good, no shorts.

If the amp passes that test, then if adjustable fixed -bias, verify -dcv at power tube socket. DCV present, set for max -dcv. 

Now with the speaker plugged in, or dummy load, put ALL the tubes in.

Passes that test, your should be good. Unplug the amp from LBL, and now if amp has adjustable -bias, set the -bias.

[Willabe]

When and for what purpose do you then use the variac?

You can adjust wall acv down, if it's high.

If doing test's and writing down the acv/dcv's, it helps to always start and keep the wall acv feeding the device under test at the same acv. So readings are consistent and relative to the same input acv. 

The e-cap forming circuit I posted in reply #11, I used a varaic to keep adjusting the power supplies dcv upward as the the cap formed.

Or anything where you need to dial in a voltage.

[Lectroid]

This is not related to startup, but--I've used a variac several times to push a known AC voltage through an unknown transformer to figure out the primary and secondary windings.  Uncle Doug has a video on doing this.     

[acheld]

Like Willabe, I use the Variac to adjust wall voltage.

All of my testing for documentation (and comparison) is done at 120VAC.   Unfortunately,  my wall voltage varies between 122 - 126 VAC, without rhyme or reason as best I can tell.   

When I deliver an amp, I deliver a chart with "standard" voltages (eg at 120VAC) and with wall voltage.

[Willabe]

....my wall voltage varies between 122 - 126 VAC, without rhyme or reason as best I can tell. 

Wall acv goes down with more usage, more people drawing current from the system, acv goes up when people stop using/turn off things. 

[Willabe]

 

[danhei]

Question on line voltages: is the wattage of the bulb selected independent of line voltage? Or do 220-240 VAC countries need to use half the wattage compared to 120?

[pdf64]

Question on line voltages: is the wattage of the bulb selected independent of line voltage? Or do 220-240 VAC countries need to use half the wattage compared to 120?

I have no experience in 120V regions, but theoretically and from the reports of others, the same wattage bulb has pretty much the same effect, whether it a 230V bulb in a 230V region or a 120V bulb in a 120V region.

[mresistor]

Like Willabe, I use the Variac to adjust wall voltage.

All of my testing for documentation (and comparison) is done at 120VAC.   Unfortunately,  my wall voltage varies between 122 - 126 VAC, without rhyme or reason as best I can tell.   

When I deliver an amp, I deliver a chart with "standard" voltages (eg at 120VAC) and with wall voltage.

What if you're checking out an amp that was originally made to work on 115VAC or 117VAC?

[Lectroid]

What if you're checking out an amp that was originally made to work on 115VAC or 117VAC?

Unless the owners all use variacs, their amps are going to be run at our higher wall voltages.  And even if an amp designed for 117V gets run at 123V, it's still less than a 5% increase.  Probably okay.

Bonus question for the Senior Wizards:  If an amp runs at a higher voltage, say, 123V, will it have a (slightly) lower current draw at that voltage?  Would it draw more current at a lower voltage?  Or am I off in the weeds?

[Merlin]

Would it draw more current at a lower voltage?  Or am I off in the weeds?

If you do nothing, the amp will draw more current as the wall voltage increases.

[Lectroid]

Would it draw more current at a lower voltage?  Or am I off in the weeds?

If you do nothing, the amp will draw more current as the wall voltage increases.

So, any electrical component does have its own intrinsic resistance, and it will only draw so much current at its rated voltage.. But if the voltage increases (analogous to water pressure), then the current must increase proportionately because it is being pushed by the increased 'pressure' in the conductor.  Is that right?

Is this condition of varying voltage mean we have to do anything special as builders?

[shooter]

I (current) = E (voltage) / R


if R is "held" constant and E goes up I goes along for the ride