Author Topic: What's the correct procedure for Variac use...  (Read 718 times)

firedome

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What's the correct procedure for Variac use...
« on: March 24, 2019, 04:11:22 PM »
for bringing up an amp that's been sitting for years? I've heard various things regarding starting voltages, speed of increasing voltage, the total length of time, &c.  I've done it by many different protocols over the years, but always wondering what's the best way...  what say ye?
Happy Motoring! from Roger in NY

electra225

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Re: What's the correct procedure for Variac use...
« Reply #1 on: March 24, 2019, 05:45:20 PM »
I'm not sure there is a published protocol for using a Variac.  All a Variac is is a transformer with an adjustable output.  Its benefits are to isolate the device under test from the AC line and to be able to vary the voltage during power up.  It is also important to have some type of way  to monitor current, amperage, wattage, whatever.  A dim bulb is typically used, is cheap to make and saves many a device from the effects of a short in B+.  A wattmeter is good, too. 


I typically start at 50 volts with the dim bulb in series with the load.  Then after I determine there is no short, I typically increase voltage in 10 volt increments.  There is no right or wrong way as long as you are successful.
I don't need Google.  My wife says she knows everything.

firedome

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Re: What's the correct procedure for Variac use...
« Reply #2 on: March 25, 2019, 03:20:05 PM »
I know what they are and do, had one and been using it for 30 years, but I've heard in the past by some knowledgeable folks that starting at too low a V can be harmful to certain tubes or other components, raising V too fast can be harmful, yada yada and I prefer to go by facts, so while I've never had a problem I've long wondered what the reasons are for those statements as I said by some very knowledgeable people ie: EE types. I should have questioned them at the time but didn't.
Happy Motoring! from Roger in NY

electra225

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Re: What's the correct procedure for Variac use...
« Reply #3 on: March 25, 2019, 04:51:03 PM »
Clock motors and record player motors are not happy much below 110 volts.  TV guys don't like using a Variac on certain TV models (don't ask me which ones!) because the horizontal oscillator will not run, thereby possibly damaging the flyback and other parts in the high voltage section.  There is a famous YouTube channel that does not use a Variac much if at all.  Just plugs them in and monitors wattage.  I worked on radios for almost 20 years before I had a Variac. 



I don't need Google.  My wife says she knows everything.

TC Chris

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Re: What's the correct procedure for Variac use...
« Reply #4 on: March 25, 2019, 08:38:06 PM »
Also, I think the "starting too low" issue has to do with when the rectifier tube starts conducting.  Until it does, you are doing nothing to "re-form" or even challenge/test that filter caps.  I would try putting a voltmeter on the B+ and cranking up the variac until you start showing high voltage.  That means the rectifier is rectifying.  You could leave it at the first voltage a while--"long enough"--and then raise it incrementally, stopping at each step and making sure the filter caps aren't getting warm.

Chris Campbell

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #5 on: April 10, 2019, 07:34:31 AM »
               If you have an adjustable regulated DC power supply with separate voltage and current meters (I know that's quite an "if" but there's an alternate method too that works just as well) you can duplicate the dielectric forming process undoubtedly used during manufacture of the caps. Lets say your unit has a multiple section capacitor all rated at 450 VDC and another multiple section cap all rated at 350 VDC. Also assume that there are voltage dropping resistors and chokes between caps but no voltage "dividers" that have resistors with one side going to ground. 
               You would connect  the regulated output plus side to the rectifier output (cathode or filament) and the minus lead to ground. (Unit being restored is unplugged from AC)  You bring the voltage up to the lower voltage, 350 VDC in this case, but you pause every 10 volts or so and look at the current meter. The current meter will show 10 mA or so while you are actually raising the voltage if you do it slowly, because you are pouring charge into the capacitor, but when you stop raising the voltage, the meter should return to 0 mA. If it stays at a value above 0, just sit patiently and watch, and after a few minutes the meter will have slowly gone back to 0. Then raise the voltage again at about a 10 mA rate of indicated current, until the meter again stays up at 10 mA during the pause. Then wait again. Repeat until you get to the lower cap voltage rating, 350 in this case.
            Then return the supply to 0 volts and disconnect whatever wire or resistor connects the 450 volt rated cap to the 350 volt rated cap. D O N  T   F O R G E T  T H I S     S T E P     !  !  !   You don't want to be unsoldering things with 350 VDC on all caps and no voltage divide bleeding it off.   Now repeat the procedure on the 450 volt cap alone and take it all the way to 450 VDC.   It takes a while but your power transformer HV winding will thank you and so will your tubes for not subjecting them to low filament voltages doing it the other way, where you just Variac the main line cord on AC voltage.  I a going to post now before this disappears and mention the alternate way without a regulated adjustable DC supply next.

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #6 on: April 10, 2019, 08:11:46 AM »
      The alternate way is to take a working unit that has a DC voltage in it about equal to the lower cap voltage rating (350, again in this case) and connect the plus voltage (and ground !!) between it and the unit to be restored, but place several 50 k to 100 k resistors in series between the plus leads of the two units. Then apply power to the working unit (but not the one to be restored) and just wait. After several minutes, measure both DC voltages, the one on the plugged in unit, and the voltage on the unit being restored. The caps will have automatically been reformed at a 10 mA or less rate. If the procedure worked you should have the same DC voltage on both units, assuming a 10 megohm DVM or a 20,000 ohms per volt Simpson 260 VOM. If you have a low resistance voltmeter, say 1000 ohms per volt, use a high scale to measure, like 500 volts, so as not to load down the resistor side of the connection and cause a big voltage drop due to the series resistors. If the caps are hopelessly and permanently shorted or leaky, the two voltages will never be equal but at least you wont have risked your power transformer secondary winding.
       There are still the output tubes cathode bypass capacitor, the one rated at 25 to 50 VDC to reform, but it is probably healthy  enough to restore itself by disconnecting the previous setup and applying no more than 100 VAC to the unit the normal line cord way and slowly variacing up to 115 or so over a 20 minute time period.  Oops Time again !!

firedome

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Re: What's the correct procedure for Variac use...
« Reply #7 on: April 10, 2019, 11:08:49 AM »
Thanks for the comprehensive procedure Ed! It's a tad more elaborate than what I've done previously given my limited electronics background- I figured on plugging the AC into the Variac and upping the line voltage by one rate or another... lots to learn.
Happy Motoring! from Roger in NY

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #8 on: April 11, 2019, 06:17:11 PM »
  Yes, I see upon rereading the mess that I should have mentioned the "easy" way first since all you need is a working amplifier with a few hundred volts DC in the power supply and a couple of 47K or 100 K resistors in series jumpering that powered up unit to the DC supply of the unplugged chassis of the unit to reform the caps on. It works just as well too, the main difference is you don't get the pleasure of seeing the reforming process take place by watching the current meter rise up and then settle back to zero.
   The regulated DC supply I have is a Lambda model 71 and whatever they may have designed it for, it is perfect for doing the cap restoration and if it doesn't work then you at least know that the caps were hopelessly ruined, usually by an ambitious seller "trying it out first" just so they can advertise the unit as working instead of unknown. Which is sad, it's like selling a fuse as tested and  working by seeing what current makes it blow.
        If I was going to use a particular radio or hifi as a "daily driver" I would probably replace the 5U4 or 5Y3 with a 5AR4 (GZ34) or a 5V3 which have indirectly heated cathodes that take as long to warm up as the other tubes so the supply caps don't sit a 450 volts or more the first minute or two after turn-on with no current being drawn. Some Heathkits had a thermistor in series with the AC line switch which changed resistance from maybe 100 ohms cod to 10 ohms hot. That gave the tube filaments a soft turn on to limit the initial inrush current and also gave a slow turn on to avoid gobs of current going to the filter caps through low resistance silicon diodes. This was their W5M mono power amp with 6CA7s and solid state diode in the power supply. They used to make a "TV tuber saver" with a similar thermistor to plug your TV set into that gave the tubes and filter caps a "slow warm-up" to reduce tube burnout "especialoy with high line voltage

voxACthirtee

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Re: What's the correct procedure for Variac use...
« Reply #9 on: April 11, 2019, 08:57:33 PM »
Since most people don't have regulated DC supplies. Or ones that do high voltages for tubes,the easiest way to "form" from the ground up is to use a plug in SS rectifier with a variac.As Chris mentioned, your rectifier won't pass DC until the rectifier tube is seeing at least 50-60% of heater voltage. And while i've never tested it, it makes sense that once you hit that point, you will hit the caps with a full inrush of current, which is what you were trying to avoid by using a variac.
I don't bother anymore. Fire'em up and GO!

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #10 on: April 11, 2019, 10:58:58 PM »
      Fire em up and go  ???   You must be a lot quicker on your feet than me !!!    When sparks erupt or popping noises pop or puffs of smoke puff, my usual reaction is to stare in disbelief with dropped jaw and bulging eyes for maybe five long seconds , then run halfway out of he room, then stop and try to remember where there might be a plug to pull out or switch to turn off. '
      There are all sorts of regulated DC supplies on the ebay an they are so handy, its like buying your first multimeter, after a few weeks you wonder how you ever did anything without it. But an unregulated supply that doesn't adjust, like something in any old amplifier chassis can be used with a large series resistor, such as a 100 k 2 watt resistor. Even into a short, the current from a 200 VDC source is limited to 20 mA which will reform without overheating a cap

voxACthirtee

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Re: What's the correct procedure for Variac use...
« Reply #11 on: April 12, 2019, 10:10:29 AM »
At this point i've built literally hundreds of guitar amps. I now use pretty much either Sprague or TAD caps.Have never had bad one. I did "form" for a while, but if anybody here thinks major manufacturers "form" they are nuts. I'd guess maybe boutique and high end companies do, but a brand like Fender? the cost of labor and cost of electricity to form caps off their bottom line? Not in this day and age. Probably not back in the 50's/60's either.
Can't really say i've ever noticed a difference in sound, nor have i had an amp come back in need of an early replacement on a failed electrolytic.
YMMV, TAD and Sprague make solid caps. If you are using 69cent electrolytics, where the built in failure rate is high, maybe not so much.

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #12 on: April 12, 2019, 07:11:54 PM »
         Yes I agree, probably nobody manufacturing amps and things is going to check that the caps are still "formed" to the correct working voltage stamped on the label, since the parts are fairly new and I think it takes years of sitting without any DC voltage on them to "deform"  Its the original manufacturers of the caps that have to do the initial forming. I think they are dead shorts when made and they just apply a low current and let the voltage increase until they get to the rated voltage.

ed from Baltimore

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Re: What's the correct procedure for Variac use...
« Reply #13 on: April 12, 2019, 09:38:03 PM »
       VoxACthirtee, do you get a lot of older tube guitar amps to restore for friends or do you mainly build new ones ?  I've got a few friends that are constantly trading in old guitar amps for other ones just as old, say like a Fender Princeton or Twin Reverb, "pre CBS" but these are usually units that have never sat idle for years so their caps at least have a chance of being good, whereas a lot of hifi radio consoles have sat for thirty years without being plugged in for even two minutes. Those  units are almost certainly in need of reforming their old electrolytic caps. 
         When I google "vintage amp schematics" images, a lot of classic old designs come up but so do a lot of new ones too. I really admire the guitar amp guys because all through the late sixties on through til today, they always insisted on sticking with their tube amps when the rest of the electronics world was going transistor and IC and claiming that there was nothing tubes could do that transistors could do better. I notice that the Fender Blues deville, if that's the right name, schematic is identical stage for stage and part value for part value with an old Fender design, when all the new effects circuits are in the disabled position. Neat

voxACthirtee

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Re: What's the correct procedure for Variac use...
« Reply #14 on: April 20, 2019, 08:05:51 PM »
Hi Ed, sorry i just saw this.
Most guitar amp companies either went solid state, or diverged to solid state being a huge part for a while in the 70's and 80's. But most have gone back.There are "good" SS guitar amps, but most are considered low end and are made to be low end as they are super cheap to make in Korea or Vietnam.
Most guitar amp companies have the tube line as the flagship line.Some are still handwired, some are PCB some are turretboard or fiberboard, but most don't vary a great deal from the old designs.There have been a lot of good new ideas and options, but i'm a fan of the simpler designs.
Personally i don't do "repairs" per se. For friends and the odd customer i've already sold an amp to, sure. But i really would hate dealing with folks who think 3 hours of your time tracking down a stray oscillation in the tremolo circuit should only cost them $25. I hear the stories from friends who do repairs. Obviously not everyone is like this, but i hear a lot of stories....