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    Do I need a VariAC?

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    mcgyver74

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    Join date : 2017-03-24
    Location : Jersey City, NJ

    Do I need a VariAC?

    Post by mcgyver74 on Wed Apr 12, 2017 2:12 pm

    So I should be done with the build of my ST120 in a few days and will be ready to do the initial startup procedure documented in the manual.

    However, I have been perusing a lot of radio forums and all seem to suggest bringing a newly built or re-build amp up slowly on a Variac.... Not sure if this is a good idea with my amp or not..

    What say Ye all? Smile
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    Peter W.

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    Re: Do I need a VariAC?

    Post by Peter W. on Wed Apr 12, 2017 3:03 pm

    mcgyver74 wrote:So I should be done with the build of my ST120 in a few days and will be ready to do the initial startup procedure documented in the manual.

    However, I have been perusing a lot of radio forums and all seem to suggest bringing a newly built or re-build amp up slowly on a Variac....  Not sure if this is a good idea with my amp or not..

    What say Ye all? Smile  

    OK - a few things:

    If you did a good job on the build, no. But there are a few things you can do to reduce risk in any case:

    a) Without tubes, any tubes at all, plug in the amp and turn it on. At most, you should be getting a very, very low hum from the power-transformer. Turn off and allow to 'rest' for a few minutes.
    b) Add the small-signal tubes only. They should light up normally. Turn off as above.
    c) Add the output tubes. They should light normally. Give this about 5 minutes, checking for anything untoward and for any type of heat-build-up in the power-transformer. If you have a temperature gun, that will be very useful. Rest as above.
    d) If, as expected, all goes well - install the rectifier tubes. Connect a speaker load and either short the input sockets or install an input - at Zero volume. Turn on the amp and watch like a hawk. You should be getting no noise at the speakers, certainly no hum. Allow the amp to warm up, then do the bias set per the manual - again, Zero volume.
    e) Presuming all is still going well, and the amp has been warmed up for 15 - 30 minutes, you may apply signal - which should be very nice!

    MINOR RANT WARNING!

    Observations on Variacs as a species and as useful tools on an electronics bench:

    1. Keep in mind that the original purpose of the Variac (Variable Auto Transformer and trademark of General Radio) is to dim lights, mostly in theaters, and act as a boost/buck transformer in other applications. They are NOT designed as diagnostic tools or slow-start tools as-found.
    2. A device with a tube rectifier will not see B+ until the rectifier tube sees about 80% of operating voltage. So, in a ST70, the filter caps will see ~375V at the onset of B+. Not exactly a slow start.
    3. For a device that pulls approximately 175 watts when in operation, even a dimbulb tester is not particularly efficacious.
    4. Meaning that you need to meter said Variac for both amps(watts) and volts, with meters sensitive enough to determine small increments. OR the device is entirely useless for the purposes you suggest.

    http://www.oaktreevintage.com/web_photos/test_gear/Heathkit_IP-5220_variable-AC_power_supply_variac_Web_small.jpg  

    I keep one of these - and it does exactly what is needed for the purpose you suggest - it will allow me to watch the onset of B+, and the overall current draw of any device attached at any voltage from 0  to  140 VAC in tiny increments (amps and volts).  There are quite a number of similar devices out there with equal-or-better precision made by a number of excellent and reliable manufacturers. These are somewhat expensive tools for the casual user, but extremely useful for the more-than-casual user. And as they isolate as well, they serve dual-duty. Isolation transformers are a separate rant, by the way.

    And in any case, you should be fine, assuming reasonable assembly care and basic skills.

    By the way, there is one on eBay as we write - not mine, no interest to me, but there it is:

    http://www.ebay.com/itm/Heathkit-Model-IP-5220-Variable-Isolated-AC-Power-Supply-/162463703965?hash=item25d397639d:g:M4oAAOSwXYtY2tVl
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    PeterCapo

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    Re: Do I need a VariAC?

    Post by PeterCapo on Wed Apr 12, 2017 4:13 pm

    mcgyver74 wrote:So I should be done with the build of my ST120 in a few days and will be ready to do the initial startup procedure documented in the manual.

    However, I have been perusing a lot of radio forums and all seem to suggest bringing a newly built or re-build amp up slowly on a Variac....  Not sure if this is a good idea with my amp or not..

    What say Ye all? Smile  

    A variac is good to have for diagnostic work.
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    sKiZo

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    Re: Do I need a VariAC?

    Post by sKiZo on Wed Apr 12, 2017 5:29 pm

    Nah ... just plug it in and have the fire extinguisher handy is all ... clown

    A variac is a handy tool on the workbench, but that's more for old stuff that's been collecting dust for years. You want to bring those up slowly to see what sort of effect time has had on the iron and electrolytics. New construction, not so much.

    One thing I WOULD do is build a dim bulb tester. That will tell you immediately if you have any amp killing issues as soon as you fire it up. Any one of a number of diagrams on the web showing you how to make one. The simplest version is simply a light bulb socket in series with the hot AC wire. Hardest part nowadays might be finding a high power incandescent bulb for it.

    And yes, DO follow the start up instructions, step by step, on first fire. That can prevent many a headache.

    ** Most important, when you think you're done, you're not. Hopefully, you've been checking off each step as you go along - I'd go over the entire build list one more time to see if you missed anything and closely inspect each connection for good solder and long tails, especially around the VTA board. Close tolerances and a bit crowded there, so good place for a short. Also pay particular attention to any of the "solder later" steps in the instructions. I highlighted those and was especially careful making sure I didn't miss any of those.

    mcgyver74

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    Location : Jersey City, NJ

    Re: Do I need a VariAC?

    Post by mcgyver74 on Wed Apr 12, 2017 5:36 pm

    Thanks everyone,

    I am checking each step as I complete it and plan to go over the build instructions one more time before I apply power. Basically I don't apply power until each step has 2 checks, Smile

    I'll take a look at the dim bulb tester and give that a shot, I think I have a 100 watt incandescent laying around
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    deepee99

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    Re: Do I need a VariAC?

    Post by deepee99 on Wed Apr 12, 2017 6:16 pm

    I have no particular use for 'em, although Roy M. swears by them.
    I would add to Peter's powering-up advice this neglected little matter on Variacs. They are totally line-voltage dependent. Input from the wall goes up by a volt, output from the Variac goes up by a volt. Ditto the other direction. Four-volt (+2 ~ -2VAC) fluctuations are common on our grid and the Variac merely passes this swing on to whatever you've plugged into it. U.S. power companies are required to provide juice within 10 percent of 120 VAC. That's a range of 108 VAC to 132 VAC.
    There's another way to go, and that's a very spendy (not to mention boat anchor-heavy) Furman SPR-20i. or P-1800 AR. I have the former. It guarantees a 119-121 VAC sine-wave output through a wall-voltage range of 113-126 VAC, using an auto-former and a bunch of other mysterious circuitry. Sure makes setting and checking tube bias a walk in the park because you're not chasing wall voltage but the price is in the $1600-$2000 range and it needs a 20-amp circuit. Their 15-amp model is about half that price but the voltage output range is not quite as narrow.
    I have no dog in this fight (i.e. I don't work for Furman or any of their re-sellers) but I've used this since Day One of going back to tube gear and consider it money well spent.



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    Bob Latino
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    Re: Do I need a VariAC?

    Post by Bob Latino on Wed Apr 12, 2017 7:01 pm

    You do not need a variac to start up a NEW amp. That said, If your line voltage is 122 VAC or above, you should probably use a variac to run your amp. Set the variac's output to 117 - 118 VAC. A variac for initial start up is recommended for older amps that have not been run for years (or decades!). While the amp sits there unused, chemical changes occur in the amps electrolytic caps and SOMETIMES the electrolytic cap will "reform" and possibly be usable again if the voltage going to the amp is brought up slowly on a variac. This is much the same thing as an automobile sitting for years and the battery going bad from disuse. Sometimes you can recharge the battery but sometimes the auto's battery will not "take a charge".

    Bob
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    j beede

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    Re: Do I need a VariAC?

    Post by j beede on Wed Apr 12, 2017 11:58 pm

    I would skip the variac. I would use the dim bulb during initial power up processes as an early warning for shorts.
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    sKiZo

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    Re: Do I need a VariAC?

    Post by sKiZo on Thu Apr 13, 2017 1:25 am

    Or instead of a variac to step down high VAC line voltage, build yourself a bucker like I did.



    I had a strong little Hammond transformer in the scrap box. The old school metal power strip was just gravy. Not everyone will be so lucky. Anyone know a good source of transformers that would work? Mine's rated 6a and has just the single 7v secondary, which is pretty much perfect for my needs.
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    Peter W.

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    Re: Do I need a VariAC?

    Post by Peter W. on Thu Apr 13, 2017 10:01 am

    All of this skims past the very relevant issue of bench safety. I am of several minds on this (Yeah, DeePee, yeah) for several reasons.

    Those doing a one-off assembly are probably not going to want to invest in the basics of a safe electronics bench, several hundred dollars (US) at the least.
    Those who do this as a hobby are probably already tooled, and take it for granted that "so is everyone else".

    In any given year, I probably have anywhere between 50 and 80 items cross my bench. Some multiple times (my stuff), some only once as a courtesy for neighbors or friends - one neighbor in particular being good for five items so far this year. That, and doing this for 40 years means that I am fairly well tooled. But my neighbor is not - he is a kitchen table hobbyist.

    Cutting to the chase: What would be the interest in suggesting bench tooling for the various levels of the hobby? This would include some very basic stuff such as NEVER, ever, nohow EVER work on a slab-on-grade concrete floor, or any concrete floor over a metal deck - and up.

    Thoughts?
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    10-E-C

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    Re: Do I need a VariAC?

    Post by 10-E-C on Thu Apr 13, 2017 12:15 pm

    Peter W. wrote:All of this skims past the very relevant issue of bench safety. I am of several minds on this (Yeah, DeePee, yeah) for several reasons.

    Those doing a one-off assembly are probably not going to want to invest in the basics of a safe electronics bench, several hundred dollars (US) at the least.
    Those who do this as a hobby are probably already tooled, and take it for granted that "so is everyone else".

    In any given year, I probably have anywhere between 50 and 80 items cross my bench. Some multiple times (my stuff), some only once as a courtesy for neighbors or friends - one neighbor in particular being good for five items so far this year. That, and doing this for 40 years means that I am fairly well tooled. But my neighbor is not - he is a kitchen table hobbyist.

    Cutting to the chase: What would be the interest in suggesting bench tooling for the various levels of the hobby? This would include some very basic stuff such as NEVER, ever, nohow EVER work on a slab-on-grade concrete floor, or any concrete floor over a metal deck - and up.

    Thoughts?


    Working on concrete with high voltage electricity is very dangerous. When we would do maintenance on the switch gear where I retired from we would always line the work area with thick cardboard, so not be standing on the concrete floor. Also the old electricians trick of having your left hand in your pocket while working on live circuits with your right hand so there wouldn't be a path across your chest to ground in case of and electrical shock.

    TM
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    tubes4hifi
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    Re: Do I need a VariAC?

    Post by tubes4hifi on Thu Apr 13, 2017 3:25 pm

    my bench sits on carpeting over carpet padding over wood flooring Very Happy
    Otherwise, a rubber mat is in order . . .
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    sKiZo

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    Re: Do I need a VariAC?

    Post by sKiZo on Thu Apr 13, 2017 4:33 pm

    ... always wear pants when soldering ...
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    deepee99

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    Re: Do I need a VariAC?

    Post by deepee99 on Thu Apr 13, 2017 4:58 pm

    Peter W. wrote:All of this skims past the very relevant issue of bench safety. I am of several minds on this (Yeah, DeePee, yeah) for several reasons.

    Those doing a one-off assembly are probably not going to want to invest in the basics of a safe electronics bench, several hundred dollars (US) at the least.
    Those who do this as a hobby are probably already tooled, and take it for granted that "so is everyone else".

    In any given year, I probably have anywhere between 50 and 80 items cross my bench. Some multiple times (my stuff), some only once as a courtesy for neighbors or friends - one neighbor in particular being good for five items so far this year. That, and doing this for 40 years means that I am fairly well tooled. But my neighbor is not - he is a kitchen table hobbyist.

    Cutting to the chase: What would be the interest in suggesting bench tooling for the various levels of the hobby? This would include some very basic stuff such as NEVER, ever, nohow EVER work on a slab-on-grade concrete floor, or any concrete floor over a metal deck - and up.

    Thoughts?

    Thought I woke up to my ears burning, Peter . . . and I was sleeping so soundly here on the Left Coast.
    My bench is dual-purpose: I hand reload rifle and pistol ammo on it, and I use it for the destruction and occasional repair or creation of electronics. It is obviously set up to be doing one OR the other at any given time. Primer caps for ammo will detonate with electrical charges, and one's soldering iron/gun should not be tempted to come into contact with smokeless powder as a very hot fire will ensue. So a couple of orange crates for each activity and a bare bench when changing modes. Some tools do come in handy for both activities, i.e. pliers, calipers, cutters, etc., and are stored in drawers.
    I even have a little basket to collect jewellery.
    A wood floor is a must, and not just for electrical safety. Kaiser, (Liberty ships, Boeing airplane skin, aluminum foil etc.) found they could save billions of dollars/year treating non-specific chronic back injuries paid through their health coverage by having wood factory floors. The rolling mills over in Spokane are over a mile long and are wood floors or, more specifically, giant concrete slabs covered with wood mill-ends, grain-up, as are I believe all their factories. Especially if you're prone to standing at a bench, an end-grain-up wood floor will pay dividends on your back in your dotage. Still hurts like the back of God's hand if you fall on it, but it is a good thermal and electrical insulator and just enough of a shock absorbent to spare your knees and hips.
    Lighting is oft overlooked and in this case, the more the better, especially as we age. Full spectrum fluorescent or halogen lights set so they're not casting shadows anywhere will make sorting resistors and diodes and wiring-up PC boards a lot easier, with the caveat being that fluorescents are electronically very noisy and send you chasing after some racket you think you're circuit is generating but isn't.
    Spend good money on a good, variable heat soldering pencil (mine's a Circuit Specialists' Station 75, which comes with all the stuff you otherwise keep running back to the hardware store for) and on a high-wattage soldering gun, the latter for those big ground and speaker posts you come across when the whole chassis becomes a heat sink.
    I run a 15-amp 12-outlet stainless heavy-duty power strip fastened to the wall above the bench. Beats fumbling around on the floor looking for a source of juice.
    As to basic tools, most of the time you're working with cutting tools. Nothing's more aggravating than a dull wire-stripper. Either keep them honed or keep a new one in supply. Wire ties, little strips of Velcro, tooth-picks and pipe cleaners also come in handy, as do a pair of those miniature pliers with the magnifying glass and a jeweller's loupe do wonders. Nice Dremel set. A point-file set as well. Jug of 90+% denatured alcohol, not the 70% stuff they sell at the grocer's which leaves as much oil behind as you just tried to remove.
    Think ergonomics, too. A comfortable chair or stool, and a wrist-pad along the lead-edge of the bench leave your mind free to focus on the task at hand rather than the aches and pains from repetitive movement. Half-inch-high fiddles attached to the sides and rear of the bench will keep stuff from falling onto the floor.
    An asbestos body-length blanket is a good idea if you're soldering in the nude. Otherwise, wear levis, not cut-offs or your senior prom attire. Spend money on decent solder, too, and keep a 55-gallon drum of de-Oxit somewhere.
    I painted my bench surface a semi-gloss enamel white, which makes it easier to find things than on a dark wooden surface. I keep a fire extinguisher within reach, too, although despite all the stupid things I've done making things that either are or are not supposed to blow up, it's never been called into service . . .





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    Peter W.

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    Re: Do I need a VariAC?

    Post by Peter W. on Thu Apr 13, 2017 5:09 pm

    tubes4hifi wrote:my bench sits on carpeting over carpet padding over wood flooring Very Happy
    Otherwise, a rubber mat is in order . . .

    I have posted as a new subject under "BENCH".
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    deepee99

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    Re: Do I need a VariAC?

    Post by deepee99 on Thu Apr 13, 2017 5:13 pm

    Peter W. wrote:
    tubes4hifi wrote:my bench sits on carpeting over carpet padding over wood flooring Very Happy
    Otherwise, a rubber mat is in order . . .
    I have posted as a new subject under "BENCH".
    Link? I don't see it in the Basket yet.
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    sKiZo

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    Re: Do I need a VariAC?

    Post by sKiZo on Thu Apr 13, 2017 5:18 pm

    Concrete floor here, but I put down some interlocking mats in the bench area. Just lay em out and you're done.



    Chairs tend to sink in a bit, but not really a problem here. The mats will spring back to shape eventually.

    Latest addition is a heavy duty turntable left over from a cabinet project. I cut several plywood squares that I'll build projects on. Rotate the work any way I want, and if it stalls, I can just pick the whole thing up and shuffle it aside ... Thought about adding handles to the boards, but it's easy enough to get my hands under the edges and just lift. I get bored, I'll add a raised border on each to keep little stuff from rolling off.

    Oh. My home not away from home ... right cozy, with everything in reach with a spin of the stool.



    (and you can NEVER have too much light, eh.)
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    bluemeanies

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    Re: Do I need a VariAC?

    Post by bluemeanies on Thu Apr 13, 2017 9:50 pm

    I agree with Bob.
    I have a Variac only b/c my in line voltage varies from 117-125 during certain times of the day. Glad I have it. Start my m125's up , adjust the Variac to 119.0 and I am set, safe and secure.
    Industrial size and equipped but only paid $100.00 on e-bay

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