I have an old tube set that I want to fire up, but know this stuff can be unpredictable, especially with old caps. I recently bought a variac (variable voltage transformer). But I have never used one for this purpose.
So, I plug in the 120vac old tube device, where should I begin with the voltages? Should I start at half voltage (60v) or lower, or go higher?
I'm asking becuse I know that the filaments will be lower, but they can handle this. Of course the DC inside the radio will be lower too, and wonder how this will work for the tube rectifier.... But my big concern is the power transformer in the device. Can too low of a voltage harm the power trans.?
So, what voltage (or percentage of power) is the best place to begin?
My biggest concern are the power electrolytics (filter caps) being shorted, (on any old tube device). Yes, I do intend to replace them, but I still want to see what this device does, before I tamper with it.
In this case, I'm testing out a Hallicrafters SX-99 radio receiver. It's supposed to work, but I dont like the idea of just plugging in somthing thats 60 years old, and waiting for smoke..... (if there is a problem).
** Well, I see it uses a 5Y3 as do many old guitar amps from Fender etc .
Based on my experience with lots of them, I would have the set open on the bench so you can see and SMELL everything !
Wind up the Variac in 10 volt steps, keeping an eye on on the 5Y3 as you go, and wait a minute or two before the next step. Bad smells, flashes or glowing plates from the 5Y3 are causes to back off fast !!
Monitor the AC current draw if possible, more than say 500mA is cause for concern.
Not a good idea to try to reform the electrolytics this way. The reason is that the rectifier tube won't start conducting until the Variac voltage is pretty high, and if the electrolytics are very leaky, you risk damaging the power transformer due to excessive current.
Best way to attempt reforming the electrolytic filters is to remove the rectifier tube, then disconnect the wiring from the electrolytics. Then, apply a current-limited DC power supply directly to the capacitors, slowly bringing the voltage up to the normal operating voltage. Be sure to monitor the leakage current through the capacitors, not letting it get much above about 10mA or so. Reforming time could vary considerably, depending on the condition of the capacitors. If they are too dried out, it might be impossible to reform them, in which case, replacement is the only recourse. Sixty-year-old electrolytics aren't likely to be good candidates for reforming. There is likely to be more electrolytics in the unit; treat them the same way if they are tied into the B+ supply line. Any low-voltage electrolytics aren't likely to pose such a big risk to the transformer.
If you want to limit the risk of damage to the unit, my advice is to replace the capacitors first, then you can be reasonably sure that you won't damage the power transformer. The transformer is the single most expensive and hard-to-find component in the set. By all means, protect it.
Forget about that variac and just put about a 100 watt lightbulb in series with the set. Turn the set on. If the light bulb is burning near full brightness after a few seconds, pull the plug and look for problems. If the bulb is not very bright, make sure you have a fuse in the set for the rted value and plug the set in.
If the set had a solid state rectifier tube, you could pull all the tubes and use the variac to see if the capacitors were shorted or some other problems.
Replace primary electrolytic filter caps if it hasn't been running for many years. Running it with probably bad caps just stresses everything starting with the expensive transformer and working down from there.
You might try a 40 or 60 watt bulb to reduce the current available. Otherwise that is the same way we power up old jukebox equipment after it has been recapped.
John :-#)#
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I don't like call backs and I don't like working on the same radio twice. And I certainly don't like replacing shit that got ruined because I was too cheap to replace a $4 part.
A lot of people do it that way, but I'd worry about stripping the HT rectifier cathode.
I'd take all the tubes out and strap temporary silicon rectifiers on the rectifier socket.
You have to bring the voltage up very gently, but only the first 2 reservoir electrolytics are directly in the firing line.
The lower Vf of silicon rectifiers mean you can bring the HT up slightly higher than normal, so there shouldn't be any surprises when you re assemble it all with the original rectifier tube.
In theory; you should leave the capacitors energised for at least 12h to re form, but there must be some means of current limiting in case they break down - I certainly wouldn't leave a variac hooked up.
With all the tubes out - the temporary silicon with a 40W bulb in series with it should prevent any dangerous current flow.
A lot of people go to extremes to maintain authenticity - for some; that means gutting the old electrolytics and hiding new modern ones in the original cans.
It doesn't save any time, but if you do a neat job its hard to spot.
Old electrolytics that need re forming isn't a fault condition - its a natural characteristic of aluminium electrolytics.
The electrolyte is caustic and very slowly etches away the oxide layer dielectric. Thinner dielectric means higher capacitance, but lower breakdown voltage.
Re forming simply grows the oxide layer back to its original thickness. The leakage current is what's doing the work - but you have to do it slowly or it overheats and/or breaks down.
BUT....those are NEW capacitors, not old ones that have been reformed.
Which is still a waste of time. Once you've "reformed" the electrolytic, you're right back where you started from. A 50-75 year old part that needs replacing.
Leaky electros get hot fast and will soon pour smoke. OTOH the power tranny will survive a moderate overload for hours while a bigger overload will pop the fuse.
** Monitoring the AC current is all you need to do.
I haven't done alot of tube stufff lately, but over the years I got the ide a that all this variac and dim bulb stuff was for solid state where it can totally fry in microseconds.
Tube stuff I would just plug it in. Of course watch the rectifier, and then whatever other tubes, especially power tubes in there. Anything redplates pull it out and start again. Eventually you get down to seeing how much rip ple is on the B+ and go from there. Things with selenium rectifiers are of course a different story, I do not need FEMA here evacuating the neighborho od and I am sure these ninnies today would do some shit like that.
Most people overlook the PTC characteristic of light bulbs that result in a switch on surge. The few microseconds it takes the filament to heat can destroy semiconductors if you're unlucky. Most modern stuff with an SMPSU has a UVLO, so the variac often isn't a viable alternative.
A plain ordinary power resistor to limit fault current is safer. Tube equipment is more forgiving - but you can still come unstuck.
That should not bother tube equipment. Possibly if it has solid state recti fiers...
With solid state stuff though that surgge save a little bit of time by lett ing the main filters charge up. In fact certain switching power supplies wo uld not start without that surge.
Bottom line is that the lowly incandescent light bulb has more advantages t han disadvantagges in this appplication. One is a fairly positive visual in dication. For example I use the bulb to discharge the main filter caps. In fact I am looking to get one (or a few) from the UK or other country that u ses 230 volts on thier mains. this because many newer SMPSes are automatic voltage select and some double the line voltage in the US, and simply don't when contronted by the higher line voltage. Sometimes the filter caps are like 400 volts, and that will eventually burn out the bulb. I have no desir e to make another jig like that to have two of them in series, plus they wo uld have to be identical as well. I think I would just rather have a Britis h bulb.
I sat on my fingers until just now. This rant goes back a few years, but is as relevant now as then.
RANT WARNING:
Isolation transformers are critical for any workbench, and after good hand-tools the first thing any radio person should get... perhaps even before said tools.
Variacs, on the other hand, are either very handy diagnostic tools or worse-than-useless anchors fit only to let the magic smoke out of an otherwise salvageable radio. I am not really sure if there is an in-between other than their original function as light dimmers.
Without both current and voltage metering, they are worse-than- useless. With the proper metering, they can be very handy tools.
Myths: A variac will help reform caps (with specific reference to tube/ valve radios).
Fact: Only on those vanishingly few radios with solid-state rectifiers that pass B+ current at any voltage, and only if done over more time than one cares to consider. Bench DC supplies are best for this typically futile purpose in any case.
In the case of a tube rectifier, most of them do not start to pass DC until the filament voltage reaches between 65% and 75% of nameplate rating. If the B+ on the particular radio is say.... 350V, that means that the fir st voltage the caps-to-be-reformed would see would be something between 227 V and 263V... not exactly a soft-start.
Myth: A variac will allow one to apply voltage gently to a radio, so as to discover problems before they become fatal.
Fact: No, not at all. True, a variac does allow a radio to see the minimum amount of current to trigger its functions... an AA5 with bad filte r caps may begin to hum at 80V rather than 120V, similarly with a transformer radio. But if the caps are not so bad as to show audible hum, the variac ce ases to be useful. And a variac will *not* tell you whether a repaired radi o is OK or not.
However, if the proper meters are put on the Isovariac, then some real diagnosis can happen: An AA5 which should, by calculation draw say...
35 watts or so (0.3A), and draws 42 watts is dissipating 7 watts of heat somewhere... perhaps the output transformer. And, unless one has calibrated eyeballs, the difference of 7 watts will not show up on a dim-bulb tester.
Comes down to a current meter with fine enough increments to give meaningful information. Such a tool is useful right down the line from init ial diagnosis to testing the completed results.
End Rant. And it could have been much longer.
There are various sorts of metered isovariacs out there. I keep a Heath IP
5220 (Nice write-up here:
formatting link
) that has two rang es 0-1A and 0-3A. So far, nothing I have had on the bench has been beyond t he 3A quiescent range - I don't do televisions, which helps. But they are a lso made by VIZ, B&K, and a bunch of other top-flight companies. Some even have a leakage function.
I am *not* a believer or supporter-in-concept of reforming electrolytic cap s more than 40 years old. That is not suggest that 40+ year old caps cannot be good - I own many such in-circuit. But if a cap at any age does not pas s ESR/Capacity then reforming is a fools' errand as it is only a matter of time. Further, that *time* will be at its most unfortunate when it arrives.
Never had that problem. But I tend to underfuse my devices. I could see it happening with anemic devices and/or dead-shorted loads with improper or no fusing.
On Monday, 13 February 2017 21:37:35 UTC, snipped-for-privacy@aol.com wrote: snips
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g. If the B+ on the particular radio is say.... 350V, that means that the f irst voltage the caps-to-be-reformed would see would be something between 2
27V and 263V... not exactly a soft-start.
That's just wrong. If reforming by running a complete set from a variac, wi nd the voltage up _very_ slowly until rectifier conduction just begins. At this point the rect won't pass much current, and any bad cap present will c onduct, keeping the voltage on the lytics low. After a while the variac can be inched up _slightly._ Bear in mind that even a 2v increase on 240v main s increases rectifier conduction many times 1%, so very slowly does it.
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