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 Post subject: More on use of variacs in powering up vintage receivers
PostPosted: May Fri 22, 2020 10:14 pm 
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Joined: May Fri 22, 2020 9:48 pm
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Variacs and vintage equipment. I have recently purchased two refurbished Hallicrafter's receivers. I plan to use them regularly.I understand the reason for the slow Variac warm-up sequence for powering up a vintage setup the first time you use it. Do use the same Variac warm-up sequence each time you use the receiver?


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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 3:47 am 
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Some people will use the variac to bring up an old radio slowly to reform the electrlytic capacitors.

Should they fail while bringing up the radio then other damage to the radio may occur.

My advice would be to replace the electrolytic capacitors before powering up the radio as they

normaly bad.

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 5:18 am 
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As I've said before, I am a big fan of variacs in vintage radio restoration (method elsewhere...), but I don't use them for regular "switch on" (after all the wax and electrolytic caps have been replaced.) However, all my tube radios and amplifiers run off 110 to 115 VAC bucking transformers (I've made up several from scrap low-voltage transformers as our local line voltage is 124 VAC.
My work bench AC supply has a separate variac and isolating transformer.
Cheers,
Roger

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 1:34 pm 
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The controlled startup is only for initial checkout after parts replacement. I use a Variac + an AC ammeter.

It is good to watch the B+ during startup. If it's over ~325 to 350 volts, then you'll need to consider increasing the rating** of the filters, reducing the B+, or reducing overall supply voltage---eg with a bucking transformer.



**A good rule of thumb is to use a rated voltage that is 1.5X the steady-state voltage. This means that the common 450-volt parts are good up to 300volts. If the transient gets much over 350, I use 500-volt parts----sometimes even 600.

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 2:59 pm 
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Right, the radios are refurbished so we assume new caps were installed, all good.
Yes, you could set a variac for an output of 110 to 117 vac. But there is always the hazard of someone turning that big knob to see what happens. Also, the variac may not be designed for continuous use like that, could get a scorch mark where the wiper touches the coil.
If you have very high voltage at the wall outlet, a bucking transformer or a capacitor circuit are better ways to permanently drop a few volts.

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 8:56 pm 
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westcoastjohn wrote:
... Yes, you could set a variac for an output of 110 to 117 vac. But there is always the hazard of someone turning that big knob to see what happens...
The "default wiring" of the several "variacs" I've acquired over the years allowed a maximum voltage 140 VAC. I've rewired mine to "max. of supply voltage" for safety. I've never found the need to run stuff up to over 124 VAC (wall supply voltage.)
Cheers,
Roger

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sat 23, 2020 9:15 pm 
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Location: Powell River BC Canada
Controlled warm ups using AC power can be enhanced if a record of the current profile
is kept.

Few have chart recorders, and the few that do fret over where to get the paper from. :(

However, marching forward, some digital multi meters have RS 232 outputs and
software suites that run on computers.

A digital storage scope, has the same. (both screen storage, and the true digital type)

So where does this go ?

For 100 + years, the electrical side has been using charts of loads to see if all is well.

(As an aside, in a past snafu, a stuck pen, contributed to multi million dollar loss,
and almost, but by grace, major loss of life). I spent more than a month, picking some
electrical aspects of this apart)


Once the profile is know, of a vintage radio type, much can be done to prevent damage.

And the before and after (restoration) records can be an asset to hand over to the new
owner of the set you are selling.

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sun 24, 2020 12:16 am 
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You don't want to over due this. Tube filaments need a certain temperature to do much of anything.


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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sun 24, 2020 3:03 am 
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J. Hill wrote:
You don't want to over due this. Tube filaments need a certain temperature to do much of anything.

Agreed..
I normally get up to about 90% within 1-2 minutes. Then I usually keep it there for a while---keeping an eye on things, looking for any anomalies

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Sun 24, 2020 12:47 pm 
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Location: Long Island NY
It depends on how you use the receivers. If these are "daily drivers," no need to use a variac with them every time you turn them on. However, if you only use these receivers once a year and they are sitting on a shelf the rest of the time, then it is a good idea.

The problem is, all electrolytics require an applied voltage from time to time to remain in good condition. So if the equipment is used every day, or every few days, or even every few months, the capacitors stay in good shape and require no further attention until they reach EOL. But if they sit unused for more than six months, they begin to develop additional leakage. After a year or more of storage, should full voltage applied it is possible the leakage current will exceed the cap's ratings, resulting in internal heating and shortened life. In extreme cases they may short out or get hot enough to vent. Even if the capacitors survive undamaged, it is still rough treatment of rectifier tubes, power transformers, and other components.

By bringing the voltage up slowly, the caps can recover more gradually. After the capacitors have had time to recover, they're fine and you don't need to "variac" the equipment again unless it goes back into long term storage. The length of time a radio should be operated at reduced voltage is a matter of some experience and judgement. It depends on how old the capacitors are and how long it has been since they were last used. With most tube radios, very little will happen below 65-75 volts because the tube heaters are not hot enough to work. Therefore you might as well start from the middle of the variac dial, not from 0. An ammeter in line with the radio is a very good idea to ensure that the set is not drawing excessive current. By bringing up a few radios that have been in regular use, you can get a feel for what voltages they begin to play; then you will know when one is having trouble coming out of "hibernation."

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 Post subject: Re: More on use of variacs in powering up vintage receivers
PostPosted: May Mon 25, 2020 2:00 am 
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Location: WESTERVILLE OH
For testing after re-capping and new filter caps, I use a Variac as described above until I am confident the restoration is a success.
For a radio with a hi-lo switch, I run on the high voltage setting for cooler running (dimmer panel lamps and filaments)
Otherwise, I add a 2 or 3 amp thermistor for a soft start and a 2 to 4 volt reduction to the radio. Acts as an in-rush limiter.
Mouser sells the CL-90 and CL-80 but they get hot, so ventilation is a consideration.
The thermistor when cold presents a resistance the reduces as the thermistor heats up in a matter of seconds - hence the soft start.

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