{Collins} Re: Variac Help...



Just to add to the good advice given by Greg:

 

When using my variac to bring up old equipment I temporarily replace tube rectifiers with plug in solid state replacements.  This allows you to truly ramp up the B+ gradually without any sudden increases.  The one caution with this method is that you will be getting B+ before tube cathodes are at full temperature so don’t take too long in doing this to avoid problems and I would pull any expensive (particularly tubes valued by the audio equipment collectors) to avoid problems.  I just brought up a Pierson KP-81 receiver using this method; I was very concerned of smoke in this case as I replaced pretty much every resistor and capacitor in the receiver and I was concerned that I could have made a wiring error or two.  Fortunately, I ended up with no smoke and a nicely performing radio.

 

An additional note, my favorite tester/restorer for old capacitors is a ZM-11 resistance/capacitance/inductance bridge.  In addition to its various measurement functions it uses a small RF oscillator to generate high voltage (up to around 550 volts) for testing/reforming electrolytic capacitors.  If the capacitor shorts or develops excessive leakage the oscillator is stopped by the load and the voltage falls-thus no smoke or explosions.  You can gradually increase the applied voltage with the ZM-11 to reform the capacitor at whatever rate is needed.  I am sure there are more modern versions of this but the ZM-11 does the job very nicely.

 

Rodger WQ9E

 

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Wednesday, December 27, 2006 5:52 PM
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Subject: {Collins} Re: Variac Help...

 

The following explanation will be overkill in order to describe what will be happening when you connect your set to the variac and begin advancing it. Very generally speaking tube rectifiers will begin conducting at around 50 volts on the variac and B+ will rise quickly with any further advancement of the variac. What you really want to know is whether B+ is normal so you should have a meter on it to make sure comes up OK and does not seem lazy. Usually if filter caps are leaky B+ will not want to rise to normal value. This indicates excessive leakage current in the filter capacitor (and or possible failing screen and plate bypass caps). Ideally you would also want to monitor filter cap leakage current while you are advancing the variac because you would want to know about 10 mills of leakage current that did not decrease as the cap attempts to reform itself but this is not very practical. If the cap is healthy leakage current may rise momentarily as you advance the variac but will quickly diminish to less than one mill with each advancement of the variac and B+ will rise. When normal operating AC line voltage is reached the cap should exhibit less than one mill leakage and B+ should be normal. I would pay more attention to the B+ than what the knob on the variac is adjusted to until the cap reaches its rated operating voltage. You might go up in 50 volt steps for example while you watch for anything unusual. If a cap is healthy it will reform at each step in a matter of seconds and B+ will quickly rise. If a cap is not healthy B+ will act lazy; it may still reform but it might take several minutes. When a cap acts like this it will not retain its reformed condition. I would probably have the chassis out of the case and standing up on its side for this. After saying all this it is very possible your equipment will all be fine.

 

73, Greg



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