Reforming caps w/light bulbs

Andyman

Andyman

Scroungus Stereophilus
Subscriber
I need a bit of a hand here. Last week I picked up a nice vintage Heathkit mono rig: FM-3A tuner, WA-P2 preamp and W-5M power amp. I haven't tested them because I've been reading all these posts about needing to reform the filter caps so they don't fail. I don't have a Variac, so i'm interested in the light bilb method. The problem is I'm not sure EXACTLY how to proceed. Is the bulb in series and are the higher or lower wattages used first? I often see mention on a resistor being addded and maesuring current with an ammeter; where and how is this done? One post over at Vintage Asylum even suggests using a dimmer as a poor man's Variac and draws mixed responses.

I'm no tech here, my expertise is more in the physical realm (replacing belts and bulbs, refoams, cabinet and pot cleanups etc) so I'm at a bit of a loss what to do. I do have my 25 year old analog Radio Shack multimeter, which I believe may have an ammeter function, but would appreciate some direction/help here.

I checked the threads and found some references, but would like some "no-brainer" direction

TIA, Andy
 
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Andy

I never messed with the bulbs since Varaics are pretty cheap. I would be more then happy to bring the pieces up on my variac for ya. If you interested drop me a message. Congrats on a superb score !

Grumpy
 
Andy,

That sounds like a nice score. The Heath w5m has a great reputation. Wire a light socket in series with the power cord to the tube amp and put the VOM on AC volts across the power cord to the amp. Try different screw in lamps starting with 25 watt, then 40, 60, 100. You want to see a steady ~50 AC volts on the amp for the first part of the bake process. That will be just high enough to get the rectifier tube working so that some B+ will be present to form the caps. Leave the amp running for 24 hours in this mode, then proceed to the next higher wattage buld and go another 12 hours and so on. Put a speaker load on the amp output with the volume control turned to minimum during the bake in to be safe. If at any time during the process you hear a pronounced hum in the speaker you have a problem, likely with a shorted tube or capacitor.
 
Ahh yes, the lightbulb trick. Used that when i was restoring the Mc240. Too bad it didnt work or else id be able to tell you how to do it...

Not that the light bulbs didnt work, but one of the output transformers were shot so thats why it didnt work.
 
Hell, you could even start off with a tiny nightlight...a little 3-watt job. This would bring it up very slowly. Power up, measure the voltage on the filter caps. It would take a while this way, but with the high resistance of a very small bulb, you could control the process very well.

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I haven't done this, but I see no reason why it would not work.
 
Originally posted by EchoWars
Hell, you could even start off with a tiny nightlight...a little 3-watt job. This would bring it up very slowly. Power up, measure the voltage on the filter caps. It would take a while this way, but with the high resistance of a very small bulb, you could control the process very well.

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I haven't done this, but I see no reason why it would not work.
Click to expand...

The reason this won't work is because you need to give the power transformer enough voltage and current to light the filament in the rectifier tube enough to start conducting. It is the presence of B+ which comes from the rectifier tube that does the work re-forming the filter capactitors. Of course if the tube amp has SS rectifiers then a smaller wattage series lamp will begin to work but a measly 3 watts is not going to be enough to overcome the cold resistance of the tube filaments. Nothing will happen except full power being dissipated in the night lamp.
 
Ya see, there ya go talking about those lame tubes again. ;)

(No, I didn't realize that the gear was tube, and it was not made clear either in the first post or subsequent posts...)
 
EW,

I did mention rectifier tube in my first post and explained the mechanism of needing enough minimum power that would heat the filament to conduction.

You must have had your head stuck in the sand. ;)
 
Um...well...yes you did.

///me sulks away to work on my reading comprehension skillz:headscrat
 
Echo,

I meant your head was in solid state amp thinking mode....you know, sand amps!
 
Grumpy,

Thanx for posting that link. It is a pretty good treatise on the subject for novices.
 
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