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Topic: Replacing filter caps on an old BF Showman |
Tom Wolverton
From:
Carpinteria, CA
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Posted 24 May 2012 11:51 am |
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I recently had my filter caps replaced on my '65 Showman head. I read somewhere once about "forming" caps by slowly burning them in with a Variac. Slowly raising the voltage about 10 V an hour up to full AC level. Does this really need to be done with filter caps?
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To write with a broken pencil is pointless. |
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Tim Marcus
From:
San Francisco, CA
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Posted 24 May 2012 12:14 pm
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I usually form them with a lightbulb limiter, which can be built from about $15 in parts from any hardware store
You can use a 25W bulb in series with the "hot" AC line - or get fancy and put it on a lightswitch like I do.
When you turn the amp on for the first time with the bulb in series, you will see it light up for a few seconds, and then gradually dim. Thats the inrush current - which you have protected the caps from the first time they charge up.
When the lightbulb is dim, you will see low voltages on your power supply. After a few minutes you can pop it out of series, and everything will pop up to where it is supposed to be.
This is also good in case you wire something incorrectly, because the lightbulb will spare your line fuse (they add up if you are popping them often)
This is the basic idea:
http://www.antiqueradio.org/art/DimbulbSketch.gif
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Milkmansound.com |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 25 May 2012 12:32 am
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Thanks Tim. I'll check this technique out. Probably easier than using a Variac, too.
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To write with a broken pencil is pointless. |
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Rick Johnson
From:
Wheelwright, Ky USA
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Posted 25 May 2012 5:09 am
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Tom
I've never did that, I doubt that it makes
much difference...just play it.
Rick
www.rickjohnsoncabinets.com |
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Tom Wolverton
From:
Carpinteria, CA
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Posted 25 May 2012 8:29 am
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Rick, it gives me something to do while you make me a new speaker cabinet for it. : )
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To write with a broken pencil is pointless. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 27 May 2012 4:30 am Re: Replacing filter caps on an old BF Showman
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| Tom Wolverton wrote: |
| I recently had my filter caps replaced on my '65 Showman head. I read somewhere once about "forming" caps by slowly burning them in with a Variac. Slowly raising the voltage about 10 V an hour up to full AC level. Does this really need to be done with filter caps? |
Definitely not. I can say without doubt that whoever replaced the caps in your amp just plugged the thing in and turned it on. Anyhow, new capacitors are always "formed" at the factory when voltage is applied during the testing process. The variac procedure you mention is sometimes done to amps that have been stored for many years as it's thought that it can sometimes rejuvenate old, dried-up caps (in order to keep the amp 100% original). Personally, I dubious about the procedure, even for that application, and I'd prefer new filter and bypass caps. A "rejuvenated" cap may work, but there's certainly no guarantee that it's electrical properties and performance will be anywhere close to those it had when it was new. |
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