[Amps] How Do You Know When Filter Caps Begin to Fail?

Wed Mar 28 19:26:01 PDT 2012

As a Novice I built a HB 6AG7/6L6 rig with surplus parts from Radio Row in 
NYC including the 400-0-400V 250ma transformer, choke and a plug in dual 8uF 
450V filter cap. I didnt own a meter.

About 5 minutes after turning it on for the first time I heard a hiss that 
kept getting louder and the 5Z3 was flashing real bad so I made a beeline 
across the basement as a tremendous bang let loose and I was peppered with 
debris. Now, Im 15 at the time but I KNEW something bad was about to happen 
and knew enough to get my young a** away FAST!

There was nothing but debris scattered everywhere and the cap had actually 
blown out of the socket and pieces of black gooey stuff plus aluminum 
shrapnel on the ceiling rafters, work bench, radio, 2 walls, floor and some 
on me. Mom came running all alarmed until she saw I wasnt hurt and 
eventually went back upstairs. Dad was at work still.

I bought an old 2nd hand Simpson and found the transformer was 
mislabled....it was 800V either side! Off to NYC the following Saturday for 
more parts. The rig worked, got me 2 FCC citations for 3rd harmonics before 
I found out I was tripling. by mistuning the pi network.

That big transformer wound up a few years later powering a 4x modified 1625 
GG amp just fine with some oil filled 2000V caps. That transformer may just 
still be around as I found the choke a year or so ago along with the loose 
1625's....56 years later.

Carl
KM1H

----- Original Message ----- 
From: "Bob K6UJ" <k6uj at pacbell.net>
To: <Amps at contesting.com>
Sent: Wednesday, March 28, 2012 7:23 PM
Subject: Re: [Amps] How Do You Know When Filter Caps Begin to Fail?

>I had one blow years ago,  It blew the white debri all over the inside of 
>the amp and
> scared the &*%$#%$  out of me.  I was lucky and only the cap went.  My 
> wife
> came running in and wondered what the hell happened, she was not impressed 
> :-)
> I decided to change all of them with new ones, hihi.
>
> Anyone else have them blow and can share with us ?
>
> Bob
> K6UJ
>
>
> On Mar 28, 2012, at 2:31 PM, Carl wrote:
>
>> A lot depends upon the quality of the cap to start with, its enviroment 
>> which also includes RF since electrolytics react poorly to it as seen in 
>> Dentron amps with only 500pf plate choke bypasses that are run hard on 
>> 160/80M.
>>
>> Many of the amps I get in for service have 80's date codes on the caps 
>> and those I always strongly recommend get changed. Subsequent testing 
>> confirms leakage is already in the twilight zone but not yet 
>> dangerous.....on most but there are exceptions.
>>
>> Since Computer Grade caps arent that expensive its good insurance. I buy 
>> all mine, no matter the amp, from Ameritron at $13.50 each for 270uF or 
>> $108 for 8 of them. These fit the Alphas, Amp Supply, Ameritron and a few 
>> others.
>>
>> Other amps such as Heath, Command, QRO, etc get the CDE 381LX series 
>> 330uF 450V 105*C caps from Mouser at $14.75 a pop but I buy the 25 qty at 
>> $9.95 since I use so many. They are also retrofitted into amps using the 
>> twist lock cans and the 30mm dia 220uf fits the NCL-2000 clips perfectly.
>>
>> Since the tolerance is far better than originals I cut down on extra heat 
>> and current by replacing 30-50K equalizers with 100K 3W MOX.
>>
>> Carl
>> KM1H
>>
>>
>>
>>
>> ----- Original Message ----- From: "Bob K6UJ" <k6uj at pacbell.net>
>> To: "amps at contesting.com" <Amps at contesting.com>
>> Sent: Wednesday, March 28, 2012 4:33 PM
>> Subject: Re: [Amps] How Do You Know When Filter Caps Begin to Fail?
>>
>>
>>> Pete,
>>>
>>> My understanding from input received on this reflector is that the 
>>> electrolytic filter caps have reached the end of their lifespan and
>>> are prone to go out after about 15 years.  This varies of course but it 
>>> is good idea to change them out if they are as old as yours.
>>> The problem with these guys is that when they go you don't normally have 
>>> any pre indication and they usually go kaboom and take out
>>> other stuff along with it.  Cheap insurance to replace them.
>>>
>>> Bob
>>> K6UJ
>>>
>>>
>>>
>>> On Mar 28, 2012, at 9:16 AM, Pete Smith N4ZR wrote:
>>>
>>>> I have an SB-220 that dates at least to 1978.  There is no external 
>>>> sign
>>>> of any problems, and the amp has been heavily used in contests for at
>>>> least 20 years.  It still delivers over 1300 watts on 80-20.  Is there
>>>> going to be any warning of a capacitor failure?  Will there likely be
>>>> any cascading damage to other components? Is it conventional wisdom 
>>>> that
>>>> I should replace them as a precaution?
>>>>
>>>> -- 
>>>> 73, Pete N4ZR
>>>> The World Contest Station Database, updated daily at 
>>>> www.conteststations.com
>>>> The Reverse Beacon Network at http://reversebeacon.net, blog at 
>>>> reversebeacon.blogspot.com,
>>>> spots at telnet.reversebeacon.net, port 7000 and
>>>> arcluster.reversebeacon.net, port 7000
>>>>
>>>> _______________________________________________
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>>>
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>
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