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Old 30th December 2010, 09:14 AM   #1
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Default Lafayette amp arcing Rectifier problem

I'm rebuilding a old lafayette la75 and changed all the capacitors and tubes but i got a small problem on the rectifier now. On a cold startup i hear a bit of a Cracle but when i turn it off, an try a hot start, it will arc in the rectifier ( 6ca4 ). I already changed the power tubes, and the rectifier.. SO i dont think thats the problem... Does anyone have any suggestions?? Does a wrong bias can couse this problem?? I dont think so but just to be sure..
Thanks for all the help.. And here is the scematic:
http://www.vintageamps.com/plexiboar...hp?id=2018&t=1
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Old 30th December 2010, 10:48 AM   #2
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I suggest initially replacing the 6CA4 with two 1N4007 and check voltage rails - if possible, use a variac to check if all is normal as AC level ramps up, before applying full mains. Add a fuse in the secondary CT leg.

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Old 30th December 2010, 01:52 PM   #3
20to20 is offline 20to20  United States
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Quote:
Originally Posted by engltje View Post
I'm rebuilding a old lafayette la75 and changed all the capacitors and tubes but i got a small problem on the rectifier now. On a cold startup i hear a bit of a Cracle but when i turn it off, an try a hot start, it will arc in the rectifier ( 6ca4 ). I already changed the power tubes, and the rectifier.. SO i dont think thats the problem... Does anyone have any suggestions?? Does a wrong bias can couse this problem?? I dont think so but just to be sure..
Thanks for all the help.. And here is the scematic:
http://www.vintageamps.com/plexiboar...hp?id=2018&t=1


Did you replace the power supply caps with the exact values, or did you increase any of them?
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Old 30th December 2010, 02:01 PM   #4
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Did you set your bias to minimum? Like 20to20 said, did you use the same size cap? Also did you check/replace the series resistors fromthe PT to rectifier?
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Old 30th December 2010, 02:01 PM   #5
kevinkr is offline kevinkr  United States
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Hot starts are a no no with many tube rectifiers, you should wait at least one to two minutes before turning the unit back on. Lower forward drop types like the 6CA4 and 5AR4 will usually arc when hot if the power is cycled quickly. Do this a few times and you will have a cathode to plate short (one or the other plate) in the rectifier.

You may place a UF4007 or 1N4007 in series with each plate lead if this is likely to be an issue in use. This should reduce the incidence of arcing.

The first (input) cap in the supply should not exceed the recommended maximum capacitance value for the tube type used.
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Last edited by kevinkr; 30th December 2010 at 02:04 PM.
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Old 30th December 2010, 02:56 PM   #6
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Originally Posted by SGregory View Post
Did you set your bias to minimum? Like 20to20 said, did you use the same size cap? Also did you check/replace the series resistors fromthe PT to rectifier?
The bias is fixed and stock... Should i change this?? whats the best way to check is.. I have a bias master fot my 6l6's and el34's...But nothing for El84. is it enough to mesure across the cathode resistor?
i didnt checked the series resistors.. can i just do that with the amp turned off?? I did use the same cap size !

Quote:
Originally Posted by kevinkr View Post
Hot starts are a no no with many tube rectifiers, you should wait at least one to two minutes before turning the unit back on. Lower forward drop types like the 6CA4 and 5AR4 will usually arc when hot if the power is cycled quickly. Do this a few times and you will have a cathode to plate short (one or the other plate) in the rectifier.

You may place a UF4007 or 1N4007 in series with each plate lead if this is likely to be an issue in use. This should reduce the incidence of arcing.

The first (input) cap in the supply should not exceed the recommended maximum capacitance value for the tube type used.
I will place the 1n4007 within a few hour to try it when i'm home.
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Old 30th December 2010, 03:02 PM   #7
kevinkr is offline kevinkr  United States
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Originally Posted by engltje View Post
<snip>

I will place the 1n4007 within a few hour to try it when i'm home.
And don't hot switch your rectifier tube, it's not designed for that sort of operation!
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Old 30th December 2010, 09:37 PM   #8
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I just tried the diodes...And ofcourse i had to try to hot start is (sorry kevinkr) .....No problem at all !!!!! But it is good to know its not made for it!! So i wont make a habit of it . In the meanwhile i found some really bad soldered wires, so it was worth the efford.. Just spend an other 4 hours of trial and error for some other problems but i guess thats the way to learn it. Now the next problem, Does anyone has an idea what to say to my girlfrind in the morning when she is asking why i was so late in bed...AGAIN!! ..Just kidding thnx all for the help
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Old 30th December 2010, 09:44 PM   #9
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Old 30th December 2010, 10:14 PM   #10
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www.vintageamps.com/plexiboard/download/file.php?id=2018&mode=view


Hoping this is a clearer view of the schemo
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