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Old 31st July 2006, 09:29 PM   #1
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Question HELP! Cathode bypass caps keep going...

I have a KT88 PP UL power amp... that I built up right off the spec sheet with run of the mill Hammond iron... keeps blowing out the two KT88 cathode bypass capacitors. It takes a bit of time, but they blow within a few days. Everything else looks OK, voltages and bias are as they should be.

I had 100v nichion electro caps in there, and when those blew, I went to more temperature stable 250v rated caps, and those blew. is it a heat issue? The chassis is kinda cramped, but nothing else seems to suffer.

i have a few Illinois 100v film capacitors that i can throw in there, but I wanted to see if anyone had any ideas.
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Old 31st July 2006, 09:33 PM   #2
ostie01 is offline ostie01  Canada
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Hi, double check polarity, schematic often show components inverted,
happen to me in the past,
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Old 31st July 2006, 09:57 PM   #3
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Quote:
Originally posted by ostie01
Hi, double check polarity, schematic often show components inverted,
happen to me in the past,

Thanx.

Checked... polarity is fine.
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Old 31st July 2006, 10:06 PM   #4
rdf is offline rdf  Canada
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Put a ohm or ten in series and measure the AC current being passed? Possible oscillation issue?
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Old 31st July 2006, 10:26 PM   #5
poobah is offline poobah  United States
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What is the value of the cathode res. and the cap?

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Old 31st July 2006, 10:34 PM   #6
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Caps are 100uF. The cathode resistors are 575r Dale 5W. Plate voltage is around 480v at idle. Tubes test fine.

I am going to replace the caps and run a signal while monitoring the plate voltage until they blow just to make sure there are no thermal runaway issues. I may drop the input impedance down a touch to see if that helps.
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Old 31st July 2006, 10:39 PM   #7
poobah is offline poobah  United States
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Well the corner frequency is 2.7 Hz. Guess it's NOT bass signal eating them up... oscillation sounds like the culprit maybe.

Could they be getting over voltaged on turn on somehow?

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Old 31st July 2006, 10:49 PM   #8
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Quote:
Originally posted by poobah
Well the corner frequency is 2.7 Hz. Guess it's NOT bass signal eating them up... oscillation sounds like the culprit maybe.

Could they be getting over voltaged on turn on somehow?


Might be. 'scope is down so I can't test for ultrasonic oscillation. There is no audible oscillation as far as I can tell.

I'll check the surge voltage at startup as well.

Hasn't blown so far with the film caps in though.

Thanx
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Old 31st July 2006, 10:52 PM   #9
poobah is offline poobah  United States
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Were the caps getting warm at all?
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Old 31st July 2006, 11:12 PM   #10
cerrem is offline cerrem  United States
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Is this Class AB amp??
WHat power levels did the caps blow ???
Well if you are using seperate cathode resistors with seperate bypass caps... Then you need to make sure the cap can handle the full AC signal current... You need to calculate the signal current and compare with the caps ripple current capability..
Many Class A push-pull amps use shared cathode resistor and bypass cap... This way the cap is there for only and residual small signal current from imbalance..Since there would be no net AC signal at that node if it were a perfect balanced amp..in theory..

Chris
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