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.
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.
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.
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.
poobah said: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
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
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
cerrem said:
Sorry, I left out details.
Class AB1. One cathode resostor/cap per cathode.
The caps are not getting any warmer than anything else, and I have run it for about a half hour. The output is slowly decreasing though. The plate voltage is dead steady at 480v still and the cathode bias voltage has not changed. Cathode current is around 55-60mA per tube at idle.
The only thing that comes to mind is that your caps have high ESR ..... The AC signal current squared times the ESR will heat up the caps... This current is much higher than the DC idle current and depends on how your plate load swings...
The quick solution is to "bypass" the caps with low ESR caps...such as AVX ceramic X7R or NPO ..These are SMD but you can solder leads to these caps....
Chris
The quick solution is to "bypass" the caps with low ESR caps...such as AVX ceramic X7R or NPO ..These are SMD but you can solder leads to these caps....
Chris
Yeah, I think that is it.
That was one conclusion that I came to as well... and i think that may be the answer. I rewired the cathodes and relocated the cathode resistors onto terminal strips to give them more room to dissapate heat, with the capacitors placed a bit further away fom them, and it seems to be doing fine now.
THANX FOR ALL THE HELP EVERYONE, YOU ARE A GREAT RESOURCE!
sgerus said:
That was one conclusion that I came to as well... and i think that may be the answer. I rewired the cathodes and relocated the cathode resistors onto terminal strips to give them more room to dissapate heat, with the capacitors placed a bit further away fom them, and it seems to be doing fine now.
THANX FOR ALL THE HELP EVERYONE, YOU ARE A GREAT RESOURCE!
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