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Old 8th May 2009, 03:48 PM   #1
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Default Capacitor voltage ratings

This post is similar to one which is already alive at the moment but I thought it was sufficiently different to warrant another thread.

I have built a guitar amplifier with 9 stages of filtering and have got to the point of testing. Obviously without any valves in there is no current drawn so the filters do not drop any voltage and all remain at the off-load HT from the recitifer which is 480V. I have only tested using a variac up to 150VAC input and all is working correctly.

The problem I potentially have is that the last three filter caps are rated at 450V and the voltage before the valves have heated up is 480V. The obvious solution here is to use a standby switch which I already am doing, but this is a university project and the person watching over me is concerned that in the event of both switches being operated simultaneously there would be 480V across the 450V capacitors which is potentially dangerous. I have noticed that pretty much all guitar amp designs I have seen use capacitors in the latter stages of filtering which cannot cope with the HT voltage before the valves have heated up.

Basically then, am I safe using the capacitors I have got and if not then why do other commercial manufactureres use similar values (the mesa dual rectifier has very similar voltages and similar rated caps and is evidently ok). Can the filter caps take a higher voltage for say 15 secs? Or is it merely a matter of just not turning the standby switch on until the valves have heated up which seems a bit crude and potentially dangerous since this is a prototype and the last thing that I need is capacitors exploding!

Many thanks for any help that can be offered
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Old 8th May 2009, 03:59 PM   #2
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Electrolytics don't fail suddenly if the voltage exceeds the rating - but leakage goes up considerably. This will cause localized heating and eventual failure if extended. Typical 450V caps have a surge rating of 500V for 30 seconds (with at least 5 minutes between surges). In an actual circuit, voltage may not get as high as the input, since increased leakage results in voltage drop across series resistors.
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Old 8th May 2009, 04:05 PM   #3
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I see, it's still pushing the limits a bit but i suppose as a backup its not too bad. Under normal use switching the switches on in the correct order leaves all the caps around 200V under their limit. So just to clarify would you say that my circuit is safe going on what I have mentioned? The 450V caps are 80uF sprague atoms and then before that is 500V 40uF atoms and then there are network to make over 700V filter caps fo the power valves.
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Old 8th May 2009, 11:04 PM   #4
jjman is offline jjman  United States
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If you are using a tube rectifier you won't have that improper startup-scenario risk. This because the PS voltage will come up gradually as the rectifier heats up. During that time at least some of the other tubes should also start conducting thus dropping volts along the supply rail. Although I think “directly heated cathode” rectifiers come up faster. Not sure on that nor how much faster.

My assumption is you are using diodes so you have instantaneous full voltage. I think you are fine but I wouldn’t run the amp with no tubes for long periods of time. There really shouldn’t be too much to test/adjust in that mode anyway.
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Old 8th May 2009, 11:21 PM   #5
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It is indeed a diode bridge recitifer I am using so there is an instantaneous voltage. As far as i'm aware, the worry was mainly for if somebody decided to switch both power and standby switches at the same time.

In most situtations in electronics it would be bad practice to use components which are subject to voltages above their specified limit. I guess in this situation things should be ok but I still cannot figure out why other major guitar amp manufacturers have roughly the same value components as I have when they would be fully aware of the problems caused if the amplfier was not operated correctly and capacitors were operated past their limits. Is there something in datasheets i'm missing which says this overload is acceptable? Tom Bavis said in this thread that typical 450V caps have a surge rating of 500V for 30 secs - could I ask where you found this information because if correct I would be fairly confident in my design!

Thank you for the comments received! Sorry for questioning everything but I know I will not be allowed to continue testing if I don't have a plausable answer to offer the person watching over me at uni!
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Old 9th May 2009, 01:18 AM   #6
Funker is offline Funker  Germany
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Hi,
to prevent the Capacitors from run over their specs during the turn on period, connect a base load of abt. 100 to 150k at the end of the supply rail. The exact value depends on the voltage on the last filter C and from the amount of all filter resistors. A load of 2mA is a good deal. That measure will keep your circuit in a safe condition.

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Wolfgang
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Old 9th May 2009, 01:21 AM   #7
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AmpBuilder,

I would agree with Tom Bavis. I hunted in my catalogues for specific information, but all I find is that voltage ratings must not be exceeded except for surges "of extremely short duration". The surge magnitude and duration time are not defined.

When you say the last three capacitors, I take it you mean 'furthest' away from the rectifier, i.e. probably after a few series resistors of some magnitude (as decoupling filters). (If this is so, one might also ask whether a high 80µF is necessary? Would it not be possible to use 16/32µF types of 500V?) In that case large values like 80µF will take a few seconds to charge up to the momentory peak, further decreasing the time of the surge. I remember reading somewhere that for the about 12s it takes for valves to start conducting, 500v surge is acceptable for 450V caps, and that was for filter input caps. Then one must also take into account that such specs are at the maximum rated temperature, usually not lower than 80 degrees C.

All of which should make your situation well within normal operation, although I cannot now quote chapter and verse for the benefit of your supervisor - I would believe he knows, and might perhaps just question to see whether you know!
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Old 9th May 2009, 02:15 AM   #8
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I found the 30 second surge rating in an older UCC catalog - don't have it at hand right now. With series resistors, the capacitors won't ever see the full input voltage, due to increase in leakage above rated voltage.
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Old 9th May 2009, 06:38 PM   #9
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Thank you to everyone for all the comments - I think the collective information gives me a few options to set my mind at ease.

Johan - I did mean the smoothing caps furthest from the recitifer. It was a while ago I designed the power stage and I think the 80uF cap was chosen to acheive the desired voltage drop and cut off point of the filter. To get the same cut off with a different capacitor would require a change of resisitor which would alter bias points etc. Obviously a different cut off point for the filter could be used and it is an option I will consider. My initial thoughts were to smooth ripple for all valves down to as close to 0Hz as practically possible - is there any problem induced when a higher cut off point is used for the input valves?
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Old 9th May 2009, 11:00 PM   #10
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No.

You are taking the cut-off point a little too academically - or I am taking it too informal! The power supply usually already has low enough ripple after the main smoothing, and I presume you are using the usual 'serial' decoupling topology, where stages down the line are decoupled in series. That would mean that any remaining ripple would become close to zero. The decoupling is mainly done to prevent feedback down the power rail, not to reduce ripple, the latter being a 'free' advantage, and as said was always chosen by me to fullfill that condition. Typically a few sections of 20K or so resistors and 16µF caps were sufficient. These days of inexpensive and small caps one sometimes go to say 32µF.
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