tube gear cap overvoltage on power up

[Bjorn Zetterlund]

If a (modern) polypropylene film cap, in this case a RIFA rated at 250V dc, sees 300V across it for about a second or two every time a piece of gear is powered up...just how naughty is that? The plate supply is a simple two big caps and two resistors job...do I really need to worry about making a switch on delay here?

Thanks,

Bjorn

 

[mcs]

I guess it may not be a problem. But I would use a 400V cap anyway...

Or perhaps you could add some zeners to "eat" the overvoltage at powerup?

Best regards,

Mikkel C. Simonsen

 

[sismofyt]

Yes, that is naughty :grin: I wouldn't recommend that, the foil may arch trough, partly destroying. Some filmcaps can tolerate this though..

No need for a delayed switch, just change the cap :wink:

 

[Bjorn Zetterlund]

Ho Ho! There are Danes about!

Well...I know it's not ideal, but these caps are right behind the valve, as in soldered to the socket...and there's 8 channels...therefore a bit of a bitch. Didn't think of zeners, though, that's actually a pretty good idea. Anyone else try that and lived to tell the tale?

Bjorn

 

[sismofyt]

Check the datasheet for that cap. RIFAs are badass and it may take the voltage.

 

[CJ]

Usually you can put two or three times the voltage across a cap and it won't even blink.
Just don't run them all day like that.
We have a hi-pot tester here at work.
It automatically limits the current when insulation breakthru is reached.
It's funny how high you can go over the ratings of most caps.
Like a 33 volt tant will take 200 volts befor it pops.
220 volt mylar takes 1000 volts!
I bet....hold on, I am gonna go hi-pot a poly for you!

 

[CJ]

OK, I just put 5 kv (our hi-pot limit voltage) on a 1200 volt spraque and I couldn't get a failure. Thats at least 4 times the dc and not even a hint of breakdown.
I bet that 250 volt poly would take at least 1000 volts dc all day long.

Another test, a spraque orange drop rated at 100 vdc just failed at 1000 vdc, ten times it's rating!

A 200 volt mylar didn't fair so well, it only took 900 volts before failing.

If that poly breaks down after a little 300 vdc everyday, I will buy you a new Lamborghini for your birthday! or however you spell it)
can you spell Castronevenes or whatever his name is?
.

 

[Bjorn Zetterlund]

Hey, CJ I'll send you a couple if you like, see if you can break 'em!

Seriously, they are Evox Rifa PHE 426, and I think are meant to be good quality heavy duty industrial stuff. Once warmed up, they sit at about 160 or something, so they don't have a bad life, really. I shall have to ponder some more...

If you're serious about blowing a few of these up, I'll send you a few CJ!

Bjorn

...you're gonna make me break the rules now, aren't you

 

[CJ]

Shoot, those are expensive!
What if I said they blew up at 1000 and then used them in a project?

Note: dc hi-pot rating will always be lower than the ac hi-pot rating.
Another reason for not worrying about those caps.
You are over-voltaging them with dc, not ac.

 

[Gus]

Chris

If you get a chance test some IC brand films like used in a guitar tube amp. I had repaired an amp that the coupling caps to the output tube grids would break down with a few volts causing the tube to go bright red. Guess what brand cap.

I don't think you could use your findings as rule. I have used a LC102 to ty to "break" some caps the leakage voltage can be adjusted from 1 to 999VDC.

 

[CJ]

No, not as a rule. It depends on the make.
IC, Illinois Cap?
Lytics don't like the hi-pot test very much!

 

[Bjorn Zetterlund]

Right, you're on! PM me your ad again...I think littlebush claimed the last one you sent me as his intellectual property...

 

[adamasd]

if whatever this is in uses a tube rectifier you could always try a rectifier with a soft start up like a 5V4, Depending on whats in there a different rectifier may lower or raise the voltage abit, but you should be able to find a tube rectifier with soft start up that should get you close enough that everything will be happy. Simple protection for those expensive caps, and no need to replace them.

adam