Identfying Specs for an Unmarked Filter Cap in a Tube Filter

krylenko · Post by **krylenko** » Fri Feb 27, 2009 12:45 pm

I have an old tube bandpass filter that has started blowing fuses.
It uses a 6au5gt power tube, and it only blows the fuse when that tube is in its socket.
Take it out, and the fuse stays intact and the heaters on the other tubes glow.

I just got and tried a new 6au5gt, which did the same thing, so I'm pretty sure the problem isn't the tube.
The socket looks OK too, no shorts that I can find.

So I'm thinking one of the filter caps has shorted.
The filter cap closest to the socket gives me an intermittent "short" readout/beep on my voltmeter -- the other filter cap doesn't.

However, neither of the filter caps is marked with values.
The one in question only says "Sprague 40-450 DC D25118 608".
It's an old tall cap, a heavy cardboard cylinder that's bolted upright to the chassis.

Any idea how I could figure out the capacitance of this cap to replace it?

Brian · Post by **Brian** » Fri Feb 27, 2009 3:13 pm

ya think it's a 40 uf 450 volt, (could be for a tube circuit) or,
a 450 uf 40 volt (more likely) in which case you could swap it for a 470 uf 50v

Nate Dort · Post by **Nate Dort** » Fri Feb 27, 2009 3:21 pm

Brian wrote:ya think it's a 40 uf 450 volt, (could be for a tube circuit) or,
a 450 uf 40 volt (more likely) in which case you could swap it for a 470 uf 50v

He specifically said it was a tube circuit filter cap, in which case it's a 40uF 450V. Make sure you get a brand new cap and not a NOS one, as modern electrolytics only have a shelf life of 5 years.

krylenko · Post by **krylenko** » Fri Feb 27, 2009 4:37 pm

Thanks!

To be precise, I should say that I *assume* it's a filter cap, since it and its twin are the largest caps on the board. There are also a couple of .1 uf 600v caps wired up below the chassis.

I'm also assuming that it doesn't matter whether a new cap can mount in the same vertical position as the existing one, correct?

Last but not least, given the symptoms I've described, what else might be the cause of this problem?

Brian · Post by **Brian** » Fri Feb 27, 2009 4:58 pm

That's probly it and you better replace it's twin too or you'll be back in a few weeks if not minutes.
other than those, the next bigger ones down the line.

If the 40-450's are connected to the larger xformer, they are the filter caps.

It doesn't matter if they are the same mount style as long as you have good connection, however, the shorter the lead lengths, the better off you are.

Boogdish · Post by **Boogdish** » Sat Feb 28, 2009 10:19 am

If I were you, I'd go ahead and change all of the electrolytics in the thing, including bypass caps (the electrolytics coming off of your cathodes) if it has them as long as you're opening it up.

You might also want to change the other tubes in the circuit as well. It might be that one of your other tubes earlier in the circuit has failed and is now drawing too much current and when you put your power tube in it's the straw that's breaking the camel's back, but not necessarily the root of the problem.

Other questions that might help: Do you have a standby switch in the circuit? If so, is it blowing fuses with the standby switch on or only when fully engaged? What AC wall voltage is the transformer trying to see on it's primary?

I'm a guitar amp tech and this hi-fi/studio tube stuff intrigues me a lot. If you could post or link to a schematic for this, that would probably help as well.

EDIT: also, double check that you're using the correct fuse, it's possible that the unit is functioning correctly and someone has put the wrong fuse in. Probably not it, but I like to try the forehead slapping things first.

Brian · Post by **Brian** » Sat Feb 28, 2009 11:22 am

Boogdish wrote:If I were you, I'd go ahead and change all of the electrolytics in the thing, including bypass caps (the electrolytics coming off of your cathodes) if it has them as long as you're opening it up.

You might also want to change the other tubes in the circuit as well. It might be that one of your other tubes earlier in the circuit has failed and is now drawing too much current and when you put your power tube in it's the straw that's breaking the camel's back, but not necessarily the root of the problem.

I'm a guitar amp tech and this hi-fi/studio tube stuff intrigues me a lot. If you could post or link to a schematic for this, that would probably help as well.

EDIT: also, double check that you're using the correct fuse, it's possible that the unit is functioning correctly and someone has put the wrong fuse in. Probably not it, but I like to try the forehead slapping things first.

No kidding on that last one, check it first, if the wrong fuse is in there and the electrolytics are good, you might fry all your good work or might not need to get overly invested too soon.
I've had that happen way more than once that it was the fuse.

The Scum · Post by **The Scum** » Sat Feb 28, 2009 3:04 pm

EDIT: also, double check that you're using the correct fuse, it's possible that the unit is functioning correctly and someone has put the wrong fuse in. Probably not it, but I like to try the forehead slapping things first.
No kidding on that last one, check it first, if the wrong fuse is in there and the electrolytics are good, you might fry all your good work or might not need to get overly invested too soon.

But don't loose sight of the fact that some of the strongest lessons in electronics are learnt when you inadvertently build an arcwelder. Those are steps in the learning process that even the best techs sometimes have to revisit.

krylenko · Post by **krylenko** » Mon Mar 02, 2009 10:04 am

Thanks for the help, let me see if I can address everyone's points.

The filter is a Krohn-Hite 310AB filter meant for lab work.
I've always been curious about test equipment for audio, and I found this one for a good price.
I don't have schematics and there's not much info on the web.

The original fuse that blew was 0.5A 125v, which I replaced with 0.5A 250v (have now blown four of these).
I assume the power tranny wants to see 110v AC since the filter's designed for US use.
There's no standby switch.

Right now I'm mostly trying to figure out how much $ and effort I want to put into this box.
Ideally I'd replace the filter and bypass caps and then maybe the tubes.
However, just the caps would be 1.5x what I paid for the filter itself and I don't know if it's worth it.

It sounds pretty cool -- good solid 24 db/oct HP and LP filters in series, but it doesn't have the liquid tube mojo I was hoping for, and the frequency range switches are annoying to use in practice.
Maybe recapping and re-tubing it would bump the sound up a notch, but I'm skeptical -- I'm sure Krohn-Hite was trying to make a clean filter from the get-go.
I also thought I could reuse the chassis and tubes for a preamp or something, but I'd need filter caps for that too. Hmm...

Nick Sevilla · Post by **Nick Sevilla** » Sun Mar 08, 2009 9:55 am

krylenko wrote:SNIP

The original fuse that blew was 0.5A 125v, which I replaced with 0.5A 250v (have now blown four of these).
SNIP

Sorry, but by putting a higher voltage fuse, you probably damaged a lot more components.

Never, ever ever replace a fuse with a higher rating. The rating is there to protect the innards.

Cheers

Nate Dort · Post by **Nate Dort** » Sun Mar 08, 2009 2:26 pm

noeqplease wrote:
krylenko wrote:SNIP

The original fuse that blew was 0.5A 125v, which I replaced with 0.5A 250v (have now blown four of these).
SNIP
Sorry, but by putting a higher voltage fuse, you probably damaged a lot more components.

Only if he plugged it into a 220VAC outlet, which would be highly unlikely.

Brian · Post by **Brian** » Sun Mar 08, 2009 2:36 pm

noeqplease wrote:
krylenko wrote:SNIP

The original fuse that blew was 0.5A 125v, which I replaced with 0.5A 250v (have now blown four of these).
SNIP
Sorry, but by putting a higher voltage fuse, you probably damaged a lot more components.

Never, ever ever replace a fuse with a higher rating. The rating is there to protect the innards.

Cheers

At least, don't replace it with a higher voltage fuse unless you KNOW it was too small via n official document, schematic, or list in the manual or service manual.

Mine had one too small and one the right size. The manual told me what size they should be. The amperage is more important.

krylenko · Post by **krylenko** » Mon Mar 09, 2009 4:27 pm

Well, that's what I get for listening to the guy at Radio Shack when I couldn't remember how important the voltage was. I did make sure to match the amperage, and I certainly wasn't plugging into a 220v outlet.

It would be nice to know the official fuse specs, but I don't have a manual or schematic and don't feel like finding and paying for one.

Anyway, thanks for the responses, but I'm out of time and money for this lil' box right now. I may resurrect this thread in a couple of months when I can tinker a bit more.

Scodiddly · Post by **Scodiddly** » Mon Mar 09, 2009 4:33 pm

The only really critical fuse rating is the current rating. The voltage rating should be at least as high as the voltage used in that part of the circuit, but higher shouldn't hurt. The voltage rating is more about isolating the voltage inside the fuse from the outside world, or more specifically the live side from the load side after the fuse has blown.

I'd second or third the suggestion to replace the electrolytics. Fix what you know is busted, or is most likely to be busted - the old dried-up electrolytic caps. If you get the thing to pass audio it'll probably sound better with fresh caps anyway.

Brian · Post by **Brian** » Mon Mar 09, 2009 8:09 pm

If you knew the make an model you could find something easily on the interweb.

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Identfying Specs for an Unmarked Filter Cap in a Tube Filter

Identfying Specs for an Unmarked Filter Cap in a Tube Filter

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