Blown fuses....inrush current?

[crbowman]

Greetings all,
I am having an issue with a recent project that I thought I would run past you guys here.
I know this forum is predominantly for builders of guitar amps, but I have strayed slightly from the norm and have attempted to build a clone of the old REDD.47 tube mic pre as was used in the old EMI/Abbey Road recording desks back in the mid to late 60’s. The layouts and schematic are from the rather controversial John Hinson who made a number of these, but has since gone out of business (and apparently to the big house).
This design uses an EF86, E88CC, and two 0A2’s for old skool voltage regulation.
My problem is this. Unit powers up just fine on the variac, light bulb limiter indicates no excessive current draw. However, plug it into the wall and hit the power switch, it immediately blows the fuse..
I brought it up on the variac last night and let it run for an hour with no problem, no blown fuse, no heat to speak of, and no blown fuses. Took it into the studio, powered it up with the variac, and it functioned beautifully. Used it for a couple of hours and it sounded amazing with no noise, stable voltages, and little to no heat. I was thinking that maybe the switch was arcing so disconnected switch, unplugged from variac, plugged it into wall and fuse blew. Doc’s specify, and I am using a .5 amp fast blow (hard to find .5 amp slo-blos around here). Swapped out all tubes and even the power transformer with the same results. All wiring has been quadruple checked. Don’t think it is a filter cap prob as it sounded fine with no hum at all.
I’m thinking this is an inrush current problem, and that an NTC thermistor may be in order but I’m not really familiar with them. I’m attaching schematic and layouts. Please be advised that there are discrepancies with these, and that I have been informed that the layouts are the more correct documents.
Any thoughts, advice, suggestions are greatly appreciated!
Thanks
CB

[Cliff Schecht]

Sounds like the fuse is a fast-blow that can't handle the initial inrush current. Your options are either use a larger FB (750mA or 1A) or find a SB that's rated for what you are doing. I've taken in more then a few amps that were reported to be blowing fuses and the problem turned out the be people were using FB fuses where the ratings specifically state a SB. Heck my Express will blow FB fuses every few turn-ons, but I've had the same SB in there for well over a year now.

[crbowman]

Hi Cliff,
Thanks for your reply.
I'm always a little gun-shy of going high on fuses (can't find any .5 amp slo-blows around here) but I may give it a try.
Any thoughts on using a thermistor for soft power up?

[billyz]

Hi Cliff,
Thanks for your reply.
I'm always a little gun-shy of going high on fuses (can't find any .5 amp slo-blows around here) but I may give it a try.
Any thoughts on using a thermistor for soft power up?

Did you try Radio Shack ?
http://www.radioshack.com/product/index ... Id=2103769

Or maybe Fry's off Mopac and Parmer.

[pdf64]

As you've found, fast blow fuses aren't suitable for high inrush current applications.
1/2 amp slow blow are available (long link as don't know what size fuseholder)

http://uk.farnell.com/jsp/search/browse ... %2B1001439

but I'd recommended inrush current limiters also

http://uk.farnell.com/ge-sensing-thermo ... dp/1351825
Pete.

[crbowman]

Thanks for your help guys!
Unfortunately I scrounged an old Fender fuse holder i had lying around and the 5X20MM fuses won't fit. I guess it'd probably be best to just abandon it and get an in-line 5X20 fuse holder seeing as they're pretty much dirt cheap.
I do think I'm going to try an inrush limiter as well, should I go CL-190 at 25 ohms or CL-90 at 250 ohms? Should I wire it in series off one leg of the primary?

Billy, that dang Fry's drives me to distraction! they're right around the corner from me and they always seem to have everything EXCEPT what I'm looking for. Thanks for the suggestion though.

[Structo]

Here try Tiny URL

http://tinyurl.com/4jepbf4

[M Fowler]

Most electrical distributors, Lowes, and other hardware stores carry time delayed fuses (slow blow).

[crbowman]

Here try Tiny URL

http://tinyurl.com/4jepbf4

Thanks Tom.
Might try that.

[billyz]

Billy, that dang Fry's drives me to distraction! they're right around the corner from me and they always seem to have everything EXCEPT what I'm looking for. Thanks for the suggestion though.

I know what you mean about Fry's, but sometimes they actually do have what you need , or can make do. There is also a Radio Shack across the street by HEB. I have found Slo BLow fuses at both. I go through so many I usually order them in bulk. I don't any 500ma slo only fast, I looked . As you are close by ,you should drop by sometime.

Or we could meet up at Fry's , ha ha.

Z

[Cliff Schecht]

Fry's also carries NTE .

I've found that at that particular Fry's, if they don't have something then they probably won't have it again for another year (when it comes to parts at least). It's almost a crap-shoot, but it's one of the only options now that M.C. Howards is out of business . One thing I love about Dallas is Tanner Electronics. I'm stoked that I'm going to be living about 5 minutes away from them over the summer.

[crbowman]

Billy, that dang Fry's drives me to distraction! they're right around the corner from me and they always seem to have everything EXCEPT what I'm looking for. Thanks for the suggestion though.

I know what you mean about Fry's, but sometimes they actually do have what you need , or can make do. There is also a Radio Shack across the street by HEB. I have found Slo BLow fuses at both. I go through so many I usually order them in bulk. I don't any 500ma slo only fast, I looked . As you are close by ,you should drop by sometime.

Or we could meet up at Fry's , ha ha.

Z

Thanks Billy. I appreciate that! I'll drop you a PM some time soon when everything shakes out around here. I need more exposure to people who actually KNOW what they're doing with this stuff.

[crbowman]

Well, lo and behold, Home Depot has .5 amp slo-blows! (Thanks Mark!)
Popped one in and she powers up with no problems.
I guess I didn't realize that power supplies kicked quite that hard on power up. Or then again, maybe it just doesn't take much to blow a .5 FB.
Either way, I guess you can count the day lost that you don't learn something.

I'm still thinking about adding a CL-190 just to minimize that initial surge. Can anyone think of any downside to this? I have a really nice old Telefunken EF86 in this thing that I'd really like to see last a good long while.

If anyone is curious, I think this thing sounds very, very nice and only cost about $500 to make.
Anyway, once again, Thanks Guys!