Another day, another blown fuse

stretchl

This is the third fuse the Yaqin MC-10t has blown in about 2 months. Here's the sequence of events:

January - Fuse blows for no apparent reason. I replace it and take the unit into the shop for a check. While at the shop, tech discovers bad stock EL-34 tube, which sends me on the quest that ended with the Psvane 6CA7-Tii's.

Early February - After a shipping mistake, I finally receive the 6CAy7's a few weeks after I ordered them. I put these in and after a few hours, while playing music at a very conservative volume, "pop." Another blown fuse. Return unit to the shop.

Late February - Tech finds a bad input tube so replace all four of these with four nice NOS tubes from Heiney. Several days of listening and all is good. Unit sounding better than ever and then...

Morning of March 1 - Turn on the system and while it's warming up, before I hit "play" on any music... "pop." Fuse blows again.

As my next step I'm considering getting the outlet wired as a dedicated circuit, which would eliminate one possible source of the problem.

Big question - Until I do that is it advisable to replace the fuse and continue enjoying the unit, or should I keep it shut down until I get the rewiring done?

Signed -

Frustrated

Thanks.

dragon1952

I'm assuming you know for sure the correct value of the OEM fuse, voltage, amperage and whether fast or slo-blo? Just saying because if you bought it used maybe the original owner placed a wrong value fuse in it at some point.

heiney9

Still could be a power tube issue. Refresh my memory, which current production power tubes did you recently get?

H9

stretchl

The fuse that blew was a 4 amp 250 volt. Folks on the Yaqin forum have indicated that I need a 4 amp 125V fuse. So I'm going to get one of those.

H9, the new output tubes are Psvane 6CA7-Tii

Thanks, guys.

heiney9

Maybe try swapping power tubes with each channel and see if the problem follows. Probably not it, but best to eliminate as many things as possible.

H9

dragon1952

stretchl wrote: »

The fuse that blew was a 4 amp 250 volt. Folks on the Yaqin forum have indicated that I need a 4 amp 125V fuse. So I'm going to get one of those.

H9, the new output tubes are Psvane 6CA7-Tii

Thanks, guys.

Since the 250V is made to a higher voltage rating I don't think that would cause a problem. Maybe check and make sure if you require a slo-blo.

F1nut

I'd have the bias resistors replaced.

As my next step I'm considering getting the outlet wired as a dedicated circuit, which would eliminate one possible source of the problem.

That's not it.

stretchl

Dragon, how can I tell if I need a slo blow?

F1, would the tech not have found this when he was giving the unit a once-over a few weeks ago? I guess I could take back in.

As of now, I have a new 4 amp 125v fast acting fuse to install. Any danger in just putting one of those in an giving it a go?

F1nut

would the tech not have found this when he was giving the unit a once-over a few weeks ago? I guess I could take back in.

You have an on going problem and it's not caused by the tubes. Whatever the problem is, I'd have to say your tech hasn't found it yet, ya know?

As of now, I have a new 4 amp 125v fast acting fuse to install. Any danger in just putting one of those in an giving it a go?

I wouldn't advise it.

dragon1952

stretchl wrote: »

Dragon, how can I tell if I need a slo blow?

As of now, I have a new 4 amp 125v fast acting fuse to install. Any danger in just putting one of those in an giving it a go?

You'd have to check with the manufacturer or the forum you mentioned. As far as the fuse you've got, there shouldn't be a problem because if you do require a slo-blo the fast-blo will go way before the slo-blo would have . Here's a blurb from wikipedia re: fast vs slo-blo.
"The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuses have different characteristics of operating time compared to current, characterized as fast-blow, slow-blow, or time-delay, according to time required to respond to an overcurrent condition. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0.1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow.

Fuse selection depends on the load's characteristics. Semiconductor devices may use a fast or ultrafast fuse as semiconductor devices heat rapidly when excess current flows. The fastest blowing fuses are designed for the most sensitive electrical equipment, where even a short exposure to an overload current could be very damaging. Normal fast-blow fuses are the most general purpose fuses. The time delay fuse (also known as anti-surge, or slow-blow) are designed to allow a current which is above the rated value of the fuse to flow for a short period of time without the fuse blowing. These types of fuse are used on equipment such as motors, which can draw larger than normal currents for up to several seconds while coming up to speed."

My Ming-Da tube amp uses a slo-blo, but like the article implies it depends on the design.

stretchl

Thank you.

I need to make double sure the right fuse is in there, as its entirely possible its been the wrong fuse. But, in any event, what's got me mystified here is that the first fuse blew with the original tubes in the amp. Then I replaced both the input and the output tubes and still blew two more fuses.

Probably will end up just taking it back to the shop...

dragon1952

Considering how intermittent the problem sounds it may not be easily duplicated or traced. Something could be intermittently shorting out, could be a marginal solder joint but it sounds like it's happened both when the amp is fully warmed up and not quite warmed up. Maybe it's something in your outlet or house power. Are you using a stock power cord?

[Deleted User]

Hello,
One of the things you might want to try is measuring the turn-on current draw. Do you have a volt/ohm meter that can measure AC current up to 10 amperes?
Regards, Ken

stretchl

Thanks, guys.

I now have a 4 amp 120 volt fast blow fuse in there and it's been powered up twice w no issue, the first time playing music for 7 hours straight w no problems.

Dragon, I'm using the stock power cord.

Ken, I'm not sure the range of my voltmeter but I'll check when I get home.

pitdogg2

do not know if this will help but my Carver TFM 45 takes an MDA 12amp 250v and I had to do some research to find out that MDA was a slo-blo ceramic fuse. My guess is that the fuse you have doesn't take into consideration the complete draw with the tubes and such. I like Jesse here am going to say your outlet has not one thing to do with this matter. you either need slow blow or need to trace down the problem yet.

michaeljhsda2

I'm a little late posting but did you know about the spare fuse that's in a little pull-out tray under the IEC socket on the amp? I had to replace the fuse on my MS-20L because I quick cycled (turned off and then back on within about 3 seconds, newbie mistake) and it blew the fuse. That's not what happened with your amp but I wanted to mention that to you so you don't make the same mistake I did.

stretchl wrote: »

I now have a 4 amp 120 volt fast blow fuse in there and it's been powered up twice w no issue, the first time playing music for 7 hours straight w no problems.

Sounds like all is well (knock on wood).

Hope do you like the sound of the amp otherwise?

stretchl

Came home this afternoon and turned on the amp. Went to the john. Came back and the amp was off and the fuse was blown. I guess I'll take it to the shop tomorrow.

Mike, I really like the sound of the amp. The new tubes have made a world of difference. Not that it sounded bad before, but the new tubes really brought out the bass without losing the mids I love or straining the highs.

michaeljhsda2

Dang, wonder what the issue is?

stretchl

Search me, Michael.

I'd consider putting in a slow blow fuse but I'm afraid of messing up the amp. Is this possible?

Ken, the meter I have goes up to 20 Amps, but I have no idea how to check the current you mention. I'm going to try to google it but I'll state at the outset that I really have no idea what I'm doing.

Phasewolf

That unit uses a F5AL250V which is a fast blow 5A fuse if your using a 4A fuse this maybe a issue. www.proshowmarket.com/en/27-yaqin-tube-amp-mc10t.html that link has a manual you can download if you don't have one but if your a amp off on the rating that is a 20% difference and that is not small.

[Deleted User]

Hello,
The meter would be substituted for the fuse. To do this you'll need to see rear of the fuse holder where you'll see two wires, remove the fuse from the holder. Then look at your meter, there might be different connections for placing the probes to measure current. I've included a photo to show how the meter might look set to measure AC current.
Place one clip lead on one of the fuse wires and the other lead on the second wire. Now the meter is going to take the place of the fuse and it will display the current that flows through it. Keep in mind there will be a peak reading as the power supply comes on, so you'll have to see what the peak reading is. Of course, it goes without saying that you should be careful and not touch any exposed components even when the unit is powered down. Leave this to a service person if you are at all uncomfortable with doing this.
I hope this is helpful.
Cheers, Ken