Fuse rating

[Stereophonic]

A couple weeks ago I had my Tube-tech SMC2b serviced by T&R Electronics out of Ojai, CA. I sent it in because it would randomly blow fuses on power up. Brian told me that the power transformer had a short in it and had it replaced. The fuse that would blow is rated at 800mA and today, out of curiosity, I decided to check the fuse that he installed. To my surprise he installed a 2A fuse. Now it's safe to assume he knows a lot more about electronics than I do, but isn't this a recipe for disaster?

[vx2000]

Originally Posted by Stereophonic

but isn't this a recipe for disaster?

You are correct, yes it is.

The fuse is rated based on the circuit design load. This does not change with a new transformer.

It sounds "fishy" to me.

Was the 800ma the original fuse?

Also, are there markings on the fuse holder area that mentions correct fuse specs?

[Stereophonic]

Originally Posted by vx2000

You are correct, yes it is.

The fuse is rated based on the circuit design load. This does not change with a new transformer.

It sounds "fishy" to me.

Was the 800ma the original fuse?

Also, is there markings on the fuse holder area that mentions correct fuse specs?

Yes, 800mA was the original fuse. I was using 750mA until I found a supplier of 800mA fuses. The rating is also screen printed on the back of the box next to the fuse holder. I did my research and the guy who made the repair seems totally legit and knowledgeable. A "certified tube-tech tech..." I emailed him my concern but based on our recent correspondence, or lack there of, I do not expect to hear from him for at least a few days. Phone calls are pointless.

[Ben F]

I generally find that the Tube-Tech gear requires a slow blow fuse, rather than a fast blow. It should always be the correct rating though. I'm not sure how a power supply would short, considering it's toroidal. It sounds like an incorrect load somewhere- is it getting hotter than normal?

[Waltz Mastering]

I've checked out T&R before when I needed some maintenance on a Tube Tech unit and they are about the only certified service for these in the US. Luckily I was able to trace the problem to a fried tube the day before I was ready to send the unit out for repair.

I know of one other unit that sounds like it had the same problem as yours, so it sounds like it might be a problem that is not unique. I'd be curious to hear why he used a 2 amp if he supposedly fixed the short that was causing the problem in the first place. I know the 800ma's are a little hard to find, but for him to go up to a 2 amp seems strange.

[Stereophonic]

Originally Posted by Ben F

I generally find that the Tube-Tech gear requires a slow blow fuse, rather than a fast blow. It should always be the correct rating though. I'm not sure how a power supply would short, considering it's toroidal. It sounds like an incorrect load somewhere- is it getting hotter than normal?

Isn't that (slow blow)as risky as sticking in a higher rated fuse than what's called for?
The unit does run hot. There is plenty of ventilation around it but still there are times when the face plate is too hot to touch for more than 20 seconds, or so. I forgot who mentioned it, but I recall reading someone on this forum popping the hood to keep it cool.

Originally Posted by Waltz Mastering

I know the 800ma's are a little hard to find, but for him to go up to a 2 amp seems strange.

It's strange indeed. I stuck an 800ma back in the box. If this things blows a fuse, well then I'm going to blow a fuse I won't know for a while because the original problem was intermittent. I could go weeks without blowing a fuse and then suddenly I'd blow 5-10 in a row. But once it was on, it would behave for weeks for each power cycle.

[dcollins]

Originally Posted by Stereophonic

Isn't that (slow blow)as risky as sticking in a higher rated fuse than what's called for?

No, the Sloblo fuse just lets you run like 150% of the rating for a period of time at turn-on.

If you still have problems, you might try a Sloblo of the original size.

The unit does run hot. There is plenty of ventilation around it but still there are times when the face plate is too hot to touch for more than 20 seconds, or so. I forgot who mentioned it, but I recall reading someone on this forum popping the hood to keep it cool.

In electronics, this is technically called "the rule of thumb." Which means if you can't put your thumb on it, its too darn hot.

But once it was on, it would behave for weeks for each power cycle.

Another reason to leave everything on.


DC

[inlinenl]

I took the top permantly off ... just to be safe ... the top is badly ventilated .. drilling holes in it or some other top could be a bit more safe ..
having damage on a unit because off running to hot feels a bit stupid ....
I've had problems in the past ....

[Stereophonic]

Originally Posted by dcollins

No, the Sloblo fuse just lets you run like 150% of the rating for a period of time at turn-on.

If you still have problems, you might try a Sloblo of the original size.

I heard back from Brian at T&R this morning: The 2Amp fuse is a fast blow type that replaces the 800 mA slow blow that tends to heat up when the unit is a few years old and the components draw more current.

What's odd is that the original fuse that came with the unit, which I still have, is not slow blow and no where on the box does it state slow blow.

Wim, are you using a slow blow fuse?

[Stereophonic]

I just blew an 800mA fuse on power up. I'm starting to feel like I was duped. I need assurance that everything is OK....

[inlinenl]

Originally Posted by Stereophonic

Wim, are you using a slow blow fuse?

were should I look ( okay in the tubetech ) .. which fuse ... ??

[Stereophonic]

Originally Posted by inlinenl

were should I look ( okay in the tubetech ) .. which fuse ... ??

The fuse on the back of the unit next to the power cord.

[inlinenl]

Originally Posted by Stereophonic

The fuse on the back of the unit next to the power cord.

Timothy,

T400L250V ... but we're in 250V europe ..

[Stereophonic]

Originally Posted by inlinenl

Timothy,

T400L250V ... but we're in 250V europe ..

Thanks for checking on that, Wim. Is the fuse fast or slow blow?

[inlinenl]

Originally Posted by Stereophonic

Thanks for checking on that, Wim. Is the fuse fast or slow blow?

shall I try ... please tell me how I can see that ... learning curve is deeeep for me ...

[Stereophonic]

Originally Posted by inlinenl

shall I try ... please tell me how I can see that ... learning curve is deeeep for me ...

Someone correct me if I'm wrong, but fast acting fuse is a straight wire while slow blow is a coil of wire, a coil of wire wrapped around a ceramic rod, a straight wire with knobs on it...or something of the sort. They look a little funky.

[dcollins]

Originally Posted by Stereophonic

Someone correct me if I'm wrong, but fast acting fuse is a straight wire while slow blow is a coil of wire, a coil of wire wrapped around a ceramic rod, a straight wire with knobs on it...or something of the sort. They look a little funky.

Correct. But they can also be in ceramic where you can't see the insides.


DC

[inlinenl]

it's the fast blow one ...

[Ben F]

I have 1.5A slow blow in the SMC 2B. Australia is 240V.

[Thomas W. Bethe]

Have you called the company and asked them what they would suggest? Guess work is never the answer. I would assume that their engineering staff selected the rating for the fuse based on real world experience so I would use the fuse rating they suggested. If there is a problem a phone call to the company will alleviate any concerns. Many times certain power supply configurations have a big inrush current that may blow a normal fuse and that is why a lot of manufactures suggest slo blo fuses.

Best of luck!

[Stereophonic]

Originally Posted by Thomas W. Bethe

Have you called the company and asked them what they would suggest? Guess work is never the answer. I would assume that their engineering staff selected the rating for the fuse based on real world experience so I would use the fuse rating they suggested. If there is a problem a phone call to the company will alleviate any concerns. Many times certain power supply configurations have a big inrush current that may blow a normal fuse and that is why a lot of manufactures suggest slo blo fuses.

Best of luck!

I emailed Tube-tech yesterday asking them if it's rated for fast or slow blow. I hope to hear from them soon. The Brian at T&R told me to use slow blow or 2A fast, the original fuse was fast and the back of the box does not state slow blow. I have other gear that clearly states slow blow. When it comes to this stuff I do exactly as I am told. Guess work is too risky. If it's as simple as using slow blow, then I'm left scratching my head wondering why Tube-tech does not state it on the back of the box or include one when the unit ships.

Originally Posted by inlinenl

it's the fast blow one ...

Originally Posted by Ben F

I have 1.5A slow blow in the SMC 2B. Australia is 240V.

Wim and Ben, thanks for checking that. Ben how did you come to the conclusion to use a higher rated, slow blow fuse?

[Stereophonic]

Here's the word (final word?) from TransAudio Group: On your Tube Tech SMC2B, your unit needs a 800ma SLOW BLOW fuse. This should work fine. The unit WILL blow a fast blow fuse on turn on due to the type of transformer used. But a slow blow will work fine, this is what is used at the factory. A 2A fuse is NOT right.
I hope this saves others from the headaches I have experienced.

[Ben F]

Originally Posted by Stereophonic

Here's the word (final word?) from TransAudio Group: On your Tube Tech SMC2B, your unit needs a 800ma SLOW BLOW fuse. This should work fine. The unit WILL blow a fast blow fuse on turn on due to the type of transformer used. But a slow blow will work fine, this is what is used at the factory. A 2A fuse is NOT right.
I hope this saves others from the headaches I have experienced.

Ok, but don't forget that is for 230 volts- if you are running 250 volts then it would be roughly 10% more power, so you many need a 1A slow blow. In Australia it's 240 volts RMS, this can fluctuate quite a lot, hence the use of a regulated UPS in the studio.

If it's drawing too much current, that would suggest a short somewhere, perhaps one of the pots/switches is worn out and slightly shorting to earth, or one of the tubes is drawing too much current. Are you 110 volt or 250 volt? if you are 110 volt it's meant to be a 400ma fuse.

[Waltz Mastering]

Have you tried the 800ma slo blow with no luck or will you try a higher rated fuse?

You can get them here:
DigiKey Corp. | Electronic Components Distributor | United States Home Page

[Stereophonic]

Originally Posted by Ben F

Ok, but don't forget that is for 230 volts- if you are running 250 volts then it would be roughly 10% more power, so you many need a 1A slow blow. In Australia it's 240 volts RMS, this can fluctuate quite a lot, hence the use of a regulated UPS in the studio.

If it's drawing too much current, that would suggest a short somewhere, perhaps one of the pots/switches is worn out and slightly shorting to earth, or one of the tubes is drawing too much current. Are you 110 volt or 250 volt? if you are 110 volt it's meant to be a 400ma fuse.

I am at 110V and according to TransAudio Group, who's in direct contact with John at Tube-tech, the unit requires an 800mA slow blow. He also stated a 2A fast blow is not an acceptable replacement for 800mA slow. The slow blow seems to be working fine.