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Old 31st May 2012, 04:57 PM   #1
Divad89 is offline Divad89  Denmark
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Default Fuse blows instantly

I've been running with my Tubelab Simple SE, for a year now without any problems whatsoever, but suddenly while I was in another room (while it was on and played music), it had blown the fuse. So I took it apart and just looked for something that could have caused it to blow the fuse, I put in a new fuse, which it blew instantly again.

So I took out the PT and OPT's (Edcor XPWR059 and CXSE25-4-5K) from it, because I had this idea to assemble a KT88 SE, from Mikael abdellah's schematics.
So now I've assembled it, but it too blows the fuse instantly, even though I been looking for short circuit several times (and when I say it blows the fuse, it shatters the fuse).

But could it have something to do with the transformers?
And if it has, would it be possible to take the PT out again, and just hook it up to the main, and check the voltage readings, or does it have to have a load on it?

---
David
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Old 31st May 2012, 05:05 PM   #2
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I'm not sure, are you using the same tubes or same transformer as before?

Do you mean the glass breaks on the fuse?

What are you using for a rectifier? (Not familiar with the SE)

You should have checked the transformers for shorts when you assembled the new one.

You can check for transformer problems without power.
I don't see a problem with, perhaps I am wrong, about removing the OPT to check the PT, remove the tubes first.

If all is well and you still pop fuses, try some different tubes.

Good luck.
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Old 31st May 2012, 05:05 PM   #3
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Divad89 View Post
I've been running with my Tubelab Simple SE, for a year now without any problems whatsoever, but suddenly while I was in another room (while it was on and played music), it had blown the fuse. So I took it apart and just looked for something that could have caused it to blow the fuse, I put in a new fuse, which it blew instantly again.

So I took out the PT and OPT's (Edcor XPWR059 and CXSE25-4-5K) from it, because I had this idea to assemble a KT88 SE, from Mikael abdellah's schematics.
So now I've assembled it, but it too blows the fuse instantly, even though I been looking for short circuit several times (and when I say it blows the fuse, it shatters the fuse).

But could it have something to do with the transformers?
And if it has, would it be possible to take the PT out again, and just hook it up to the main, and check the voltage readings, or does it have to have a load on it?

---
David
You don't say what fuse it blows?
On B+ or mains supply?

If its the mains fuse..you can put the OP leads into block connectors to isolate them and put a mains fuse in connect to the mains then check V out if all is OK check the diodes on the rectifier with power OFF...Remember some meters won't read on the Ohms range so you need diode check to overcome the .5V drop..

If the mains fuse blows with no outputs connected.....they must be isolated from each other...not shorted togeather..LOL
Then the power Tx is stuffed....Remember to put an earth on the Tx..Just in case its a short to Gnd on the Tx and don't hold it while testing..LOL

Regards
M. Gregg
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Last edited by M Gregg; 31st May 2012 at 05:18 PM.
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Old 31st May 2012, 05:54 PM   #4
tomchr is offline tomchr  United States
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I think you committed the cardinal sin of debugging. Modifying the circuit to an entirely different circuit in an attempt to fix the problem with the original circuit. Always fix the circuit, then modify it. Just my two cents worth...

It takes a significant amount of energy to shatter a fuse. I'm assuming you have the fuse in series with the power transformer primary.

Disconnect the power transformer from the circuit and mains. Measure each winding (both on the primary and secondary) with an ohmmeter. Expect to see resistances on the order of 10's of ohms maybe upward of 100 ohm. If any of the high voltage windings measure close to 0 ohm, the power transformer is shot. A high current filament winding will probably measure a few ohms. That's OK.

Now isolate all secondary wires using a terminal block. Leave them all disconnected. Connect the primary to AC mains through a fuse. Turn the power on. If the fuse does not blow, the power transformer is OK - you can verify the secondary voltages to be sure (be careful HIGH VOLTAGE!!).

If the transformer passes so far, the fault that causes the fuse to explode must be in the amp. I'm betting on the supply.

Make sure all supply caps are fully discharged. Pull all tubes. Using an ohmmeter, measure across the main reservoir cap. If you measure close to 0 ohm, the cap has failed. You should see a relatively high resistance that appears to be rising if you leave the ohmmeter connected for a while.

Measure the diodes - preferably with a DMM that has a diode setting. In one direction, you should see a 0.7 V drop. In the other, the meter should read infinite (no conduction).

Repeat for remaining supplies (bias, filament, etc).

If all this checks out, connect the power transformer to the supply but leave out the tubes in the amp section. Turn power on, measure B+.

I suspect either the rectifier diodes or the reservoir cap have failed.

~Tom
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Last edited by tomchr; 31st May 2012 at 05:57 PM.
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Old 31st May 2012, 06:24 PM   #5
Divad89 is offline Divad89  Denmark
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@Alvis:
I haven't even got to put in tubes in yet, but I have planned to use the same tubes (except in the start, where I plan to use some cheaper tubes...) and it is the same transformers as before.
And yes the glass break, and there is practically smoke everywhere...
Both in the old and in the new I'm using a tube rectifier (5U4-GB in the new and 5AR4 in the old)

@tomchr
I'm not modifying the circuit, I've started from scratch and have just taken the transformers and the tubes from the old amp, and started building an entirely new amp.
And yes I have the fuse in series with the power transformer primary.

But I will try disconnect the PT from the circuit, and measure the windings...

I have taken the power supply from this design:
HTML Code:
http://diyaudioprojects.com/Tubes/KT88/
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David
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Old 31st May 2012, 10:58 PM   #6
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First you need a light bub tester:

Light Bulb Tester
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Old 31st May 2012, 11:48 PM   #7
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You didn't test the transformer first? Tsk tsk.

Judging by your description,it instantly blew the fuses in both amps? I'm gonna take a wild guess and say that your transformer may be bad. (shorted turns,or something.)
Take it out and test it with a DMM,come back and post the winding resistances.
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Old 1st June 2012, 12:01 AM   #8
7N7 is offline 7N7  United Kingdom
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I don't know this amplifier but regarding diodes (if it has diode rectification) I had one a few weeks ago that I repaired: it has a hybrid bridge - two damper diodes and two silicon diodes. Took it to the chap's house plugged it in and am embarrassed to say that the fuse did not blow but there was a loud buzzing from the power transformer. I switched off very quickly and found the transformer to be very hot. A 1000V diode had let go and was short-circuit.

Worth a look

Paul
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Old 1st June 2012, 12:35 AM   #9
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when fuses shatter, its usally becuase the voltage rating of the fuse is too low for the circut in question...common on the B+ side of he circut. The issue is if the rating is too low....the current rides the melted fuse element a little longer and continues to maintain the circut. Whereas a higher voltage rated fuse will break the circut as soon as the fuse element is compromised.
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Old 1st June 2012, 08:59 AM   #10
M Gregg is offline M Gregg  United Kingdom
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Well...

What have you found?

Regards
M. Gregg
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