5e3 stopped working - blows fuses

[andyo64]

Looking for some guidance. I have a 5e3 that I bought as a kit and it had worked great for the past couple of years but the other day I turned it on and it immediately shut off and wouldn't go on again. Found that the fuse had blown. I opened it up, didn't see anything, I have a Tube tester and tested the rectifier and 2x power tubes which all show as good. I'm thinking my next step is to check the power transformer. My question is this - do I need to remove the transformer from the circuit to ohm out /verify if there's a short? And the 2nd question should this be the next thing to check? Any input would be appreciated, thanks Andy

[xtian]

Pull the power tubes and test voltages.

[martin manning]

By removing all of the tubes and the pilot lamp you can disconnect all the transformer secondary windings. You can then measure resistance on both halves of the HV secondary and the filament windings, then power up to measure voltages.

[frankdrebin]

Use a lamp bulb in series with primary,secondaries disconnected,you can't get any conclusions with an ohm meter.

[andyo64]

Thanks for the helpful recommendations. Hopefully I'll be able to test without blowing another fuse - down to 3! This was my 4th build, I guess I forgot about initial power on testing, I also have a current limiter I made "light bulb version"" to keep current down.

[sluckey]

What size fuse? Fast or slow?

There's a chance that there is nothing wrong with the amp. Fuses usually blow because of a failure inside the amp. But fuses may also blow simply because of the inrush current at turn on. A slow-blow fuse usually prevents this kind of blowing. And sometimes fuses blow for mysterious reasons that will never be known.

Put a 2A SLO fuse in it and see what happens.

[Mark]

It the amp fixed yet?

[andyo64]

So-I did some testing today. Found that the fuses are slow blow 250v 2a. Pulled all power tubes, rectifier and pilot bulb. Installed a new fuse and powered it on; herd a little pop - it was the fuse again. Does this mean that the power transformer went bad?

[martin manning]

It's not looking good... Is your circuit the same as the original 5E3? Did you try powering up with a bulb limiter?

[andyo64]

So the last question: is it the same 5e3 ckt, Yes. As for current limiting; I hooked up my 300w light bulb current limiter and was able to turn the amp on. I was able to see voltage on the low voltage connection to the rectifier tube socket but the high voltage was showing 0 volts DC. I think this settles it - I need a power transformer. I'll tell you the outcome once I get the new one installed unless you guys can think of something else? Thanks again for the help.

[JazzGuitarGimp]

So the last question: is it the same 5e3 ckt, Yes. As for current limiting; I hooked up my 300w light bulb current limiter and was able to turn the amp on. I was able to see voltage on the low voltage connection to the rectifier tube socket but the high voltage was showing 0 volts DC. I think this settles it - I need a power transformer. I'll tell you the outcome once I get the new one installed unless you guys can think of something else? Thanks again for the help.

I assume the light bulb limiter was a fairly bright glow? And, you did this test with the 5Y3 removed from its socket, yes? If you want to be absolutely certain it’s the PT (and you haven’t done this already) disconnect both HV red wires from the rectifier tube socket and measure again. If still no AC from red to red, then yes, I would assume the PT is fried.

Good luck!
Lou

[andyo64]

Yes the light was bright, I was thinking I should have said this which alerts you to a short. The other question is yes the 5Y3 was out. I'll try the last test with the VOM set to AC. I was thinking of trying a Hammond 290BBX as a replacement.

[hagglund]

My cousin once built a 5e3 that blew fuses. I found that the OT was shorted to it case that shorted to ground when it was in place. I did not blow the fuse when rectifier was out but as soon as I put the rectifier in the fuse blew. Measured all the OT windings and they where ok. They where connected and some ohms as they should. When measured to the transformer case I found the short. There shouldn't be any connectivity there.

Just an idea if the PT is ok.

[Mr. dB]

Before you buy a transformer, check the power switch. Those Carling switches can fail and short a pole of the switch to the threads - connecting your AC to the chassis.

I'm not saying that's what's happened here, but it is a possible failure mode I've seen.

[boots]

Power transformers often turn out to not be the problem. I think a lot of PT's get changed without curing the problem.

I would not feel comfortable spending money on a new PT until I tested it with all the primaries disconnected as Lou mentioned above. If you are not getting any high voltage with the secondaries totally isolated, THEN you can feel confident there is a problem with the PT.