5E3 Blowing Fuses

[rdh006]

My 5E3 clone blew a fuse at a gig last night. Nothing unusual happened to cause it, the amp just went down mid-number. No immediate visual evidence that anything has fried. I know the most likely causes are rectifier tube failure or power tube failure. I replaced the fuse and watched the tubes on power up. The rectifier tube glowed blue and the fuse blew again.

Hopeful that I had found the fault, I replaced the rectifier (NOS 5Y3GT), put in a new fuse (2.5 Amp slo-blo) and tried again. No arcs or blue glow, but the fuse blew again.

Hoping that the experts here can help with what to look at next, or a troubleshooting procedure?

Any help appreciated.

[alexradium]

try first without any tubes,with a light bulb limiter in series,if the light is fully bright the power transformer is toasted.
Since its a self made amp,check all wirings for potential short circuits.

[rdh006]

Thanks for the help, alexradium. I did as you suggested, light bulb limiter, no tubes. The bulb doesn't glow at all. What is the rationale for that?

[alexradium]

change the fuse

[nosaj]

Thanks for the help, alexradium. I did as you suggested, light bulb limiter, no tubes. The bulb doesn't glow at all. What is the rationale for that?

Transformer is not shorted is the rationale. Next step add just the rectifier still using the lightbulb limiter and watch the dimbulb. and report back.
nosaj

[rdh006]

alexradium -- I did change the fuse -- the pilot light is on.

nosaj -- thanks for your help, too. Did what you said -- the bulb first glowed bright, then dimmed down a bit to maybe half brightness

[nosaj]

alexradium -- I did change the fuse -- the pilot light is on.

nosaj -- thanks for your help, too. Did what you said -- the bulb first glowed bright, then dimmed down a bit to maybe half brightness

That would be a sign of your filter caps charging. So far so good, now turn off put in the outputs 2x 6v6s I believe. then turn on and see what happens.

nosaj

[rdh006]

The explanations are very helpul, nosaj -- thanks for that. Put in the outputs (6v6s). Bulb acted the same as with just the rectifier in -- glowed bright then dimmed down about half.

[nosaj]

The explanations are very helpul, nosaj -- thanks for that. Put in the outputs (6v6s). Bulb acted the same as with just the rectifier in -- glowed bright then dimmed down about half.

All right next you have your phase invertor to do then the preamps...do them in the same way. Once they all pass it tells you there is nothing majorly wrong with the amp. take it off the light bulb limiter then give it a whirl with straight mains AC.

nosaj

[rdh006]

I put the remaining tubes back in, one at a time, bulb glowed bright and then dimmed. I connected directly to mains -- the rectifier glowed blue again, and the fuse blew again.

[g1]

The bulb limiter keeps the voltage down. It's possible a filter cap that is ok at the reduced voltage is shorting at full line AC. A bad power tube might act that way too.
Ideally you would add the tubes one at a time at full line voltage, the same way you did on the bulb.

[rdh006]

Thanks, G1. I didn't get very far with your suggestion. I took all the tubes out and started again at full line voltage with just the rectifier tube*. The rectifier starts to glow as soon as the standby is switched to on -- first red, then blue, then the fuse blows. All that happens pretty quickly -- I can flip the standby before it gets to blue and before the fuse blows, if I am quick enough.

*this is the second rectifier tube that I put in as a replacement in post #1.

[Chuck H]

There's a small chance you had two bad rectifier tubes even if they were back to back on the assembly line. My guess is that you have a bad main filter that is causing an overcurrent situation at full line voltage and full amp current.

[Jazz P Bass]

Thanks for the help, alexradium. I did as you suggested, light bulb limiter, no tubes. The bulb doesn't glow at all. What is the rationale for that?

If you have no tubes installed then there really is nothing to draw current.

Hense the unlit limiter lamp.

I think you have a bad power tube (s) and it took out the rectifier & the fuse.

[rdh006]

Thanks Chuck H and Jazz P Bass. With these two different possibilities, any ideas on how I should proceed without blowing any more tubes?