5E3 Blowing Fuses

[Chuck H]

IMO the limiter test showed that the amp was behaving right as to the health of the filters and rectifier. Of course I can't see the actual amount of illumination and it's a pretty blunt (but useful) test. So I'll ask the obvious questions...

How many hours, approximately, on the power tubes and what sort of service do they do? That is, do you crank the amp and control with the volume on your guitar so the power tubes are clipping often? Do you usually have the amp relatively clean and run with pedals keeping things below max volume? That sort of thing. AND, when was the last time the electrolytic caps in the amp were changed? Most electrolytic caps have a use life of ten to twenty years. The range is large because it depends on quality and how the amp is used, or not used. Some older caps could live for a VERY long time indeed, as much as thirty or forty years! But that era is coming to an end and more modern construction caps, while more consistent and generally with closer tolerance throughout their useful life, don't seem to have the legs that the old power supply caps did. So if your filter caps are original or more than ten years old there may be reason to suspect those as well.

[pdf64]

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.

Yes, maybe another bad rectifier, or, if the standby is 'hot switching', between the rectifier and HT reservoir, then it may be that the HT reservoir cap is bad, eg excessive leakage at high voltage.

[mozz]

Take it to a tech if you're not one yourself. You have bigger problems than a fuse or rectifier tube.

[rdh006]

Thanks, guys.

mozz -- I live on an island in the Caribbean -- no techs, no parts, no nothing. I have to order stuff in from the US, and I am reliant on you kind folks here for some assistance in troubleshooting. I built the amp, but I'm no expert. I'm OK with safety/discharging caps etc, but I'm no amp tech, and I have no troubleshooting experience.

pdf64 -- thanks for the suggestion. It looks like the B+ and reservoir cap are on one side of the standby, and the rectifier is on the other.

Chuck H -- I use the amp on 2-3 gigs a week, say 6-10 hours per week. Power tubes are TAD 6v6s which have been in for about a year. I run the amp quite hard and control it with guitar volume. The amp has a VVR and is run pretty much all the time at reduced voltages. Electrolytic caps: a couple have been replaced about 3 years ago, most have been in since the amp was built 10 years ago.

[Enzo]

I missed where we tested the power tubes. Did we fire it up without the bulb, but with a rectifier tube? And did the fuse hold? Then install just one 6V6, fuse hold? Now install the other 6V6, fuse hold?

[rdh006]

Hello Enzo -- I can't get that far. If I power up with just the rectifier, the rectifier glows red, then blue, and the fuse blows.

Edit: I replaced the rectifier tube and the same thing happened again.

[Enzo]

Then there is a short on the B+. Possible shorted or miswired filter cap, possible short in output ttransformer.

[eschertron]

So you tried no tubes with light bulb limiter = OK
Rectifier in with light bulb limiter = OK
Rectifier in (no other tubes) direct to mains = POW!

did you try no tubes, direct to main? this should eliminate the PT if it's OK. ...wait. I see Enzo does not suspect PT. As soon as the rectifier starts to 'make' DC (blue glow), the fuse pops. Did you say what was between the rectifier and the standby switch? Is there a switch?

Who built the amp? Is there someone around with the skills to unsolder and lift wires to components such as the OT?

[rdh006]

Enzo -- thanks for the help -- you did mean OT and not PT?

eschertron -- thank you.

no tubes with light bulb limiter = OK -- correct
Rectifier in with light bulb limiter = OK -- correct (worked through all tubes with the limiter, bulb dimmed down in each case)
Rectifier in (no other tubes) direct to mains = POW! -- correct

no tubes, direct to main -- I hadn't tried that, but I just did -- fuse holds.

I built the amp, but I'm no expert. I'm OK with safety/discharging caps etc, but I'm no amp tech, and I have no troubleshooting experience. I can unsolder and lift wires, I just don't have the understanding/logic.

Also, it is a long time since I used the bulb limiter. It is definitely glowing bright and dimming under the tests above, which I thought indicated no shorts. But Chuck H's mention of brightness makes me wonder if it perhaps previously dimmed down further -- IOW it is perhaps brighter under the tests above than when I originally used it. I have been using a 100 watt bulb.

Edit: It looks like the B+ and reservoir cap are on one side of the standby switch, and the rectifier is on the other.

[eschertron]

Enzo -- thanks for the help -- you did mean OT and not PT?

eschertron -- thank you.

no tubes with light bulb limiter = OK -- correct
Rectifier in with light bulb limiter = OK -- correct (worked through all tubes with the limiter, bulb dimmed down in each case)
Rectifier in (no other tubes) direct to mains = POW! -- correct

no tubes, direct to main -- I hadn't tried that, but I just did -- fuse holds.

I built the amp, but I'm no expert. I'm OK with safety/discharging caps etc, but I'm no amp tech, and I have no troubleshooting experience. I can unsolder and lift wires, I just don't have the understanding/logic.

Also, it is a long time since I used the bulb limiter. It is definitely glowing bright and dimming under the tests above, which I thought indicated no shorts. But Chuck H's mention of brightness makes me wonder if it perhaps previously dimmed down further -- IOW it is perhaps brighter under the tests above than when I originally used it. I have been using a 100 watt bulb.

So a 25W bulb would show some tests dim more or less than others. The less dim results indicate too much current. But I think you're close enough here to not repeat all the testing.

Edit: It looks like the B+ and reservoir cap are on one side of the standby switch, and the rectifier is on the other.

Did you try the rectifier in with the switch open? If I read you correctly, there would be no caps or anything else downstream in this case. More importantly, did you do the limiter in/limiter out tests with the standby switch in the same position?

edit: If you lift the OT lead off the power supply string, you can test to see if you get the same results with and without the OT in the circuit. Eliminating the OT as a culprit would be a good thing. Here's where a more sensitive dim bulb (25W vs 100W) might help. I don't want you to have to burn up all the fuses in the Caribbean!

[rdh006]

eschertron -- ha, yes, I'm down to my last 4-pack of fuses and I have one more 5Y3GT which lives in another amp.

All the previous tests were powered up first with the standby open then, after checking the rectifier and the light bulb, the standby was switched to on. The amp got through the bulb tests with each of the tubes added sequentially.

When connected direct to the mains, the rectifier glows red/blue and the fuse blows only when the standby is switched on/closed.

With only the rectifier tube in, and the standby switch open, the rectifier lights up normally and the fuse holds.

I can't lift the OT lead right now, I can do that a little later -- what exactly should I be testing before and after doing this?

Thanks for the help -- much appreciated.

[Enzo]

Lifting the OT center tap tells us whether the OT has a short to frame or to secondary. If lifting it stops fuse blowing, then the OT is involved. Of course the OT could also be fine, and a power tube socket has pin 3 shorted to ground.

[The Dude]

Since it's a homebuilt amp, it doesn't have flyback diodes does it?

[rdh006]

Thanks, Enzo.

Dude — no flyback diodes. Thanks for responding.

[rdh006]

The amp is working, so I wanted to wrap this thread up, post the final solution, and say a big thankyou to everyone who helped.

Reading back through all the suggestions, Chuck H's questions about how long the tubes and filter caps had been in made me realise that they are overdue for change. I first checked all the tubes in a known good amp and they are all working well.

Given that filter caps was one of the potential causes of the problem, I decided to replace them. If I replace the caps and the problem is solved, then great. If the problem is not solved, the caps needed changing anyway. So as a first step, I replaced the first filter cap on the board and re-tested with the bulb limiter. The bulb lit up then dimmed right down as it should.

Hopeful that the problem was solved, I connected direct to the mains. No such luck. The fuse held and all the tubes looked normal, but no signal through the amp, the only sound was a fairly loud hum. You guys taught me before that distinguishing between 60hz and 120hz hum can be difficult, but this sounded very much like 120hz -- around Bb on the 5th string. I checked the filter cap grounds, which seemed especially likely given that I had just replaced one of them. Turned out there was a broken ground wire at a tag strip connecting to the chassis.

I rewired that connection, and the amps works perfectly.

I still need to replace the rest of the filter caps, and I'll put in a new set of tubes now that they're not blowing, but the fault appears to be fixed.

So, I have a gig tonight, and I can again use my favourite amp, although I will definitely be taking a backup amp.

Thanks very much, guys, for helping me get this amp up and running again -- the time and expert help that is so freely given here is truly appreciated...