5E3 Build Blowing Fuses

[Munga]

Ok, I finished it, but it won't start. In fact, it is blowing fuses. I turn it on, and test lugs 4 and 6 on the rectifier tube socket, and the MM averages about 450-700v. And then, the fuse pops. Any ideas what could be wrong?
BTW, this is with no tubes installed.

[Mars Amp Repair]

5e3 blowing fuses

Hello,
If the 5E3 is like the schematic listed in Schematic heaven, then you're exceeding the ratings of the filter caps . The filters are rated at 450VDC. They must be getting hot before the fuse blows, too.
I would have to guess you have the wrong PT for that unit, if once again it is like the schematic on schematic heaven for the 5E3. glen - mars amp repair

[Munga]

I was using the 5E3 weber kit.

[Enzo]

I don't think he'd over voltage the caps with the rectifier tube missing.

If it blows fuses with no rectifier tube, then one or more of the secondaries is loaded down too much. Or the power transformer is defective. I'd bet on something you wired. I would look carefully at the heater wiring. Make sure you didn't ground both sides of that.

Also, make sure neither pin 4 or pin 6 of the recto socket is grounded. A stray bit of solder or of wire ends could do that.

If nothing else, disconnect the power transformer wires from the recto socket, from the heaters and from the recto heater. We want all those wires not connected to ANYTHING. If it still blows fuses, then we have to suspect the transformer. If it holds a fuse that way, then reconect the HV wires to pins 4 and 6 of the recto. Obviously have the amp unplugged from the mains while rewiring.

If it blows a fuse now, then something about the socket and wiring is bad. If it still holds a fuse, then reconnect the recto heater wires. Again if it mow blows fuses, we know where the problem lies. If it still holds a fuse, we move on to reconnecting the tube heater wires. If one of these steps revealed the problem area, then we know what area to search for the problem. And if we get it all back together and it no longer blows fuses, we have bot only cured the problem, but we also learned it must have been something in the wires.

[Tom Phillips]

… averages about 450-700v. BTW, this is with no tubes installed.

You are measuring the AC voltage of the end to end HV secondary. 450 to 700V is a very wide range. 700 is a bit high and 450 is way low. You should get a steady reading unless something is wrong with your meter or the transformer. With no tubes installed and if you are using the correct size fuse (2A) then I’d say that you either have a short in the wiring or a bad transformer. You could disconnect all the secondary leads and tape them off. Then apply power again. If the fuse still blows then I’m afraid that you have a bad PT. If the fuse does not blow, then carefully check your wiring.

Good Luck,
Tom

[Munga]

I don't think he'd over voltage the caps with the rectifier tube missing.

If it blows fuses with no rectifier tube, then one or more of the secondaries is loaded down too much. Or the power transformer is defective. I'd bet on something you wired. I would look carefully at the heater wiring. Make sure you didn't ground both sides of that.

Also, make sure neither pin 4 or pin 6 of the recto socket is grounded. A stray bit of solder or of wire ends could do that.

If nothing else, disconnect the power transformer wires from the recto socket, from the heaters and from the recto heater. We want all those wires not connected to ANYTHING. If it still blows fuses, then we have to suspect the transformer. If it holds a fuse that way, then reconect the HV wires to pins 4 and 6 of the recto. Obviously have the amp unplugged from the mains while rewiring.

If it blows a fuse now, then something about the socket and wiring is bad. If it still holds a fuse, then reconnect the recto heater wires. Again if it mow blows fuses, we know where the problem lies. If it still holds a fuse, we move on to reconnecting the tube heater wires. If one of these steps revealed the problem area, then we know what area to search for the problem. And if we get it all back together and it no longer blows fuses, we have bot only cured the problem, but we also learned it must have been something in the wires.

So, for the first step, according to the layout I provided, disconnect the 2 pure red, 2 pure yellow, and 2 pure green wires? Just double checking. I hate getting stuff like this wrong when I think its right.

[Enzo]

Yes, that is what I am suggesting. But first, look over the 6v wiring to see if it is grounded more than one place, resulting in both sides of it grounded.

The two yellow wires should be the 5v winding for the recto heater. If there are any other yellow wires - like yellow with a stripe - DO NOT GROUND IT.

If the two green ones also have a green with yellow stripe friend, and he is grounded, then NEITHER side of the 6v should be grounded. The green/yellow is the center tap, and it is normal for THAT one to be grounded. The same for the red/yellow one. But the yellow body wires do NOT go to ground.

[Munga]

Ok, with the help Jim McGee, I have figured out that the PT isn't the problem after all. Yipee! I guess.
I need some more slowblow fuses. But I am at least happy that it wasn't the PT. After I get the fuses, I am gonna try and rewire the PT leads and see if that fixes anything. Thanks for all of your guys's help!

[Tom Phillips]

While you still have the PT wires disconnected you have a good opportunity to ohm out the chassis wiring to see if you find a short that you missed by visual inspection. It’s not clear yet if you are using the original heater wiring scheme with one side grounded or upgraded to a grounded center tap arrangement. Which is it?
Tom

[Munga]

While you still have the PT wires disconnected you have a good opportunity to ohm out the chassis wiring to see if you find a short that you missed by visual inspection. It’s not clear yet if you are using the original heater wiring scheme with one side grounded or upgraded to a grounded center tap arrangement. Which is it?
Tom

I got a Weber kit, so I am using this layout almost exactly:
https://taweber.powweb.com/store/5e3_layout.jpg


Alec

[Tom Phillips]

Alec,

OK. That wiring uses a grounded center tap on the heater winding (The green/yellow wire) so with the green leads from the PT disconnected, the tubes removed and the pilot light bulb removed you should measure near infinite resistance between:
1) The two legs of the heater wiring
2) From either side of the heater wiring and the chassis

With the red HV leads disconnected you should measure near infinite resistance between:
1) Pin 4 of the rectifier tube socket and any other pin of the socket
2) Pin 4 of the rectifier tube socket and chassis
3) Pin 6 of the rectifier tube socket and any other pin of the socket
4) Pin 6 of the rectifier tube socket and chassis

Hopefully this will help isolate a wiring problem that your eye doesn't catch.
You may need to wiggle the wiring while the meter is connected if all tests are good on the first try.

Tom

[Enzo]

Just a thought. Make sure the pilot light holder does not touch one side of the green wires to chasssis. In other words, both terminals of the light socket must be insulated from chassis, and make sure the sodler tabs on it are not rotated around so they touch either the frame of the light socket or the chassis itself.

If one side of the light socket gets grounded, then teh 6v winding will be grounded at both the center tap AND that sode. That is a dead short across half the 6v winding

[Munga]

Ok, heres the progress. I soldered everything from the PT except for the green leads, and no fuse popping. Then I soldered the green leads to the light, and still no fuse popping. Then, I installed the Weber Copper Cap, and a hum started coming from the light, and pulled the plug before the fuse could pop again. This is what happened before. Any ideas?

[Tom Phillips]

Now that's new information.

Either the Weber Copper cap is bad or something downline from it is drawing too much current.
Is the standby switch in the "standby" position when this happens?
If it is then it's the copper cap or a bad switch (low probability but not impossible)

If you disconnect the lead from the rectifier socket pin 8 to the standby switch and the fuse still blows, then it's a bad copper cap.

Tom

[Munga]

Yeah, its in standby. I will test that right now.