Weber 6A20 kit blowing fuses

[rstreber]

Hello - first post here, and I hope my question isn't to newb-ish.

I just finished assembly on a Weber 6A20 (BFDR) kit. It's my first build, but I thought I could handle it having built other tube and pro-audio projects with success in the past.

Anyway, the whole thing is assembled, wired cleanly, I've double and triple checked all connections for correct continuity throughout the circuit and inspected the whole thing for any shorts or other problems.

I can't find any issues, and I've followed the Weber layout precisely. However, when I power it up, it immediately blows the fuse. This is with no tubes but with the Weber WZ34 rectifier installed. It also blew the fuse with no rectifier installed.

First I used the fast-acting 3A that came with the kit, then I tried a slow-blow; both blew within a couple of seconds.

I've checked all power-supply connections for shorts to ground, and I can't find anything. The filter caps look fine, and I don't see (or smell) any signs of component failure.

If anybody has some suggestions on places to look for possible problems, I'd greatly appreciate the advice!

[CitizenCain]

Does this have the Elon branded filter caps? There was talk on several forums, including Weber's, about the smaller caps inside the outer shell. Sometimes they're rated lower than what the outer shell states. Maybe one of them has failed already?

[rstreber]

Thanks so much for the response! Yes, this does have the Elon filter caps. I checked all of them for their resitance from + to -, and all of them were in the 4-5 M ohms range and showed no signs of failure. Any other ways I should be testing them?

[rstreber]

This is probably a very dumb question, but I'm learning as I go here: is it normal for there to be continuity between the heaters and ground? I'm tying the heater CT to ground as specified, so I assume that this is correct, but it surprised me at first. Just thought I'd confirm...

[Diablo]

You need to hook up a light bulb limiter for testing. Do a search on this forum. You probably have a short, the question is where. You may have a shorted power transformer, especially since it blew the fuse without a rectifier in place. Disconnect all secondary leads on the PT, then power up the PT with the light bulb limiter. If it glows brightly and stays bright, you have a short in the PT.

[CitizenCain]

Sorry, the Elon thing was just a shot in the dark. Don't know of any other way to test them for this type of problem. It sounds like a short somewhere as Diablo mentions. The lightbulb test is a good place to start.

[CitizenCain]

PS - You've seen the post linked at the bottom of the page, about the Weber 5E3 kit blowing fuses? http://music-electronics-forum.com/t812/

He had a shorted power transformer.

[rstreber]

Thanks for all the responses, and thanks to theunrulychef who seems to have written a lengthy response that isn't showing up here (but that I got in an email notification of the response.)

I'm going to try to get over to Home Depot today to get the stuff to make a lightbulb limiter, and then I'll start going from there. I'm kind of scared that it's a blown PT. I still haven't found any shorts anywhere or any mis-wirings, and I've been over this thing again and again, but obviously something is wrong somewhere.

[CitizenCain]

Could you post theunrulychef response here? I'd like to see what he had to say. Thanks!

[rstreber]

Sure:
theunrulychef wrote:

"Try to isolate the problem. Does it blow when the standby switch is open? If so, then it's something before that switch. What about if you disconnect the red wire at pin 8 of the rectifier? If it blows with the switch open, but doesn't blow with that wire disconnected, then you know it's between the 2 points (those two 16uf caps & the wiring between would be suspect & should be checked etc). Still blows with that wire disconnected, then you are having a problem at the socket or earlier (check the 5v wiring, any shorts to ground on the rectifier socket? PT Wiring 110% correct?). Standby switch open keeps it from blowing, then you've got a problem after the switch (Filter caps, choke ok?, power rail connections shorting to ground?).

"I'm pretty theory & math challenged (especially compared to the smart & helpful people on this forum), but I can fix most of my problems by figuring out where they heck they are located & going from there.

"The 6.3v heaters will have continuity to ground, yes, though the 5v heater wiring should NOT have continuity to ground. If the 5v heater supply is shorting to ground, that would blow your fuse I believe."

[theunrulychef]

Could you post theunrulychef response here? I'd like to see what he had to say. Thanks!

Sorry, I pulled it down after seeing Diablo's reply. As usual, I was missing the obvious info staring me in the face that without the rectifier tube in there, there's no connection to any of the filters etc. What I said about finding the location of the problem still stands of course, but I was looking on the wrong side of the rectifier.

[CitizenCain]

Yeah, I'm still challenged in all aspects of amp design, that's why I'm always interested in what anyone has to say

Hope you get it sorted, rstreber!

[rstreber]

Thanks again, guys! I can "connect the dots" pretty well, but when stuff doesn't work, I'm at square one. I'm very eager to learn, though, so I guess in a perverse way it's good when I have a major fail like this.

In any case, after my session today, I'm hoping to build a bulb limiter and go to town this evening, so I'll report back once I've made some progress (or not.)

[Bruce / Mission Amps]

Don't waste time with this

I usually start my new builds with a variac... You say it blows the fuse with out the tubes. OK a good start.
However, in this case you simply will not know what is happening now until you disconnect the power transformer secondary leads from the the circuit.
The tubes are the major current draw but even a shorted filament wire on any socket or pilot lamp holder will blow the main fuse.
So, I'm assuming you are 100% sure the primary leads are wired correctly... I say this because many of us have wasted WAY too much time with novice builders who say they built this amp 100% correct, triple checked their wiring and there is no way in Hell they have made any mistakes whatsoever ... only to find out they made a colossal mistake in the main wiring. Novice bravado.
I can say this because I've done it.

If you remove ALL (do not cheat) the PT's secondary wires from the circuit and it blows the fuse you have a primary lead short, bad on-off switch, bad fuse holder or a bad transformer.
Simple as that.
If it is the PT, you have to accept this and replace it.

Let's think the PT is not bad.
If it doesn't blow the fuse with the secondary wires off, then put one set of secondary wires back in place at a time and check again.
If it blows the fuse with any on or sets of the secondaries going back in the amp, you have to trace it out and find out where the short or incorrect path to ground is...
If it never blows a fuse again, you probably wired it back together correctly this time and just could not see the forest through the trees.

[Diablo]

I'm kind of scared that it's a blown PT.

Nothing to be scared of, and it's an easy fix. If it's a bad PT, Weber will send you out a new one - no charge. They're very good at sending out replacement parts quickly and without a hassle.