Build Your Own Clone Message Board

I just finished completing the assembly of my amp and with just the rectifier tube in the fuse blows after 20 to 30 seconds. The rectifier shows signs of life until the fuse blows. I've checked a lot of the connections around the tube sockets, but nothing jumps out at me as being wrong.

Anyone have any ideas what I can check?

It blew the fuse twice by the way.

Ron

Ron I would post some good clear shots of those guts showing the wiring and the turret board.

Does it blow without the rectifier tube in? If not then your rectifier tube is bad.

+1.

Amp Troubleshooting 101.

Fuse blowing??

Remove tubes.

Turn on the amp with speaker plugged in.

Fuse doesn't blow?

You have a shorted tube, most often power or rectifier tubes.

Can the tube blow if you wired the rec tube wrong, ie. the tube could be fine?

Just curious to learn something here.

Here's a shot at the chassis and turret board. I replaced the rectifier tube with a known good one from another amp and it still blew the fuse. I've doubled checked all the connections and everything looks fine, but still blowing fuses with just the rectifier tube, no other ones plugged in. It doesn't blow the fuse without the rectifier tube.

Sorry, here's a picture of the chassis.

Are you sure that the wiring from the AC power cord is correct? The wire colors are quite different from mine, which were light brown, light blue and white. The white on mine is the ground wire and the light borwn is the "hot", but I see that your white wire is connected as the "hot" wire to the pull switch, and the turquoise colored wire is connected to the chassis ground lug. Are you sure that's correct?

I can't see where you capped off the Red/white wires. Those are capped and separated?
-P

I had to use a different power cord as I nicked one of lines in the original. I did notice that colors were different and made sure that they matched up correctly. The green one is ground on my power chord.

I fired up the amp last night and this time the fuse didn't blow, but the output transform got really hot. I then measured the DC voltage at the positive end of the 16uf cap and a got 0 voltage. I'm thinking there is a short somewhere or the cap is bad.

Any other ideas?

Thanx,
Ron

Ron, did you separate the red and white and cap them like patrickbrose mentioned?

If it's me I'm taking some good clean shots of the wiring for the OT for the guys so each wire can be traced to where it is connected. But I'd surely drain those caps first before I start poking around.

TNBlueshawk,
Which red and white wires? Where are they suppose to be coming from? The only wires I capped off are the brown and yellow wires coming from the output transformer.

Thanx,
Ron

Ok, my bad, it looks like you have the 120V single primary PT, so there will not be the Red/White wires. The thing that is concerning is that you are not getting a voltage at the 16uf cap. It should be about 425V, that leads me to believe that there is an issue between the Power supply, PT, and rectifier getting to that cap. The fact that the OT is heating up leads me to believe that the heaters are working, but something is amiss with the B+.
Someone out here probably has a better idea than I do though.
-P

I had the 240v like you on the PT. I forgot about that.

Sorry Ron. That explains why we didn't see it We got the PT from another source.

Here's an update on my problem:
1. I checked all three of the power supply caps to make sure they are still good and they are.
2. With no tubes plugged in and the amp turned on, the Power Transformer (larger one) will get warm after about 2 to 3 minutes.
3. With just the rectifier tube plugged in and the amp turned on, the PT gets really hot after about 30 seconds, too the point that you can't touch it. Also, I still can't measure any voltage on the plus side of the 16uf cap. Also after a couple of minutes either the PT or the rectifier tube starts to make a crackling sound. At that point I turned it off and let it cool down. I don't think the PT getting real hot is normal behavior, does anyone else know about this.
4. I've tripled checked all the connections around the rectifier and PT and everything checks out fine.

Thanks,
Ron

Yeah, a PT getting that hot that quickly is definitely not normal. If it does the same thing with more than one rectifier tube, sounds like there must be a short somewhere. I wonder if maybe there's a short in the OT? That might explain why you can't measure any B+ voltage at the positive side of the first filter cap. Try measuring the resistance between that B+ turret and ground. Should be very high; it's off the ohmmeter scale on my DMM.

I measured only 2ohms between the b+ turret and ground, with the power off.
Ron

Well, that's certainly not good! Something connected to that turret is shorting to ground. You said you had checked that 16uf filter cap and it's OK, and the 10K resistor going to the adjacent turret should be a non-issue. So that would leave either the OT or a short somewhere on the output side of the rectifier socket itself. Try disconnecting the red primary wire from the OT from the turret and check the resistance between the end of that wire and ground.

I disconnected the red wire from the OT and it still reads 1.7 ohms. I even removed the rectifier tube and it still read 1.7.

Ron

What is "it"? The B+ turret?

Yes the b+ turret.

Disconnect the red wire from the rectifier socket to the B+ turret, and read the resistance from the turret to ground again.

Yes, in fact that wire looks a little suspect solder-wise, though I can't tell very much from the picture.

I disconnected the red wire between the rectifier tube socket (pin and the B+ turret and the ohm reading went through the roof. If I touched the wire to the turret it went back down to 1.5 ohms.

Ron

OK, I think we're zeroing in on the problem now. Sounds like it must be a problem with the socket shorting to ground. With the rectifier wire still disconnected from the B+ turret, check the resistance between pin 8 of the rectifier tube and ground. If that's very low, then I'd suggest carefully inspecting the rectifier socket for a problem--maybe some stray solder or wire touching a grounded surface.