Burning Fuses

[Rick1114]

I have a 1957 GA-5 I recently aquired. Of course I been experimenting with tubes and such for it and last night it started sounding a little funny and then wham - it just died. I smelled the scent of burnt components, although no actual smoke or anything.

To make a long story short I replaced all the tubes and the the amp still blows the fuses a second or two after power up. I opened it up & It looks like the amp had a cap job at some point because I saw those shiny blue & white electrolytics in there(actually they were pretty old looking too, but not "vintage"), but noticed the big 'ol 20uf cap said "mallory" on it - that can't be good. Nothing looks "burnt up" though - I think the smell was from those old transformers heating up more than usual.

Can anyone tell what the next troubleshooting step is? I've got a decent multimeter & soldering iron to work with. If it's the tranny I'll bite the bullet and go to a tech, but if it's not, this amp is so so simple, I might try this one myself. But I'd like to determine that part first before making a decision!

Thanks,

Rick

Here are some pics:

[Don Moose]

Start with a good rundown on the personal safety issues of messing around inside tube amps. The voltages therein are genuinely lethal.

Next, the best source of general Tube Amp Debugging information is R.G.Keen's page.

Specific amps may have specific common complaints, and experienced people will jump in to the thread to help.

Lastly, popping fuses is a _symptom_, not a problem. It means something else is bad wrong. Just jamming in fuse with a higher rating is a 'hold ma beer an' watch this' move. (It seems you know that one already)

Proceed slowly, keep at least one hand in a pocket if the amp is open and plugged in, and take the time to build that 'light bulb limiter' (carefully).

Hope this helps!

[Twist]

On an amp that old you might as well replace every electrolytic and coupling cap in it. Unless there's a faulty tranny you'll save a lot of time and the results are worth it.

[Enzo]

Troubleshooting is troubleshooting. Forget that the amp is 50 years old, forget what the caps look like, etc. Just look for the problem.

Sure old caps are suspect, but let us not make assumptions.

Does it blow fuses with no tubes in it? If not, then tubes are involved. If so, it points towards power supply. If fuses hold without tubes, then install the rectifier tube ONLY and see. If it now blows fuses, then either the rectifier tube is bad or B+ is shorted - now filter caps are likely suspects. Not because they are blue or large, but because they are the first thing to think of after the rectifier.

it is also possible you have bent the leg of a tube socket so it touches something - look closely. Also check the pilot light assemby, make sure the terminal lugs there are not touching something or each other.

[Rick1114]

Troubleshooting is troubleshooting. Forget that the amp is 50 years old, forget what the caps look like, etc. Just look for the problem.

Sure old caps are suspect, but let us not make assumptions.

Does it blow fuses with no tubes in it? If not, then tubes are involved. If so, it points towards power supply. If fuses hold without tubes, then install the rectifier tube ONLY and see. If it now blows fuses, then either the rectifier tube is bad or B+ is shorted - now filter caps are likely suspects. Not because they are blue or large, but because they are the first thing to think of after the rectifier.

it is also possible you have bent the leg of a tube socket so it touches something - look closely. Also check the pilot light assemby, make sure the terminal lugs there are not touching something or each other.

Thanks EZNO

That's some good info. I will test that once I buy more fuses. Also everyhing is LOOSE inside this amp pin sockets etc... so I will be checking that closely as well. Still hoping I didn't hurt the transformers tho!

Rick

[tubeswell]

+1 for Dons advice about safety.

Make sure that the amp is switched off and unplugged from the wall and that the filter caps are fully discharged before you go putting your fingers inside.

[Rick1114]

I checked today & the amp is still blowing fuses with no tubes installed - boom - right away. What is the next step - cheking the power transformer? I know how to drain fliter caps (I use a wire with alligator clips on pin 1 of the preamp tube & chassis and test the caps witha multimeter for good measure.)

What's the eaisiest way to test the tranny with a multimeter?

Thanks,

Rick

[Enzo]

RG describes a transformer tester on his Geofex site.

But you can disconnect the wires fropm the transformer to the rectifier socket. Tape the ends and position them so they touch nothing. Also disconnect the heater wires from the transformer and likewuse tape and position them to touch nothing. NOW plug the amp in and flip it on - so you have nothing but the transformer itself piowered up now. If it still blows fuses, it is bad.

[Rick1114]

RG describes a transformer tester on his Geofex site.

But you can disconnect the wires fropm the transformer to the rectifier socket. Tape the ends and position them so they touch nothing. Also disconnect the heater wires from the transformer and likewuse tape and position them to touch nothing. NOW plug the amp in and flip it on - so you have nothing but the transformer itself piowered up now. If it still blows fuses, it is bad.

Thanks Ezno - that's the info I was looking for. I'll try disconnecting the two red wires to the rectifier circut. How to I identify the correct heater wires?

Thanks for all your help.

Rick

[jrfrond]

.....I think the smell was from those old transformers heating up more than usual.....

In general, not really a good sign for the transformers.

[Enzo]

That's true.


The heaters run on 6VAC, there will be a couple wires trailing socket to socket feeding the heater pins. There will be a pair of wires - hopefully green - coming from the transformer that connects to this pair. That is them.

[Rick1114]

Enzo -

Tonight I unsoldered all the wires from the transformer to the rectifier socket. There were 2 red wires, 3 yellow wires and 1 red/yellow wire (all vintage cloth probably original) all going to the rectifier pins.

The only wires I left connected from the transformer were 2 green going to the light bulb, and 2 black going to the switch.

The fuse blew immediately as usual. Does this confirm mean the PT is fried?

You can see the yellow & red wires I'm describing in the pic on the left side coming from the hole in the chassis...



Thanks again,

Rick

[metalangst]

That does point to a bad power transformer...but, it doesnt prove anything. Prove it. Disconnect the two green heater wires (I like to cut the wire leaving a little insulation so I can see where they went). Even better, disconnect the two black wires, and run A/C straight into the PT. This way you will know if it's the transformer, and not the crusty power cord, the floppy power switch, or anything else connected to the primary or secondary. Of course before connecting A/C to that hunk of iron, you'll want to build some troubleshooting tools. A suicide wire (an A/C plug with aligator clips) plugged into a current limiter(lightbulb in series with an A/C socket) is what I use. At least a fused A/C cord, so you have some indication of excess current. It only takes a second to flip a power switch on then off to "see" if it's bad.

[Don Moose]

Thanks Ezno - that's the info I was looking for. I'll try disconnecting the two red wires to the rectifier circut. How to I identify the correct heater wires?

Thanks for all your help.

Rick

The heater wires are usually (but not always) green. The secondary wires come in pairs or in threes (center taps), so 2 reds + a red with a stripe, or 2 greens and a green with a stripe. I'd suggest looking for those striped wires and taping them off as well. If they're not there, they're not there. When you go to reconnect, the unstriped wires of a color are interchangeable, but the striped one must go back where you got it.

Was there a tube rectifier in this amp? The 5VAC filament winding would be another pair/triplet to look for and tape off.

Hope this helps!

[Don Moose]

A suicide wire (an A/C plug with aligator clips) plugged into a current limiter(lightbulb in series with an A/C socket) is what I use. At least a fused A/C cord, so you have some indication of excess current. It only takes a second to flip a power switch on then off to "see" if it's bad.

Hint - there's a reason it's called a 'suicide wire'. Jacket those clips, keep hands in pockets.