melted heater wires

[thermion]

got an amp (blackfaced fender) in the shop. owner said the amp was hummier than normal but worked fine, then smoked and faded out. he promptly switched the amp off.
open it up: pilot light wasn't hooked up (before the incident), heater CT was grounded/ no artificial ground resistors, and the wiring on the heater secondaries is what burned. looking down the heater lines the insulation on the heater lines is melting together between the wires all the way back to V1 in the preamp. put it on the current limiter "as is" and the bulb glows bright. i disconnected the heater lines from the heater secondary tap and the unloaded transformer doesn't make the lamp glow.
my guess is somehow the heater lines got shorted out, causing massive current draw (and probably the extra apparent hum) until the wires got too hot, melted together, and burned some insulation off.
thoughts here?

[Willabe]

looking down the heater lines the insulation on the heater lines is melting together between the wires all the way back to V1 in the preamp.

What tubes were in it? Did he put EL34's or 6550's in place of 6L6's? 6L6's in place of 6V6's?


                         Brad        

[sluckey]

looking down the heater lines the insulation on the heater lines is melting together between the wires all the way back to V1 in the preamp.

There is/was a dead short near or on the V1 socket. Or a tube was plugged into V1 socket that had a short across the filament pins. Either way, you gotta replace the entire heater string. While the string is removed, pull all the tubes and measure resistance between the filament pins on each socket. Should be infinity. This will check to see if the short is gone.

I melted the filament string on the very first amp I ever built!

[jjasilli]

Ditto to sluckey's analysis.  Though I would 1st check UNloaded filament voltage (actually maybe all unloaded voltages), to make sure the PT, especially the filament secondary, is still good. 

[thermion]

i certainly will check unloaded voltages just to be sure the pt isn't toast. i'm really looking forward to rewiring that whole heater string!
i'll put V1 (sov 12ax7LPS) in the tester to check for shorts.
and replace the socket, just to be sure, and submit it for forensic failure analysis.
the output tubes were sov 6l6WXT+; I don't think he had anything else in there.
you guys are awesome...thanks for your insight!

[stingray_65]

looking down the heater lines the insulation on the heater lines is melting together between the wires all the way back to V1 in the preamp.

There is/was a dead short near or on the V1 socket. Or a tube was plugged into V1 socket that had a short across the filament pins. Either way, you gotta replace the entire heater string. While the string is removed, pull all the tubes and measure resistance between the filament pins on each socket. Should be infinity. This will check to see if the short is gone.

I melted the filament string on the very first amp I ever built!

wouldn't you read the filiment windings in the PT? or if it had 100 om resistors for an artificial CT?

OH! and pull the lamp bulb too

Ray

[sluckey]

wouldn't you read the filiment windings in the PT? or if it had 100 om resistors for an artificial CT?

OH! and pull the lamp bulb too

Well maybe on the first big tube socket, if the PT wires are still connected. But I said "while the string is removed...", meaning there ain't no filament wires going from tube to tube to tube, etc. IOW, the tube sockets' filament pins are naked, except maybe the first big bottle. He also said there are no artificial grounding resistors, and the pilot light was not connected.

[darryl]

If the heater secondary has a centre tap to ground, then the short may not be across the entire 6.3 volt secondary - it could be a short from one side of the secondary to ground. If the wiring string is melted all the way to V1, then the problem is very likely to be at the V1 socket.

[thermion]

update- tested V1 on my mercury tester; the short indicator would not light no matter how much i poked and prodded the tube while testing. all i had time to do last nite...i can do more probing with the meter tonite and report back.
thanks again.

[HotBluePlates]

Then measure at the socket with the tube removed.

[HotBluePlates]

Somebody do the calcs, wouldn't the fuse blow before this happens. (probably not)  (6.3 volts 15 to 20 amps to cook them wires)

No calculation required...

When I was first starting out, I made a dumb wiring mistake trying to convert an old Fender amp with 6L6's to allow EL34's. Almost instantly after switch-on, the cloth on the heater wires burned right off!

But since you asked for calculation:
If VA in = VA out, then the current in the heater winding should step-up by the same ratio the voltage steps down.
Secondary Volts / Primary Volts = 6.3v/120v = 1/19.05 = Ratio
Secondary Current * Ratio = 20A * 1/19.05 = 20A/19.05 = 1.05A Primary Current

That probably won't even stress the fuse, especially the slo-blo type normally used.