Blown Tube [solved]

[squirreldude]

Hey, Paul, can I have you clarify what is considered "stock configuration" as far as repairs go? Obviously some of the capacitor pr0n elsewhere in the forum would be exempt, but can I leave the speedball upgrade installed? I have one resistor attached to each RCA jack as well. Would these need to come out?

I hope to redo my voltage and resistances tomorrow after removing those added black wires on the terminal strips. I can't imagine that's the culprit, but it's worth a shot.

 

[Paul Birkeland]

Hey, Paul, can I have you clarify what is considered "stock configuration" as far as repairs go?

Yeah, cap porn gets pulled out.  Chokes get pulled out, etc.  The Speedball is still considered "Stock".

I hope to redo my voltage and resistances tomorrow after removing those added black wires on the terminal strips. I can't imagine that's the culprit, but it's worth a shot.

You're creating a shorted winding through part of the stack.  This is actually kind of a horrible thing to do to a power transformer.  (Like running your car on jet fuel)

 

[squirreldude]

I hope to redo my voltage and resistances tomorrow after removing those added black wires on the terminal strips. I can't imagine that's the culprit, but it's worth a shot.

You're creating a shorted winding through part of the stack.  This is actually kind of a horrible thing to do to a power transformer.  (Like running your car on jet fuel)

Yikes! Well if it's analogous to running your car on jet fuel, it could explain why I keep getting sparks in the power tube. I pulled out the two wires connecting the ends of the terminal strips and did resistance/voltage checks. Everything looks the same as before; still up to spec. The only difference is that terminals 11, 17, and 22 are no longer part of the ground path. They do not give a resistance reading. Thus far, I haven't heard the dreadful noises either  ;D. I'll give it a week or two of listening and see if it shows up sometime.

I've been thinking about why the "shorted winding" problem might not have appeared in the voltage checks I did before. Perhaps you can correct my understanding of tube amps and power transformers. My guess is that the voltage going into the tube was higher than it should have been, but my 6AS7G was working overtime to properly control the outgoing voltages. In contrast, the unusually high voltage was too much for the 6080s and they failed. I wonder why the voltages weren't too high upstream of the tube though. I'd be interested to hear some reasoning from the experts.

Finally, do you think these 6080 tubes are toast? If they are as "incredibly rugged" as you say, is there a chance they could still function?

Voltages
1      78.2
2      180.8
3      0
4      179.2
5      78.9
6      0
7      110.3
8      0
9      107.7
10    0
11    0
12    0
13    180.8
14    0
15    198.8
20    0
21    218
   
A1    78.4
A2    0
A3    1.5
A4    0
A5    0
A6    77.1
A7    0
A8    1.5
A9    0
   
B1    77.0
B2    178.5
B3    109.7
B4    78.3
B5    178.2
B6    107.4
B7    0
B8    0

 

[Paul Birkeland]

It looks like you now have a working amplifier. 

If you trace the paths made by your black wires, screws, and transformer cover, you'll discover that they make loops through the transformer stack.

 

[Paul Birkeland]

Just an update to this thread:

This Crack came in for repair, and we found that a little too much solder led to several cold solder joints, whose intermittent connections were causing this issue.

 

[Doc B]

To further this, often the solution to cold joints is not to add more solder but to pay close attention when you reheat what is already there. Make sure that the solder flows over both parts being soldered together and has a smooth and shiny finish with fillets (i.e. the solder draws into the space between the parts) instead of lumps. If you see a thin line or little blobs of rosin around the edge of the solder joint it might also be in between the wire and the solder, creating a cold joint. A little more heat will help it to reliquify and get the solder in direct contact with the metal.