Something blown after changing power tube

[Umustbekidn]

Hi folks,

The crack has been used daily since we got it running a few days ago, and I'd swapped out numerous 12AU7 tubes - so I know the current tube had been working (all with the same power tube). Today I received a tung sol 5998 from ebay and proceeded to swap power tubes...

When I powered up with this new(old) 5998 tube, I saw a much brighter than normal glow from the 12AU7 tube, then saw what seemed to be a flash from the 5998 power tube. I also heard a faint pop from somewhere in the amp.

Thinking the 12AU7 had blown, I replaced it with a known good tube. It did not glow normally at power on, and the new 5998 was glowing very brightly - and not producing sound.

I unplugged everything and ran resistance checks, results below. A few things have changed. I also smell something faintly like burned plastic. No visible damage I can detect.

It is unclear if the new tube was bad and blew the circuit, but it was the only thing I'd changed prior to this event. Can someone help determine what went bad, so I can replace things? Can a bad tube do this, or could this just be bad luck? Wondering if I should return to the seller as damaged.

Terminal = Reading
1 = 11k and rising (was No Reading at assembly)
2 = Less than 0?! (was No Reading at assembly)
3 = 0 ohms
4 = Less than 0?! (was No Reading at assembly)
5 = 20k and rising (was No Reading at assembly)

6 = 0 ohms
7 = 3k ohms
8 = 0 ohms
9 = 3.4k ohms (was 2.9k at assembly)
10 = 0 ohms

12 = 0 ohms
13 = 2k ohms and increasing
14 = 0 ohms

20 = 0 ohms
22 = 0 ohms

B3 = 3.4k ohms (was 2.9k at assembly)
B6 = 3.4k ohms (was 2.9k at assembly)

RCA centers = 100k+ ohms
RCA grounds = 0 ohms

Thank you kindly for your help.

 

[Paul Birkeland]

Put the 6080 back in and a known good 12AU7, then power up the Crack and check your voltages.  It might also help to post some photos of the underside, as we may be able to see something amiss.

If the 12AU7 isn't glowing and the 6080 is, then the wires leaving B7/B8 and going up to A4/A5/A9 aren't properly connected.  Doing a lot of tube rolling will aggravate cold solder joints and make these issues come to the surface pretty quickly.

-PB

 

[Umustbekidn]

Alrighty here are voltages and some pictures. I installed the original tubes for this measurement. Both tubes are glowing, but the 6080 is a lot brighter than usual. The voltages look almost identical to the as-assembled voltage test... nothing odd there.

Terminal = VDC
1 = 90v
2 = 205v
3 = 0v
4 = 205v
5 = 85v

6 = 0v
7 = 120v
8 = 0v
9 = 120v
10 = 0v

Both LEDs are on, but seem to be glowing more brightly than seen at assembly.
Thank you for your help!

 

[Tom-s]

Based on the pictures i'd touch up the joints going from the terminal strip near the 8 hole socket to the headphone jack. Be sure that pin 9 on the 9-pin socket looks as the others (can't see it).
The socket saver could be another potential weak link.

 

[Paul Birkeland]

What's the black plastic thing in your 9 pin socket?

Based on the voltages you have, the 9 pin tube must be glowing.

Your connections at B7 and B8 look not really be soldered at all.

 

[Umustbekidn]

Paul, the black plastic thing on the 9 pin socket is a "socket saver". It's got gold plated connectors and is easier to roll tubes with (they tend to insert/remove more easily). The voltage measurements below were taken with that socket saver in place.

You are also correct, the 9 pin tube is glowing. Picture below.

I re-inspected B7 and B8, and all the socket connections. They all seem fine; I can see solder on them all, and none of the wires are loose. But I found other problems...

After spending some time pondering my work, I have discovered a couple more mistakes:

6L was unsoldered
Red wire at 13U was unsoldered

So I resoldered those joints, and resoldered several other questionable joints: 10L, 12L, 13L, and 15L.

Voltage and resistance checks below. After powering on, I note both tubes glowing, pictures below.

Resistance
Terminal = Reading
1 = 30k and rising
2 = 4k and rising
3 = 0 ohms
4 = 5k and rising
5 = 50k

6 = 0 ohms
7 = 3k ohms
8 = 0 ohms
9 = 3k ohms
10 = 0 ohms

12 = 0 ohms
13 = 5k ohms and rising
14 = 0 ohms

20 = 0 ohms
22 = 0 ohms

B3 = 3k ohms
B6 = 3k ohms

RCA centers = 100k - 200k ohms

-------
Voltages

Terminal = VDC
1 = 80v
2 = 190v
3 = 0v
4 = 190v
5 = 85v

6 = 0v
7 = 115v
8 = 0v
9 = 115v
10 = 0v

Thank you once again for your help! Really appreciate it.

 

[Umustbekidn]

One other thing has been on my mind... My initial measurement of my home voltage led me to wire the amp for voltage under 115volts. I am using a cheap analog VOM. Was tempted to buy a nice fluke digital meter, but I just don't use these things often enough to justify buying a nice one.

I am wondering if I should re-do the amp for the next higher voltage (above 115v)?

I can't say how accurate this cheap VOM is, though I do try to zero it properly before I start measuring. Just wondering if you've ever had anyone switch the input wiring, just in case my measurements are off by a few volts....

 

[Paul Birkeland]

Analog meters are extremely inaccurate for that kind of reading.  The cheapest digital meter at Harbor Freight (it's often on sale for $2) is very accurate and worth buying to verify your line voltage.