6080 sparking

[rehhy]

I simultaneously built two Cracks. The one works perfectly, no issues at all; The other has an issue with arcing inside the 6080 tube. Both heaters turn on just fine - after heating up for a few seconds sparks can be seen inside the tube.

 I thought it could be the 6080 that was included with the kit, so I ordered 2 more tubes from Brent Jesse. After plugging one of the new tubes in, the same sparking happened. I decided to try this same tube in the other (working) Crack, and it also sparked there - I am wondering if it is possible that the "broken" amp is damaging the tube permanently when it arcs.

 I have checked multiple times the to make sure there are no shorts, as well as comparing the entire circuit to the other working amplifier. I cannot do the voltage check as I turn it off after I see the arcing to make sure that I don't damage any other components.

 I only have one 6080 left which I have tested and is working perfectly on the functioning amplifier, but I don't want to damage this one and have to order more.

 My two suspicions: There is either a bad cap, or that it possibly has something to do with the 12au7.

 Any ideas?

[Paul Birkeland]

How were your resistance checks?  Can you post photos of the build?

[rehhy]

Sure, is an imgur album okay for photos?
(not sure if you'll be able to see the link because I am still a newbie)

imgur.com/a/TB7Ks


 Resistance seems fine. Everything matched the other one when I did the check - I just don't want to leave the amp on to do the voltage check so I don't send anything else into orbit.

 I've since lost the link to my copy of the manual -- I stupidly downloaded it on my phone (temporarily) without realizing I only had one chance to download it, but I followed the whole procedure with the first amp.

[Paul Birkeland]

One thing we can do for starters is to have you pull the 6080 and run the Crack.  The voltages at terminals 1 and 5 should drop down to somewhere around 100V, terminals 2 and 4 should be about 200V, and the LEDs on the 9 pin socket should light up. 

Let me know how that test goes.

-PB

[rehhy]

Alright the test is done:

LEDs are lit.
1 and 5: 130.6vdc
2 and 4: 130.1vdc

For curiosity's sake, I tried a Tung-Sol 12au7W that I had, and both voltages read around 120vdc.

Another thing to note is that during this test the transformer was vibrating quite heavily.

[Paul Birkeland]

Can I see a photo or two of how the power transformer is mounted from the top side of the chassis?  It does look like there's a bit of extra length on the transformer screws.

6080s aren't the issue, they are reacting to a different problem.  I have seen this only once before.

[rehhy]

Sure thing:

https://imgur.com/a/5qpB2
imgur.com/a/5qpB2

I have it mounted as such:

screw -> washer -> bellend -> lock washer -> transformer -> plastic sleeve (firmly located in the hole of the xformer) -> chassis -> black plastic sleeve (located in hole) -> solder tab mount -> self locking nut.

[Paul Birkeland]

So the only other time I saw this, a builder had added wires connecting each of the terminal strip lugs on the 6 lug strips that has screws passing through.  This created some shorted turns through some of the lamination stack and caused said Crack to go haywire.

You don't have these wires or a power transformer mounting error that would mimic that thankfully, but you have the same symptoms and your PT is humming/vibrating with almost no load.  You are also making very, very little voltage with no load on the transformer, and somehow at the same time we aren't seeing the 12AU7 draw any current down across the 22K plate load resistors.

Can you power up the amplifier with no tubes and listen to the power transformer to see if the noise you observe changes?  Can you check the AC windings again as shown on page 29?

Also, what is your incoming AC line voltage (and what country are you in)?

-PB

[rehhy]

I am in the US and my AC line voltage is 119v.

With no tubes plugged in, there is no hum or vibration on the transformer.

 Just to be sure - the "spiked" lock washer must go between the bell end and the lamination stack, correct? I want to make sure that the potting on the lamination stack has not been damaged by something as silly as over tightening. Also, if the black washer on the bottom side of the chassis is slightly out of alignment, would the threads of one of the screws touching the chassis ground cause this issue?


 Could this be an issue of a failed diode in the full wave rectifier? It is my understanding that if a diode were to fail, it would act as a half wave rectifier and cause the voltage to drop by 1/2. My meter has a diode test - I could desolder them and test to see if that could be the issue.

 Would it also be possible to get a 2nd download link of the manual? I downloaded it in a private browser session on my phone and have lost access.

[Paul Birkeland]

With no tubes plugged in, there is no hum or vibration on the transformer.

Good to know.

Just to be sure - the "spiked" lock washer must go between the bell end and the lamination stack, correct? I want to make sure that the potting on the lamination stack has not been damaged by something as silly as over tightening. Also, if the black washer on the bottom side of the chassis is slightly out of alignment, would the threads of one of the screws touching the chassis ground cause this issue?

Yes, that's where that washer goes.  Generally the screws can be about as tight as you want.  The screws themselves will break before the transformer will be bothered.  If the shoulder washer is slightly out of alignment, it isn't the end of the world, especially if it's just one of the washer, but usually you can loosen that transformer screw and nudge the shoulder washer back into the hole.

Could this be an issue of a failed diode in the full wave rectifier? It is my understanding that if a diode were to fail, it would act as a half wave rectifier and cause the voltage to drop by 1/2. My meter has a diode test - I could desolder them and test to see if that could be the issue.

We don't generally encourage this.  I have never, ever seen a UF4007 diode fail in any Bottlehead kit, and they have been in use since the start!  Removing and testing parts can just cause other issues to arise from the process of installing and uninstalling parts.

Would it also be possible to get a 2nd download link of the manual? I downloaded it in a private browser session on my phone and have lost access.

Our forum rules suggest having your first name and last initial in your signature.  If this was there, Josh could send you a link when he runs into this post.  Since it isn't, you'll need to contact [email protected] and provide your name, order date, and order number, then we can provide another download link for you.

-PB

[rehhy]

Our forum rules suggest having your first name and last initial in your signature.  If this was there, Josh could send you a link when he runs into this post.  Since it isn't, you'll need to contact [email protected] and provide your name, order date, and order number, then we can provide another download link for you.

 I've gone ahead and added that.


 I tested the rectifier in the circuit, it's functioning as it should. Other than this I am out of ideas. I'll go around once more and make sure there are no dry solder joints anywhere.

[Paul Birkeland]

You'll want to test the AC voltages on the power transformer.

[rehhy]

Done:

The heater winding comes in at 6.5vac
The high voltage secondary comes in at 177vac

I also double checked all of the resistances, everything is all good there.

[Paul Birkeland]

Is that test with or without tubes installed? 

Pull either the red or black wire from the 160V winding (that measures 177V) and recheck for transformer noise with tubes plugged in.

[rehhy]

 Yes, without tubes plugged in the secondary was measuring 177vac.

 With the red wire desoldered, and both tubes plugged in, there is no transformer hum or vibration.