jeffg
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Posts: 10
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Post by jeffg on Jan 27, 2020 8:27:48 GMT -7
The facts: this is a COMBO config'ed Z Wreck. Running Russian 6P14P power tubes, and the Sovtek GZ34 rectifier.
What happened: bad noise, plus smell. Opened it up and checked around, found the two 1.5k grid stopper resistors on the 'brown' pair of EL84s were both burnt, and both of those tubes now test as failed with an Orange tube tester.
I pulled all the power tubes out and powered it up, there is NO DC current leaking through that side of the PI's 47nF coupling cap. Replaced the two burnt 1.5k resistors and loaded a new quad of Russians and... you can guess what's coming.
Same thing happened. Exactly. Two burnt 1.5k grid stoppers, two failed EL84s on the 'brown' side, again!
I am puzzled, to say the least. In order for the 1.5k grid stoppers to burn, there has to be voltage across them. Either leaking from the PI plate through the 47nF coupling cap, or due to an internal tube inter-electrode short of some kind.
Regarding the first case, leakage from the PI, power on testing with the power tubes pulled shows NO leakage from the PI coupling cap.
Regarding the second case, tube failure, why would two separate pairs of 6P14Ps fail on the SAME side? And if there was an interelectrode short placing high voltage on the control grid, you've got the 220k grid resistors completing that path to ground. Almost all the voltage from an internal tube electrode short would be felt over the 220k grid resistors, and very little over the 1.5k stoppers...
I should add that the 220k resistors have been checked and are correct. If anybody has any notions of what may be going on here, I'd love to hear them!
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Post by purpletele on Jan 27, 2020 9:30:15 GMT -7
The facts: this is a COMBO config'ed Z Wreck. Running Russian 6P14P power tubes, and the Sovtek GZ34 rectifier. What happened: bad noise, plus smell. Opened it up and checked around, found the two 1.5k grid stopper resistors on the 'brown' pair of EL84s were both burnt, and both of those tubes now test as failed with an Orange tube tester. I pulled all the power tubes out and powered it up, there is NO DC current leaking through that side of the PI's 47nF coupling cap. Replaced the two burnt 1.5k resistors and loaded a new quad of Russians and... you can guess what's coming. Same thing happened. Exactly. Two burnt 1.5k grid stoppers, two failed EL84s on the 'brown' side, again! I am puzzled, to say the least. In order for the 1.5k grid stoppers to burn, there has to be voltage across them. Either leaking from the PI plate through the 47nF coupling cap, or due to an internal tube inter-electrode short of some kind. Regarding the first case, leakage from the PI, power on testing with the power tubes pulled shows NO leakage from the PI coupling cap. Regarding the second case, tube failure, why would two separate pairs of 6P14Ps fail on the SAME side? And if there was an interelectrode short placing high voltage on the control grid, you've got the 220k grid resistors completing that path to ground. Almost all the voltage from an internal tube electrode short would be felt over the 220k grid resistors, and very little over the 1.5k stoppers... I should add that the 220k resistors have been checked and are correct. If anybody has any notions of what may be going on here, I'd love to hear them! Jeffg, you should copy your note and send it to Don in repairs. Your issues are very unusual and most likely beyond the forum responses. Let us know the results, it is interesting.
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jeffg
New Member
Posts: 10
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Post by jeffg on Jan 27, 2020 9:48:12 GMT -7
Thanks, I will.
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Post by DRZ on Jan 27, 2020 10:06:46 GMT -7
Sure does sound odd. Check to make sure you have continuity from the 220K's on the board to pin 2 on all 4 tubes. Check for 50 Ohm bias from ground to pin3 on all tubes. Then check OT , Red Center tap to each plate connection pin 7.
Can I ask what is the serial number? And were did you buy it ?
Z
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Post by steiner on Jan 27, 2020 10:10:21 GMT -7
Your understanding is far better than mine. At this point, I'd be loading the car. When I got to the amp epicenter, all I could tell them is I let the smoke out...
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jeffg
New Member
Posts: 10
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Post by jeffg on Jan 27, 2020 13:45:38 GMT -7
This is serial number Y31444. a combo, with a date of 11-25-14 on masking tape on the chassis side. I've got solid continuity from all the power tube cathode pins to the 50 ohm resistor, indicates 50 ohms to ground. All the feed sides of the 1.5k stoppers indicate 220k to ground. Of course until I fix the two fried 1.5k stoppers, I can't show that from the actual grid pins. All the screen grid resistors indicate nominal 110 ohms, and the DC resistance of the OT primary sides are Brown: 45.7 ohms, and Blue: 38.3 ohms. Again, the 'brown' side is the problematic side.
I should add, this is not my amp. I am working on it for a friend. I am a retired EE who has worked on tube guitar amps since 1981 as a sideline.
Thanks, Jeff
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Post by DRZ on Jan 27, 2020 14:02:45 GMT -7
Not much left to check. I guess it isn't impossible to have two bad sets of tubes causing the same problem, not very probable but possible. What happened to cause this amp to get into your hands ? How was the failure explained to you ?
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jeffg
New Member
Posts: 10
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Post by jeffg on Jan 27, 2020 14:13:26 GMT -7
The failure was explained in the usual manner: noise, smell, fuse, no workee. Not much to be gleaned from there. I'm going to encourage the owner to send the chassis in to you all for repair -- I really don't see what the deal is here, and the fact that it ate two pairs of power tubes and two pairs of stopper resistors on the same side within a few weeks of each other makes coincidence seem not too likely.
Edit: thought I'd mention, I have the chassis pulled on the bench now, so if photos of anything would be of use, let me know!
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Post by Chilly Gibbons (Todd T.) on Jan 27, 2020 14:20:51 GMT -7
Jeff, what do you mean by brown side? Just curious. Thanks
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jeffg
New Member
Posts: 10
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Post by jeffg on Jan 27, 2020 14:28:42 GMT -7
Chilly, you've heard of the brown sound, right? No, just kidding. The output transformer has three primary leads, it's a fairly standard color code assignment: one end of the primary is terminated using a blue wire, the other end of the primary is terminated using a brown wire, and the center tap goes to a red wire. Hence, the 'brown side' refers to the two power tubes that are tied to the brown wire end of the OT primary. 'Blue side' would be the other two.
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Post by Chilly Gibbons (Todd T.) on Jan 27, 2020 14:57:22 GMT -7
I thought that’s what you meant but wanted to make sure. I’m unfamiliar with the Wreck and also wondered if there was possibly some sort of voltage attenuator, or brown side in the power section. Thanks!
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Post by premiumplus (Dave) on Jan 28, 2020 20:27:13 GMT -7
That's a head scratcher for sure. Either you've had really bad luck with 2 pair of tubes or I would recommend that you never, ever play the lottery again. This amp wins for the most bizarre set of symptoms I've ever heard.
Please do let us know what the final diagnosis and solution are. Weird.
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Post by purpletele on Jan 28, 2020 23:11:23 GMT -7
That's a head scratcher for sure. Either you've had really bad luck with 2 pair of tubes or I would recommend that you never, ever play the lottery again. This amp wins for the most bizarre set of symptoms I've ever heard. Please do let us know what the final diagnosis and solution are. Weird. My money says he has an OT problem.
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Post by Chilly Gibbons (Todd T.) on Jan 29, 2020 3:03:22 GMT -7
Please let us know of your findings in this one. Thanks.
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