Possible causes for TubelabSE failure - diyAudio
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Old 26th May 2010, 06:04 PM   #1
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Default Possible causes for TubelabSE failure

Hello all,

About a month ago I completed my TubelabSE build and it has brought me great listening pleasure. I constructed it as per build instructions with no modifications whatsoever. I wanted to build it this way to establish a baseline for future modifications.

Yesterday I left the room to make something to eat and when I returned (not sure how much time elapsed) I smelled that awful smell....the electronic death smell. I immediately raced over to the amp and noticed the left 45 with a nuclear orange glow on the plate and heat damage around R5. As fast as I could I cut the power and waited for the unit to cool down.

I think I have narrowed down the cause of the failure, but I wanted to cover all my bases and have someone double check my thinking so this doesn't happen again.

What I think happened was simply thermal runaway. I have had meters monitoring the bias of both 45's ever since it was constructed and noticed one of the tubes was always 1 to 2 ma above or below the other and would always have to adjust the bias point to match. I chalked it up to the tubes being over 70 years old and to be honest I wasn't quite sure how stable this should be since the power supply isn't regulated and the mains power fluctuates between 117 and 122.

I checked for a shorted bias pot, but I didn't think this would cause such thing unless it shorted wide open allowing no bias voltage at all, but the pot tested fine. I then checked the mosfet for shorts as this is all i can do as I don't have a transistor checker. The mosfet tested fine. All other resistors, capacitors, and semiconductors tested fine.

I think the tube went into thermal runaway (yes, they were run above spec at 320V and between 26 and 28mA although George hasn't had a problem with any tubes at this operating point) and the massive current drawn by the tube overwhelmed R5 causing it to heat up dramatically. Amazingly it did not fail and still tests ok (of course it will be replaced though) and did some damage to the board since it was mounted on the bottom of the board.

Let me know if my logic here sounds correct or if you have any better ideas I'm game. Thanks!
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Old 26th May 2010, 11:11 PM   #2
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I don't know the circuit so I can't make an intelligent assessment. However, if it's resistance coupled, I would replace the grid coupling capacitor. What happens there is a bit of leakage as it warms up tends to raise the grid voltage a bit. That makes the tube draw more and warms up the area, which increases the leakage in the capacitor, and there goes runaway.

This also may be why you have seen variations in bias, maybe temperature related.
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Old 26th May 2010, 11:26 PM   #3
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Binaural,

Did you mount all of you components on the bottom of the board? If so make sure that your FETS have plenty of heat sink. Replace them anyways even if the measure fine. I had mine do a similar thing as a result of the heat sink being loose. FET quit working and bias went through the roof.

The amp uses a FET follower which results in a very low grid impedance (assuming it is not fried) so runaway would not be my first focus. Most likely the FET failed or bias was lost do to a bad connection or other component. bob91343's comments are also another possibility regarding the cap if it is leaking. They are close to the FET heat sinks and could get quite toasty. Also check very carefully your solder connections.

FYI, I have abandoned my bottom mounted Tubelab SE board, not because it didn;t sound good or its design, but the thermal issues in having the components flipped made it to unreliable. George does not recomend doing this.
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Old 27th May 2010, 01:18 AM   #4
rknize is offline rknize  United States
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+1 to the FETs. Also, FETs can go bad without "blowing" like BJTs and diodes often do. I wouldn't trust them. Bias drift may be indicative of the FETs getting really hot. However I wouldn't rule out the tube either. I have a couple of persnickety 45s whose filaments kink slightly when operating (not enough tension from the spring hook?) and come very close to touching the grid wires.

Another consideration are those trimmers. They are cermet pots, which really aren't meant to endure a lot of tweaking...usually in the 100s of operations. I've replaced mine with carbon trimmers for this reason.
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Old 27th May 2010, 01:24 AM   #5
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SGregory,

My second choice would've been the mosfet. Other than the tubes, they are the only thing mounted on the top of the board, so in this case heat wouldn't have been an issue. The coupling caps are mounted on the bottom mainly because they are so big (Audio Note .47uf copper in oil caps.) I haven't checked the tube yet to see if it's ok. It has lost some of it's slivering but I assume that's because of all the emission inside the tube from the glowing plate. If it hasn't killed it I know it has at least knocked some life out of it, so I'll probably have to go shopping for a new tube as well.

Like I said, I built it with no mods for a baseline and was planning on upgrading R5 with a choke, so I guess now would be the time to do it! I was also planning on replacing the B+ power supply caps with motor runs, so they will no longer be on the board. I think the final amp will have the tubes on some kind of extensions to mount them on the top plate of the chassis so this should allow everything to be mounted on top of the board. I'll probably have to mount the 5842 grid stoppers off of the board since the extra space between them might cause oscillations since this tube will oscillate if given the chance....

Thanks for the suggestions. I was planning on replacing the fets just in case they were the culprit. They're cheap and will hopefully save me much head scratching and pain in the process.

Joe
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Old 30th May 2010, 05:47 PM   #6
w5jag is offline w5jag  United States
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Just take the PA tubes out and measure the voltages at the socket to see what is going on.

When I lost a FET, my bias voltage went so negative the tube was completely cut off.

Sounds like you might have had a runaway, particularly in light of exceeding the published spec. That's always great fun, but sometimes not without consequences.

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Old 3rd June 2010, 06:36 AM   #7
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I am having a similar problem. After months of operation today I heard a bop, and bad smell. What is happening is that, if I measure the voltage on the grid of the 300B, it starts to go down, and then suddenly it gets all the B+. I hope I was fast enough to switch it off and the tubes are still safe.

I though the problem was only on the left channel, so I replaced only the mosfet on the left. Nothing.

So I run out of FET, and I replaced the Toshiba K3563 with two IRF840. I am not sure if they can work there, but I had the same behavior.

After I removed the coupling caps, but still the same.

Any hint ? I am out of ideas.

I think the first failure was caused by a lose screw on the heatsink.

D.

P.S.: Any chance that the tubes are still OK ?
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Old 3rd June 2010, 10:26 AM   #8
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Now I am really puzzled. The right channel is OK.

On the left I do not manage to put the bias below -15 V. I replaced all the resistors, the trimmer, the coupling cap and the mosfet.

I am doing the measurements without the rectifier. I really do not know what to think any more. Please help.

D.
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Old 3rd June 2010, 01:44 PM   #9
rknize is offline rknize  United States
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Measure the DC voltage at the gate of each FET and compare that to the source.
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Old 5th June 2010, 03:22 AM   #10
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On the right channel I have D: 418 V S: - 70 V G: -70 V Voltages are not precise, as I wanted to be quick.

On the left D:418 V and S & G: 357 V.

On the left channel the coupling cap is not connected, but this should not make any difference.

I am really puzzled, I changed the FET many times, It's a bit difficult, as I am running high in voltage, and without tubes I can go well over the max voltage of the FET.



Thanks,

D.
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