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Old 23rd October 2008, 02:48 PM   #11
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How is it? Mine is holding up so far but noticed higher noise level (hiss & buzz). Itís not confirmed whether itís the new diode or something else so Iím wondering.
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Old 23rd October 2008, 04:12 PM   #12
wicked1 is offline wicked1  United States
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Mine is still dead silent. (as far as my ears can tell)

I actually think it sounds better than ever, but that's just because I've been listening to an inferior amp for the past week.

(tho I did re-bias, and am using a different brand rectifier, but I doubt I can tell a real difference from that)
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Old 23rd October 2008, 04:17 PM   #13
kevinkr is offline kevinkr  United States
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One thing that seems to help the current crop of 5AR4 is to put a high voltage silicon diode in series with each plate. I generally use something like the UF4007. This prevents arcing during warm up and under most transient conditions, it effectively increases the PIV rating of the rectifier by close to the rating of the diodes used. (depends on diode reverse leakage current) It basically prevents the rectifier from seeing much of the pk inverse voltage it would normally see on the non-conducting plate. I have had relatively few failures with 5AR4 since I started doing this and it does not otherwise impact the performance of the tube rectifier.
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Old 28th November 2008, 07:42 PM   #14
wicked1 is offline wicked1  United States
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Hello,
I fixed my amp, and now it has done it again. I didn't have a fuse on the amp before, and now I do, so at the moment the 5ar4 arced, the fuse blew.
The problem is... when the amp started to fail last time, I could quickly see which parts were overheating.
Now I don't know how to track down the problem.

One thing it could be.. after the last time, r5 darkened. I tested it, and it was still at 270 ohms, so I left it. Now, r5 is completely dead. Would the negative supply rail failing cause the 5ar4 to fail? Or maybe its just that when whatever failed on the amp, r5 heated up again and burned out before the fuse blew.
(there's a link to the schematics on the first post)

This time both diodes are still OK. The caps (c6 and c7)are ok, as far as I can test, but I don't really know. They tested ok last time too, but I replaced them anyway.

One last thing, after the 5ar4 died this time, I thought maybe it was just a bad tube, so I replaced it. The amp powered up, sounded pretty distorted for a minute, and as I was walking over to check it out, the new 5ar4 arced and died.

So.. let me know if anyone has any ideas what could be wrong.... again.....
thanks!

Oh, KevinKr, I did not put diodes on the plates of the rectifier. I was just looking at the board, and I can cut a couple of traces and do that. But seeing as how 3 have died now, its not the tube failing first, but something else causing it to arc.
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Old 28th November 2008, 08:12 PM   #15
Cobra2 is offline Cobra2  Norway
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Too large C4 will kill any 5AR4/GZ34... the diode-mod is really recommended, and makes even a cheap 5AR4 last longer.

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Old 28th November 2008, 08:32 PM   #16
wicked1 is offline wicked1  United States
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c4 is only 47uf.. I think the tube specs say it can take up to 60uf.
I will definitely put in the diodes when I repair my amp this time. I just need to figure out what is causing the problem.
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Old 28th November 2008, 09:44 PM   #17
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Didn't someone else here have essentially the same problem? I'll search around and see if I can find a related thread...

Edit: here it is, except the board in question was a Simple SE, not a Tubelab SE. Might still be some useful info, though...
Putting the Simple SE back together - blown PT - Flashing Rectifier
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Old 29th November 2008, 01:24 AM   #18
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Quote:
Originally posted by wicked1
Would the negative supply rail failing cause the 5ar4 to fail?

One last thing, after the 5ar4 died this time, I thought maybe it was just a bad tube, so I replaced it. The amp powered up, sounded pretty distorted for a minute, and as I was walking over to check it out, the new 5ar4 arced and died.
I'm going back and reading your thread more carefully. The Tubelab SE is a fixed bias design. I don't have one myself; I have a Simple SE. The Simple SE uses a different bias strategy.

Judging by the schematic, I'd expect if you lost the negative supply rail you would also lose the negative bias voltage on the output tubes. They would go into full conduction at this point. Current draw on the 5AR4 would be enormous - I doubt it would last long under those conditions. When it finally arcs and dies it I wonder if it might take C4 with it.

If you power up a fixed bias amp with no negative bias voltage it would sound very bad indeed. Since the tubes are pushed all the way over to full conduction it is unlikely they are operating in their "most linear" region. Not to mention it's mighty hard on the output tubes (red plate) and absolute murder on the rectifier tube.
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Old 29th November 2008, 01:31 AM   #19
wicked1 is offline wicked1  United States
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thanks a lot! thats is exactly what I was wondering.

So.. I hope that resistor failed, bringing down the negative supply, which killed the rectifier.
That's easy enough to fix, and the first thing I was going to try anyway.
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Old 29th November 2008, 02:24 AM   #20
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I'd be asking myself why that resistor went out. Resistors might drift in value, but they don't just go open unless there is a lot of current going through them.
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