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Old 21st December 2009, 03:48 AM   #11
roggom is offline roggom  United States
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Just wondering, is the 10m45 installed backwards? since it is installed under the board, K should be to r13 and A to r14. Pinout is GAK http://ixdev.ixys.com/DataSheet/98704.pdf.

Just a thought.
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Old 21st December 2009, 04:02 AM   #12
RDL_AK is offline RDL_AK  United States
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Also I noticed that the 150 ohm 3 watt R2 in the schematic looks to have gotten a little warm but it checks out fine with my meter


Is this a typo? R2 is 150,000 ohms not 150 ohms. This will surely cause arcing in the rectifier if that is what is actually in your amp.
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Old 21st December 2009, 10:32 PM   #13
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Is this a typo? R2 is 150,000 ohms not 150 ohms. This will surely cause arcing in the rectifier if that is what is actually in your amp.
Nope thats what it is. You have found it!!!!! I just removed it from the board and it is toast. I guess when I ohmed it out it was picking up R4 in the circuit.
Wonder what else is toast???
Looked at my digikey order from the parts and shure enough I forgot the K $#%$# !!!

I ordered some 150K and some new 10m45's and some new caps just incase.

I also would guess that I will need a new 5AR4??
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Old 22nd December 2009, 03:24 AM   #14
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Wonder what else is toast???
R2 is a 150K (150,000 ohms) resistor. Its sole purpose is to provide a discharge path for the power supply caps. This "bleeds" off the stored charge in the capacitors when the amp is turned off. Operating the amp with a 150 ohm resistor subjected the 5AR4 to an extreme overcurrent situation, which made it protest with fireworks. It is likely damaged, but I have seem some tubes survive this. R2 is a redundant bleeder resistor. The combination of R3 and R4 will also discharge the capacitors but at a slower rate. You may try the amplifier with R2 removed, but it must be replaced before putting the amp in service. I like to have a bleeder on each power supply cap in case a wire gets disconnected, or the choke fails.

It is not possible to figure out how you have the 10M45's wired from the pictures. I dont believe that the 10M45's are damaged.

I would try the amp again with R2 removed, If it still sparks out the 5AR4 is likely toast. If there is no fireworks, measure the voltage on the coupling caps, and report back.
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Old 22nd December 2009, 12:09 PM   #15
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Did a quick check with just the 5AR4 in and no arcing at all. Checked the voltage at the coupling caps and I am showing 540vdc. Off to work for now. Everyone have a good day.
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Old 22nd December 2009, 02:46 PM   #16
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Checked the voltage at the coupling caps
Install the 12AT7 and measure the voltage at the coupling caps (the white ones). You should have zero volts at the end connected to the output tubes, and 150 to 250 volts at the end connected to the 10M45's. If this voltage is OK the 10M45's are OK. Try adding the output tubes and see what happens.

Do not run the amp for extended periods of time without all of the tubes in place. As your voltmeter tells you the power supply can produce over 500 volts when it has no load. This is more than the 500 volt rated capacitors can take for extended periods of time. They can be real messy when they blow up too!
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Old 22nd December 2009, 08:41 PM   #17
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Originally Posted by tubelab.com View Post
Do not run the amp for extended periods of time without all of the tubes in place. As your voltmeter tells you the power supply can produce over 500 volts when it has no load. This is more than the 500 volt rated capacitors can take for extended periods of time. They can be real messy when they blow up too!
I always worried about that. I wish there were an easy source for 550 (working voltage) rated snap-in electrolytics. It seems the only affordable recourse is to go with motor run caps and mount them somewhere off the board.

I think the Panasonic caps I put in mine (the recommended part from Digi-Key) is surge rated for 550 volts. Unfortunately, I've never seen a length of time specified for the duration of the surge.
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Old 22nd December 2009, 09:10 PM   #18
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I think the Panasonic caps I put in mine (the recommended part from Digi-Key) is surge rated for 550 volts. Unfortunately, I've never seen a length of time specified for the duration of the surge
I have never seen this specified either. The (over) voltage handling ability of most electrolytics are highly temperature dependent.

I can say that I hooked up a big Antek 400-0-400 volt toroid to a Simple SE board to see what happens. I got about 530 volts of B+. The EH KT88's seemed to deal with this OK and nothing fried. I ran the amp for no more than an hour at a time for about a week before dismantling it. I use that same board for all of my extreme experiments and I have blown it up at least a dozen times, but the original Panasonic caps are still hanging in there.

While finding the upper limit of Pete Milletts "red board" I managed to explode a 450 volt Nichicon electrolytic. It operated at 450 volts for an hour right next to some toasty sweep tubes before I decided to crank up the power supply. The cap exploded rather violently at 600 volts. Hey I wound up extracting a reliable 250 watts from an amp designed for 18 WPC. Sacrificing a few parts in the name of science is OK by me, but most Simple SE builders would rather not have their amps explode.

Posted new P-P power amp design
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Old 23rd December 2009, 10:15 PM   #19
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Well I got my 150k ohm restitor and checked with my VM. Got it install put the 5ar4 fired it up no arcing, shut it off put the 12at7 in after awhile fire her up and starts to arc hmm?? So the quest continues.
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Old 23rd December 2009, 10:34 PM   #20
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The 12AT7 and its associated components can not possibly draw enough current to arc a healthy 5AR4. The 12AT7 gets its supply voltage through a pair of 10K resistors. Assuming the resistors are indeed 10K, everything after that (10M45's and 12AT7's) could be shorted out and the 5AR4 shouldn't arc. I must assume that your 5AR4 was injured by the arcing that it saw with the 150 ohm resistor.
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