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Old 20th August 2006, 06:02 PM   #1
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Exclamation Quick urgent Help plz !

Hi, there

I just build my first tube amp (6l6GC PP 2 channel class A) I`m ready to test it, when I turn on the power switch everything goes correctly, filament circuit is ok, no problem. But when I turn on the beast with the standby switch, I saw glow red my power resistance (270 ohm 20 W) and a bit of smoke come out of it... so it`s too much for thoses resistance. Here`s some pics of the inside and the psu schematic.

Please tell me what should I do to make it run. Thank

Schematic : 6L6 Push pull -- Power tranny question
(see post 44, the CCS I use is the one proposed by refference post 48-49)

Inside : see attachement

Thank a lot !
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File Type: jpg inside.jpg (74.7 KB, 228 views)
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Old 20th August 2006, 06:04 PM   #2
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Here`s the zoom of the psu
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Old 20th August 2006, 07:02 PM   #3
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Could we see a schematic of the amplifier? do both resistors smoke? or just one?
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Old 20th August 2006, 07:06 PM   #4
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Have you scheme for the amp to post here....?

Sounds like you have too much current flowing through 6L6 tubes...Assuming the Hot 270 Ohm, is the Cathode resistor.....

What voltage Negative do you have on the Grids of the 6L6 tubes....Again still assuming the 270 ohm cathode resistor is the 'Hot' one!
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Old 20th August 2006, 07:16 PM   #5
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Both smoke, its realy the power resistor of the psu,

Thank again
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Old 20th August 2006, 08:38 PM   #6
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Do they get hot without the 6L6 tubes fitted?

Are the 6L6 tubes Glowing anodes?

270 ohm does seem to be quite a high value resistor to have in the supply to the O/P stages......
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Old 20th August 2006, 08:49 PM   #7
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Default I think you've gone tooooo fast!

Beautiful wiring BTW. However, you should not be having basic
problems like this by the time the wiring is pretty and the lovely
wood case is made. Lesson learned: You are a victim of your
own project management.

One thing I dont like are those 20W sand-cased resistors sitting on top of the bank of electrolytics. For one, electrolytics seriously lose life expectancy over 50C. By using them as heatsinks for the power R, you are just compounding a failure more. Second, those power R's need a much better thermal contact to wherever they are sending their heat... I'd get rid of them and get RH25 or RH50 rated aluminum bodied, finned power R's. www.handme.com has them. You will need two more M3 or #4 screws on your top plate to secure the power Rs. Further, apply a thin film of handy CPU heatsink grease to the bottom of the Power R to give it some nice thermal contact... The RH25's only cost about $5. They are a much better part than sand-encased speaker resistors.

Finally, if your resistors are cooking hot then you have a DEAD SHORT somewhere. Start measuring resistance to ground through your B+ path.. You're going to have to unsolder some of that lovely wiring to find the problem.

-- Jim
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Old 20th August 2006, 09:32 PM   #8
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Could we see a picture of the top of the amplifier?
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Old 20th August 2006, 10:53 PM   #9
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Sounds like you have a short somewhere, normally a 20W 270 ohm resistor would not have a problem at the load currents we are talking about here. (say 100mA) Are your electrolytics wired backwards? Check for other potential miswiring.

Incidentally a nice set of chokes would provide more ripple rejection and better voltage regulation under load.
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Old 20th August 2006, 11:47 PM   #10
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Thank for your input, I'll check for short and miswiring tomorow, the power tranny is 270-0-270 @ 450 mA.. if it's help. For picture, you'll get some tomorow.
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