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Old 4th April 2008, 11:48 AM   #1
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Unhappy Help! My amp stopped singing...

Hi guys,

I have to say that my knowledhe of electronics is still pretty limited but, with some help, I did manage to build a pair of 300B amps, Thorsten´s full monkey style, which I have been enyoying big time for the last three years without a single glitch.

Last week my audio nirvana came to a sudden stop when one of my monoblock amps stopped working . I turned it off fast and took it to the bench.

When I turned on, the 7K5 resistors got really hot and melted the solder! Time to stop and check.

I went and checked everything that I could check and I still haven´t found the fault.

Comparing with the healthy amp, I get 710V right after the rectifier, without output tubes, with the faulty one I get 570V before I have to hurry and turn it off...

Help guys, maybe I´m overlooking something that I can fix myself before taking the amp to a technician.

Thank you and best regards!!!

Luis
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Old 4th April 2008, 11:49 AM   #2
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Old 4th April 2008, 11:51 AM   #3
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Old 4th April 2008, 11:52 AM   #4
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Sorry, one thing...is the 1K resistor that got real hot melted the solder...

Thanks again!!

Luis
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Old 4th April 2008, 01:03 PM   #5
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12W...methinks there's more than that coming from a 300B at times, so maybe go for a larger resistor and chassis mount it?
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Old 4th April 2008, 01:16 PM   #6
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Hi,

Under normal working conditions, the 1K resistor should dissipate around 4 watts. I got a 12W mills unit in there actually, which I think is a safe margin.

Maybe I´m wrong but I don´t think that´s the problem...

Thanks.

Luis
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Old 4th April 2008, 01:18 PM   #7
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A few things come to mind, the good thing is you have a good mono-block to compare your readings to first off I would make sure the tubes are good, possibly swapping them, second I would measure the primary and secondary of the audio transformers, Third I would kake sure the capacitors going to the B+ and the filament of the 300B are good, not shorted (the 50 and 3.3uf caps). You might as well check the 4 capacitors that are in parallel with the 2 7.7K resistors also for shorts.


Hope this helps.

Sal
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Old 4th April 2008, 01:31 PM   #8
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Hi,

I swapped tubes and also tested capacitors with a capacitance meter.

What I didn´t do was to check on the audio transformer primary and secundary. What is the procedure there?? what should I be looking for?

Thanks for your help!

Luis
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Old 4th April 2008, 01:34 PM   #9
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Measuring grid to cathode voltage on the 300 ought to be diagnostic.
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Old 4th April 2008, 01:51 PM   #10
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Fearing for my tubes I plugged them in to test grid to cathode voltage on the 300B. It was 110V when it should be around 60V...yikes, I hurried and switch off.

Were are those 110 Volts coming from? I checked resistors, cabling, caps...I even swapped the anode chokes!!

Could it be that the output transformer is fried?

Thanks for the help!

Luis
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