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    Building, troubleshooting and testing of these amplifiers should only be
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Help! My amp stopped singing...

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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 :mad: . 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!!!


Amp Schematic


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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.

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!

Hey, thanks for the help!!

No luck yet though...I´m getting a bit frustated.

I rewired and checked the input stage including the 15uF cap and bypasses. I switched anode chokes. Arrghh!:bawling: nothing!

Futher test on the 300B revealed only 30 volts anode to cathode (should be around 350V) and a crazy 530V on cathode to ground.

Could the output transformer be fried? How can I test for that?

Thanks, thanks for the help!

Your output transformer is unlikely to be the cause. In a circuit like this, almost anything upsets the basic operation of the whole rest of the circuit. So you'll need a bit of patience here.

Let's assume that because the amp did work at one time, the wiring is correct (if you've rewired, double-check what lowrider suggested). Clearly, you're underbiasing the output tube (that was the measurement I asked you to do earlier). It draws a ton of current which drops the rail voltage, heats the resistors, and takes the tube to its perveance limit.

Don't just check the caps for capacitance (both the 15u and 50u clusters!), you need to look at leakage, too. Lacking a means to do that, I might be inclined to replace them with something cheap'n'cheezy of roughly the right value and voltage rating to see if the problem is resolved. Otherwise, you can test cap leakage by removing them from the circuit, charging them up with a nice high voltage, and waiting 5 minutes. Then briefly stick a high impedance voltmeter across to see how much they've discharged and back-calculate the leakage resistance from R = t/C.
I am only trying to help...

I know! and I tried to thank you! Sorry if I sounded like I wasn´t!


Good news it that I found it!!

I took out the bypass caps for the 50uF and voila! The amp is working again. One of them was shorted or something.

Thank you guys for all the pointers!! great to have this kind of help.

Regards and see you around!

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