Help! My amp stopped singing...

[ligascon]

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

[ligascon]

Amp Schematic

[ligascon]

Supply Schematic

[ligascon]

Sorry, one thing...is the 1K resistor that got real hot melted the solder...

Thanks again!!

Luis

[Psychobiker]

12W...methinks there's more than that coming from a 300B at times, so maybe go for a larger resistor and chassis mount it?

[ligascon]

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

[Sal Brisindi]

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

[ligascon]

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

[SY]

Measuring grid to cathode voltage on the 300 ought to be diagnostic.

[ligascon]

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