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

Amp Schematic

 

[ligascon]

Supply

Supply Schematic

 

[ligascon]

Correction

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

Thanks again!!

Luis

 

[flyinglemur]

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

 

[SY]

OK, pull the output tube and check the first stage over. I'd look at the parallel cap combination and the choke first.

 

[ligascon]

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! 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!

Luis

 

[Lowrider]

Disclaimer: I know nothing about this stuff...

But it looks to me that B+ is connected to the cathode instead of to the anode...

 

[DigitalJunkie]

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

To me,This suggests that a cathode resistor is open,or a much higher value than it should be.
The cathode should not be at/near B+!

 

[ligascon]

I wish it would be so easy...but it isn´t.

It´s connected correctly, as it was when it was working three years straight without problems.

But thanks, please keep giving me ideas!

Luis

 

[SY]

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.

 

[Lowrider]

I am only trying to help...

I had a problem that bias doubled in my amplifier when I put it straight, everything measured ok on the bench, upside down...

I still dont know what happened, better double than half, at least nothing burned, probably some piece of metal moving around...

 

[ligascon]

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!

Luis

 

[Sal Brisindi]

Thats great news Luis! Glad you found the problem.

Sal

 

[Lowrider]

Good it is fixed, I wouldnt mind your amplifier...