My Marshall JCM900 blow tube.

[Alan0354]

I was using the Marshall JCM900 Dual Reverb to drive 60Hz to break in the speaker, it blew the fuse again.

Before all these, the amp blew fuse, I change the fuse and tested ok. But I did not see one of the tube is blown. Today when I tried to see the output to 12V into a 8ohm speaker, I couldn't get there. So I start troubleshoot and found the left 6L6 had power in it and not lighting up. So I just change to another tube and it worked.

I got it to 12Vrms and ran for over an hour. I went to snooze and when I wake up, the amp was dead. The fuse blown and after I change the fuse, I couldn't get 12V again. I looked, the new tube blown also. So this is no longer coincident anymore.

Any suggestion what to look for. I did check all the voltage a month ago before replacing the fuse. What can make the tube blow. It is the same left tube that blown. Obviously I did not troubleshoot yet, but it's strange only one tube blown. And it will work for at least an hour before the new tube blown.

[g1]

Which fuse is blowing, the OPvalve fuse? If so, you are probably losing bias to that tube. Either a bad connection between the bias circuit and the grid of that power tube, or a leaky coupling cap between the phase splitter and that power tube.

[Alan0354]

It blew the main 4A fuse. The tube cathode 500mA fuse was fine.

[g1]

You said the first one that blew no longer lit up (no heater?), was that the same thing with the second one?
Check for arcing at the socket.

[Alan0354]

You said the first one that blew no longer lit up (no heater?), was that the same thing with the second one?
Check for arcing at the socket.

The first had greyish powder at the bottom. The second one seems to light up differently.

[Chuck H]

Is this a four tube or two tube amp?

[Alan0354]

Is this a four tube or two tube amp?

It was 4, I pull the two outer two.

[Chuck H]

If you're only using two tubes in the amp when it blows one on the same side then I'm with g-one. Check the bias resistor and it's connections. Check the PI coupling cap for leakage. Check the tube socket for a loose grid pin pocket.

It sounds like you had two different tube failures. But both probably blew due to over dissipation.

[Alan0354]

If you're only using two tubes in the amp when it blows one on the same side then I'm with g-one. Check the bias resistor and it's connections. Check the PI coupling cap for leakage. Check the tube socket for a loose grid pin pocket.

It sounds like you had two different tube failures. But both probably blew due to over dissipation.

Thanks, I have to get to it later as I am busy in breaking in the speaker for the moment. It's on the sixth hour, 30W 60Hz.

So 8 hours and I am done?

[Chuck H]

Maybe? But that's on another thread. Let's keep this one on topic.

[52 Bill]

This one lasted for 6 hours of 50 to 80Hz at about 30W. I check it once every half hour and I notice the cone does not move as much. So I read with the meter and got only 6V or so. I know one tube is blown.

I understand what you are trying to do, but I think that trying to use an amp to continuously drive a speaker for hours at a time is asking for trouble. I'm surprised that they have lasted as long as they have.

[Alan0354]

I deleted the last post about I blew the second amp. I went around, change tubes, scoped the signal around the power tubes, I cannot see the problem. I put back the pair of tubes that I though had problem, everything looks right. I got the full wattage. I swear I measured the voltage dropped by half with the meter before I shut it off. But I cannot duplicate the problem.

[Chuck H]

Maybe you should check you speaker connections for this break in method.

[Alan0354]

The Marshall is blown. But for the life of me, I cannot find the problem of my custom amp. Everything looks normal now, the tubes that I used for breaking in are working, I measure all the voltages, scope traced all the voltages from PI, through grid, to plates, to speaker output. Everything looks normal. I jiggled the wires, nothing loose. I even question the generator and hooked it up. I got full power.

The speaker cable is very simple, I have a female 1/4" jack connected directly to the speaker which I am still using even in the cabinet. I plug the output of the amp directly to the speaker with a 1/4 jacked cord. I was not seeing things as the loudness of the speaker got lower, so I measured the voltage with the meter, instead of getting 15V, I only got about 6.6V at the speaker yesterday before I terminated the testing.

[g1]

Maybe the input signal to your amp is what dropped out. You wouldn't know unless you measured it while the problem occurred.
I still haven't heard any good reason why you shouldn't just connect the speaker directly to the variac and adjust for 15VAC out. Yes you should be careful, but it will work (unless your variac has a VERY low power rating).