My Marshall JCM900 blow tube.

[Alan0354]

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).

I checked the signal generator, I check everything, it belong to the X-File. I only have a smaller variac, it's good for doing power scaling for a Twin reverb, but lower to 15V and drive 30W? I don't know the small winding can take 2A. Remember even the variac is 100W, but it does not mean it can provide 30W and 15V as the current is a lot higher. I have to go look in the surplus store to see whether I can find a step down transformer cheap. Then I can crank the voltage of the variac higher and be safe.

[g1]

You need a variac with a current rating of at least 2A. The current spec. of the variac is for any voltage it is set at.
If your variac is not rated for at least 2A, you can't use it for this.
My variac is rated for 10A. It can deliver 10A whether I have it set at 120V or 6V.

[Alan0354]

You need a variac with a current rating of at least 2A. The current spec. of the variac is for any voltage it is set at.
If your variac is not rated for at least 2A, you can't use it for this.
My variac is rated for 10A. It can deliver 10A whether I have it set at 120V or 6V.

That's the difference. I only have a smaller variac. It's cheaper to get a 15 or 18V transformer and use my own variac.

But all that said, I really tested the speaker today, it is too bright. I don't think I'll get there even with another 8 hours. I already have 7 hours already, the effect tails off after the first half of the period, it's like you should be at least 75% there after 1/2 the break in.

[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.

Well, I have been using my amp the whole day, everything sounds normal, nothing wrong. I checked the signal generator, it's fine. The new speaker is working, it's loud and clean. Everything is working!!!! Yes, the Marshall still burned.

[Alan0354]

I opened the Marshall, I check the continuity around the grid to the PI stage and wiggled the components, I see not bad connections. I have not change the coupling caps as I have to take the pcb out. But nothing looked wrong.

My own amp works perfect after two days of tinkering with the circuit to adapt to the new speaker.

Any other suggestions on the Marshall? The first tube that burn was a 5881, I replaced with a 6L6GC rather than 5881 and it burn after an hour. Does that make a difference?

[Chuck H]

There may be an intermittent bias condition for that socket. Intermittent problems can be snarky and difficult to detect. I can't think of anything else that could cause two different tube failures on one specific socket with two different tubes operating below clipping. Ignore that another amp stopped functioning while doing the same duty because as it turns out, that amp is fine. So that part is just a confounding coincidence.

I would reflow all the joints related to the bias for that tube. I would check the bias resistor and tube socket slot for the bias connection for that tube. I would check the bias voltage for that tube while wiggling and jiggling a bunch of stuff. After this, if everything checks out I would plug in a pair of known good tubes using a current limiter. If voltages checked out I would remove the current limiter and recheck voltages. If everything checks out I would assume the problem is fixed until and unless it resurfaces. And if it does I would hope for more indicative symptoms for guidance.

[g1]

The first tube that burn was a 5881, I replaced with a 6L6GC rather than 5881 and it burn after an hour. Does that make a difference?

I'm still not sure what you mean by "burn". You said it was the mains fuse that is blowing, not the power tube cathode fuse. Has the heater gone bad? Have you checked the sockets for arcing?

[Chuck H]

I think he indicated that the mains fuse opened on the first tube failure. I'm not sure if it opened on the second. But even if it did I think the description implies two different tube failures. I'm thinking the first tube shorted plate to heater and that's what took out the mains fuse. It's hard to know. I can't imagine an engineer with an amp on the bench that's blowing the same tube finding nothing wrong either though. I get the feeling he hasn't checked everything we've suggested so far because his engineering experience tells him he doesn't need to.

[Alan0354]

They both took out the main fuse. The first tube have greyish color powder inside, The second tube looked like it did not light up. I jiggled all the wire, keep changing the tubes. I used a stick to poke the components, the wires around. I measure all the voltage.

I am convinced it is NOT fixed, more like I mod the amp and get decent sound. So I just button it up for the moment and use it as a reference and work hard on my own amp. I figure I have plenty of fuse and 3 more 6L6 to go. So I just left it alone for now unless I have other suggestions. I have a lot on my plate right now, I spent over an hour tracing the circuit, banging the boards, the components and the wires. If the problem still refuse to show up, I couldn't afford to waste any more time.

I don't think the coupling cap is leaking, I got the correct bias voltage, and I bend the cap back a fore measuring the circuit. I have seen cap failed, but I never seen intermittent leaking.

BTW, when both fuse blown, I change the fuse, it did not blow the fuse even with the bad tube. So the tube is not shorted after it is burnt.

[Chuck H]

Well it certainly seems that you've checked all the obvious stuff. Could just be a coincidence with a couple of bad tubes in a row. It sounds like you're taking them from a pile, I dunno. I recently used up two pair of salvaged tubes in a build. Both were young, tested fine, and both blew a tube within a week in a good working amp. It happens.

[Alan0354]

One question, Could it be running 30W from two 6L6 CONTINUOUSLY with a constant signal of 60Hz is too much? Remember:

1) This is a guitar amp that only has instantaneous signal, never continuous.

2) At 60Hz, the OT is not the most efficient. I adjust 30W at the output, but the 6L6 might be working a lot harder.

Even on my own amp, it failed even thought it did not blow a fuse, the output power drop. I notice the excursion of the cone was a lot less, so I measure the voltage across the speaker. I got 6.6V or so, it's about half the voltage I set. I went to the back of the amp. I could swear one of the tube light up very strange. I saw a line of orange glow down the greyish body inside the tube. So I hurry up shut the amp down. But I could not duplicate the problem on the bench later. I actually spent more time poking my own amp as that is important. I changed tubes and all. Couldn't find the problem.

I was not drinking!!!! I am not drunk!!!! This is puzzling.

[Chuck H]

That's actually a very interesting observation. The idea of a continuous 60Hz @ 30W doesn't sound at face value like it would be problematic. Now that you've described the nature of the failure with your other amp, including the red plating, I would speculate that you may be right about the amps being capable of instantaneous power, likely continuous music program, but not a continuous LF sine wave at a little above half the amps power. But I'll also note that this problem could be exacerbated by a hot bias. I'll wager that if you set the bias up just null the crossover notch while driving 60Hz at 30W the amp would run forever.

[Alan0354]

Yes, Both the Marshall and my amp was bias hot. The Marshall was biased at -45V( don't have a bias probe) with +B at 475V. That is quite high. My amp is a little better. +B=400V and bias at -43V. I am not trying to bias my amp hot. It was meant for 6V6. I stuck a 6L6 in it and change the tap on the output to get higher power. -43V was as high as I can adjust. So yes, both are biased hot. I did think about this afterwards and I adjust the bias to -48V on the Marshall before I button it up. Now is like wait and see. The Marshall is not going anywhere except serve as a reference, I have quite a few fuses and 3 more 6L6!!!

I did not think at the time it was red tube with my amp, I just saw a thin line of glow. I was not thinking at all at the time. My mind was all into breaking in the speaker. BTW, I think the speaker has a lot more bass and loosen up quite a bit. I have been playing with it for two days already.

I suspect the kicker is the efficiency of the OT fall off at 60Hz. Remember I even ran at 50Hz. I notice I had crank up the volume when lowering the frequency. I really did not stop and think at the time, but assuming the power from the tube is the same, then the drop off can be with the OT. But the poor tube has to drive much harder at lower frequency.

[Alan0354]

I finally have time to go back and look at the two blown tube and took a picture. I plugged into the amp to verify the tube is not working.



1) The tubes are not working.......obviously.
2) The filaments did not light up BUT both tubes got hot. I think the filament is still intact, my guess they loss of vacuum.
3) From the picture, you can see the bottom of the glass on the left tube and the top of the right tube has a whitish fog like coating inside the glass. I had seen this when the tube loss vacuum before.
4) I check the voltages on the plate, screen and grid, nothing shorted, I have all the voltage on them.
5) I pulled two of the four power tube and I did not change the tap of the OT. So the two power tubes are seeing half of the required plate load impedance. That put more dissipation on the tubes.

My guess is this.

The tubes are over heated due to excess power dissipation because:
1) Running at 30W continuous for an hour.
2) Running at low frequency between 55Hz to 70Hz where the OT is not very efficient. To get 30W output, the two tubes must be running like 40W.....almost max.
3) I forgot that I had the grid bias set at -45V which is hotting than a normal PP class AB amp.
4) The +B of the Marshall is +475V.

All these add up to be over stressing the 6L6 and 5881 tubes. Maybe if it is EL34, it might survive.

[jazbo8]

When you pulled the two tubes to lower the output power, did you also change the output impedance/tap on the amplifier? Also, did you verify the amp was indeed putting out 30W into the speaker?