Troubleshooting after power tubes die.

[Chill]

The power tubes in my main gigging amp died recently. They were JJ E34L's around a year or two old. Some history leading up to the failure: at a gig, I left the amp on and idling after we soundchecked. About 5 minutes later the 2A slo blo mains fuse was blown. I replaced it with a 4A slo blo from a friend and made it through the gig. I replaced that with a 2A slo blo before playing it again. This time it made some humming noises and then started to drop in volume. Looking at the power tubes, one was redplating. I turned the standby switch immediately but not before the B+ fuse died. So I replaced the power tubes with a new set of E34L's. The B+ fuse dies within 30-60 seconds of powering up past standby and the amp now makes zero sound/ noise through the speakers.

The amp is basically a 2204/ ax84 Hi-Octane which usually runs at close to 490Va. 5500 primary. Metering around, the DC at the rectifier output is about 330V. I took out the power tubes, measured, then disconnected the OT primary CT, measured, then disconnected the screen supply lead and measured, each still showing 330V at the rectifier output.

Measuring resistance of the filter supply caps shows ~2M and falling. The diodes in forward bias show about 2Mohm each and read as open in reverse bias. The screen resistors measure about right at 1.1K or so, and the grid resistors similarly at 2.2K. With the screen supply lead reconnected, there's about 275K ohms between the PI side of the grid resistors and pin 4 for the screens, which made me consider arcing on the socket, but even with the screen lead disconnected the problem is still there. The OT primary measures about 49 ohms on one side and 53 ohms on the other, no connection to ground, and no reading from end to end although it does make a quiet popping noise when the meter is connected across the primary ends.

Any thoughts about what the issue(s) is?

(and sorry about the length of the post! )

[Enzo]

First, let's not make it complicated. If the OT measures from either end to the CT it is OK, the inductance of the whole winding is fooling your meter. Also, if there is plate voltage to all the power tubes, then the OT is not open. After all it is the path from B+ to those plates. The impedance of the thing is not likely to have changed, nor would it blow fuses even if it were incorrect.

Your bad tubes did some damage. It may be that the show must go on, but putting that 4A fuse in there gave whatever the fault was the opportunity to do more damage than it would have.

Does your meter have a diode test function? That is way better to use than the ohms scale. I admit usually the diodes are either shorted or they are OK. SO if they are not shorted now, f0rget them.

The amp blows fuses, that is the problem to solve. Does it blow the fuse with the power tubes removed? That would be a powerful clue right there. If so, then disconnect the CT of the OT primary. Now does it blow fuses. If not, then there is an issue with the OT. If it still blows, then the OT is not involved - leave it disconnected until we find what IS wrong. We want to isolate the problem.

Just because tubes are new does not mean there was not a bad one in the box.

If the amp without tubes blows fuses, then we have a short somewhere. SHorted filter caps are rare, but it happens. SHorted rectifiers are not common, but they happen. Arcing sockets are more common. Turn out the room lights and fire it up. it helps make the little blue spark visible. Look both top and bottom sides. A socket might only arc with a tube in the socket, so consider that too.

Arcing will never be measured with an ohm meter and the amp cold.

If the amp holds a fuse without tubes, then:

If all the screen resistors are OK, we still need to know that there is B+ at both pins 3 and 4 of every power tube socket.

We need to verify that bias voltage is at pin 5 of every power tube socket.

We need to verify that pins 1 and 8 of all the power tube sockets are grounded.

Doing this without power tubes will tell us the amp is ready for tubes.

If those are OK, and the amp holds fuses, then any fuse blowing is likely the tubes' fault.

[R.G.]

Tube Amp Debugging Page
http://geofex.com/ampdbug/ampdebug.htm

I'd guess a biasing issue, but it's worth going through the whole procedure to avoid missing something.

[stokes]

If the amp normally has 490v on the plates which is what I think you are saying,and you only get 330v at the rectifier with no tubes in,something is loading down your B+.It sounds like your problem was originally a power tube that went out of spec and was drawing too much current,the redplating you described.Putting that 4amp fuse in an amp with a redplating tube was,well Enzo already said it.I would disconnect the rectifier from the B+ rail and see if the voltage comes back up at the rectifier.With no tubes in,the only thing connected to your B+ would be the filter caps,and that would seem to be a likely first suspect.

[Enzo]

Ah, I overlooked that voltage. it could be loading the 490 down to 330, but when I see numbers like that the first thing the voices in my head scream is - loss of filtration. Unfiltered DC of about 346v will result in about 490v once filtered. That is pretty close to your 330.

So either way - the filter cap is super leaky and dragging the B+ down, or the filter cap is open and not filtering at all - you ought to check that first filter cap out thoroughly.

Scope the thing. Is the DC clean? No scope? Measyre that 330vDC again, then flip the meter over to AC volts and measure it. On clean DC there will be an AC voltage of about zero, but if you read a large AC voltage, that is the ripple from a lack of filter.

[Chill]

The amp still blows the B+ fuse with:

-no power tubes,
-the OT's CT disconnected, and
-the screen lead disconnected from the filter cap which is just past the choke.

In the dark I did not see any arcing or blue spark on either side of the sockets and it blew the B+ fuse each time. The only voltage connections to the sockets right now are the filament leads and the bias/ grid resistors, which measures about right at -54V (I reset the bias cool when I put the new tubes in). RG, your tube amp debug was the first place I checked!

With the B+ fuse blown, the rectifier output measures about 330V DC and about 180V AC, which I assume is because it is no longer connected to the first filter cap. I believe the fuse blew by the time I metered the DC voltage past the fuse, as it was <1V past the fuse and ~330V before the fuse. The blown fuse measured several M ohm.

The first filter cap meters at about 4-6M ohm and falling with the meter's red lead to ground and about 8M and falling with the meter's black lead to ground. Is there another diagnostic for that short of replacing it?

Rectifier and bias supply diodes tested normal with the diode test function, ~.5V forward, no connection reverse.

I agree the bigger mains fuse probably helped the problem along, but it got me through the gig, and got me into troubleshooting the amp. I figure I built it and I can rebuild it, but there is probably a simpler solution which more experienced or knowledgeable folks can point me toward.

And thanks for the input so far, any suggestions for more diagnostics or for the fix? Disconnect the bias supply to rule it out? Replace the first filter cap?

[Enzo]

Your bias supply is sitting there steady at -54v, so it is fine.

Did not realize the standby was between the rectifier and filter. SInce the rectifiers ddon't measure shorted, and they make DC without burning up, we can assume they are OK.

Your filter cap doesn't measure short, but your meter puts only about a volt across it. It could very possibly be super leaky at anything over say 10v. The meter would not see that, but ti would blow fuses.

If the OT CT is disconnected, then no B+ gets to the power tube sockets, so they won't be arcing from pins 3. SInce you disconnected the screen supply, there is no B+ on the screens, so there is no arcing on pins 4 of the power tubes either. Nothing else there has the balls to arc.

If I read you right, at this point you have remaining the first filter cap, the choke and the second filter cap wired after the stanby switch. The screens are not connected to the second filter now, and the CT is not connected to the ded.

While the choke wires are disconnected at BOTH ends, measure resistance from the choke wires to the frame. SHould NOT be continuity. Of course that is that 1v from the meter again. The choke could have insulation breakdown and internally arc when high voltage is applied.

SO if it doesn't measure shorted, connect one end of the choke to the first filter where it belongs, but leave the other wire disconnected. Now fire it up. If the fuse blows, the choke is arcing to ground. If the fuse holds, turn it off and move on.

Now connect the other choke wire back to the second filter cap. REmember, we have disconnected the resistor to later stage filters and also the screen wires. Nothing connects to the choke wire but that cap. Fire it up. If it blows fuses, the second cap is bad.

All I did here was move down the circuit connecting one thing at a time until I run into the thing causing all the trouble. Follow?

[Chill]

All I did here was move down the circuit connecting one thing at a time until I run into the thing causing all the trouble. Follow?

Yep, I think so. Make sure the choke is not shorting to chassis with the 1V from the meter, when disconnected from the caps. Then power up with only the first filter cap connected, then first filter cap and choke, then first filter cap, choke, and second filter cap. When the fuse blows, we found a problem.

[Enzo]

Yep.

[Chill]

It's the first cap in the supply:

I disconnected the choke first, it metered about right at 83 ohms across the choke, and no connection to chassis. Powered up with only the first filter cap connected, blown B+ fuse. Disconnected the lead to that cap and jumpered from the far end of the B+ fuse to the choke. ~330V with a ~165V ripple, no blown fuse. Jumpered from the B+ fuse to the screen cap, 492V with 120VAC at the mains input, measures right and no blown fuse while leaving it on for a couple minutes.

The dead cap is a 50uF+50uF, 500V rated JJ cap can. I have another here and can replace it. Any benefit to looking for one rated at 600V+? Does big current kill caps?

ETA: What about dropping the supply voltage by 16V or so with a zener to make sure that the DC is closer to the 500V cap rating?

[Enzo]

A 500v cap is just fine for a 490v B+. That is a "working voltage" rating. it will have a surge rating somewhat higher. No I wouldn't bother looking for expensive 600v caps. if I wanted 600v caps, I would wire 350v or 400v caps in series to make them. Twice as many caps that way, but still probably cheaper than 600v caps.


8,000,000 MArshall amps using those caps at similar voltages, and they are just fine. PArts fail. it is not necesarily because you busted specs.

[Chill]

OK, thanks for the help Enzo and gang. New cap will go in before the weekend ends. And I'll have my main gigging amp back.

[stokes]

I'd be more concerned using another one of those JJ can caps.I was helping someone trouble shoot an amp online and he went thru 3 of them,they kept shorting,before he went and got some Sprague caps and fixed his amp.I have never used the JJ's myself,but after seeing his experience with them,I wont touch them.

[Chill]

Well it's not back quite yet...I replaced the cap and got it to hold a fuse with the PSU connected but not the OT CT or the lead feeding the screen resistors. With those connected (still no power tubes), it blew the fuse again, which seems to suggest the issue is with one of those two connections.

But here's the rub: tonight I disconnected the OT CT, the lead feeding the screen resistors, and the choke's connection to the first filter cap. With only the OT CT connected, no blown fuse. With only the lead feeding the screen resistors connected, no blown fuse. With the choke and rest of the PSU connected, no blown fuse. With the replaced first filter cap connected, blown fuse. I believe that rules out the OT, the screen connection, arcing on the sockets (they were both connected to the power tube sockets), and the choke and supply downstream from the choke.

Admittedly the replacement was a used cap, and I will replace it with a new one this time, but it was odd that a similarly rated cap in the same position died in pretty short order. Any ideas what else could be causing the problem?

[Enzo]

yeah, scope the raw B+ to make sure the rectifiers are not leaky. ANy AC on those caps will not make them happy.

I would also say that if you replace something with a used part, you ought to mention that.

If you remove the first filter and connect the other stuff, it removes a lot of stress from those other parts. The power supply now becomes a choke inut filter and will run at a lower voltage. If it is still connected that way, measure it. What voltage appears at the CT without the first filter but with the choke and second filter connected to screens and all the other stuff? What voltage appears at the screen node? Does that first filter ALONE still blow the fuse now?

COnecting the stuff after the first filter should not affect the filter cap itself. If anything, they will draw the B+ voltage down and reduce stress on it.

You have a two section cap in there now? ANd one side when used as the first filter pops the fuse? Move the wire over to the other half of the thing and wire that half as the first filter for tests. Does it still blow the fuse that way? if so, the cap is not the issue since it worked as the second filter a moment ago. If it works that way, then the other section was bad. You say the cap was used. OK, why was it pulled from something in the first place? I don't usually remove parts that are good.

And just not to assume anything, we are sure we used the proper terminal for ground on it?