Is it possible and/or likely for a preamp tube failure to cause a mains fuse to blow?

[mort]

Welp, I've built about 25 amps now and this is the first time for an outright failure out in the wild. I tend to keep them for a couple weeks before sending them out, and I play them hard and even put some light gigging on them before sending them out, in hopes that if a failure is going to occur early, that it have a good chance of doing so while still in my possession. Well my customer got his 5e9-a tweed tremolux combo a couple weeks ago, loves it, says it's easily the best sounding amp he's ever played through, and now it's blowing fuses. I've only spoken with him briefly so far on it but it appears that a new rectifier and 2 new power tubes will not solve the issue. It could be a transformer I suppose but is it even possible for a preamp tube to do it? he says nothing is obviously cooked except for the fuse itself. I've had little to no issues with tubes and never a bad transformer. Thoughts?

[gui_tarzan]

You know the basic troubleshooting steps so I won't bore you with those, but check the bias circuit to see if it's failing as well as the plate caps on the PI. I just repaired a SF Twin that had bad PI caps and it was causing too much voltage to go to the grids.

[pdf64]

I suppose anything is possible, but a pre-amp tube blowing the line fuse seems unlikely.
I guess that Fender style, the line fuse is the only one, no B+ fuse etc?
A slow release gremlin may be in there.
Can the user check the power tube cathode current easily, eg back panel test points?
How long does a fuse last in it now?

[Enzo]

If your preamp tubes are like most, they have a 100k or greater plate resistor. It doesn't matter, but let us assume the B+ for the preamp is 300vDC. If the plate dead shorts to ground - hard to do inside the tube - then we have 100k ohms across 300v. That is only 3ma drawn from the supply. If we grounded the grid pins, that wouldn't cause much excess current, and neither would shorting the cathode to ground. So in short (pun intended) I don't see any way for the typical 12AX7 to blow ANY fuse, mains or otherwise. I guess if the heater dead shorted across itself, but how would that happen inside the tube.

If he has actually swapped the power tubes and recto tube with known good ones, then we can probably eliminate tubes. Doesn't mean something has not happened under a tube SOCKET, but that is another story. Transformer failures remain the last thing on any amp repair list.

It wasn't something simple like a bad tube, so unless you want the guy digging inside it with a soldering iron, bite the bullet and get the amp back. It might be something real simple. NOBODY has 100% of their amp product work 100% of the time. It happens.

[mort]

Just made contact with him. It blows the fuse with no tubes installed. He paid for the Mercury transformers too.. what a shame if it's bad iron. I told him to send me the chassis at first opportunity.

[drewl]

I think once I had a bad preamp tube that caused the mains fuse to blow.

It was a long time ago, so I forget or didn't bother to find out why.

[Chuck H]

It's still not necessarily a transformer. Thee's still the rectifier (tube or diode), the filter caps and the (very slim ) possibility that a wire whisker or something has found a bad place to settle. So take heart. Even though it's a new build, and one of yours, it's still a repair at it's root. Since the amp has BEEN working, what are the odds that the PT is only now showing a flaw compared to the other possibilities? No point stressing before you get it on the bench.

[drewl]

More anecdotes!

I did have a Weber PT die after only an hour or so use.
At a gig even, but I always bring a backup amp as fixing something is the last thing I want to do when I'm out playing having fun.

They replaced it for free.

[mort]

Welp, looks like the PT HT windings are scrambled. It's not shorted to ground but looks like it's shorted straight through. I'm getting 10 ohms from red to red, and 26.6 ohms & 36.7 ohms from each red respectively to the CT. I don't know what it's supposed to be from red to red but I know it should be the sum of the two other halves, which should be equal to each other, so it's cooked. I guess it just happens sometimes.

[mort]

I just happen to have one of these circuits with all the exact same parts that I built for myself at the same time I built this one, so I checked the PT on that one and it's 55ohms/112ohms/57 ohms

[pdf64]

A 6L6 tube short on my 6A20HP took out the OT and PT, even though I'd taken precautions and added fusing to the B+ dc line.

[mort]

A 6L6 tube short on my 6A20HP took out the OT and PT, even though I'd taken precautions and added fusing to the B+ dc line.

ouch. that failure went right for your wallet!

speaking of B+ fusing, is it true that a fast blow fuse should be used there? And a slo-blow on the mains?

[Randall]

Welp, looks like the PT HT windings are scrambled. It's not shorted to ground but looks like it's shorted straight through. I'm getting 10 ohms from red to red, and 26.6 ohms & 36.7 ohms from each red respectively to the CT. I don't know what it's supposed to be from red to red but I know it should be the sum of the two other halves, which should be equal to each other, so it's cooked. I guess it just happens sometimes.

So much for the Mercury hype.....

[mhuss]

speaking of B+ fusing, is it true that a fast blow fuse should be used there? And a slo-blow on the mains?

B+ - fast blow yes, always
Mains ... it depends. US amps tend to use SLO-BLOW fuses and UK ones tend to use normal ones, but for a Tremolux, might as well use SLO-BLOW like Leo.

[mort]

Thanks mhuss for the clarification.


Something else that has me thinking.. Is there anything I can do to determine what caused the PT failure? If it was a tube short then it stands to reason that if he puts the same tubes back in that the problem could repeat as soon as he powers it on, right? He only sent the chassis as per my instruction, as I have tubes here to power it up with once the new transformer goes in. I wasn't thinking about possible tube failure once he had found the amp blowing fuses with no tubes installed. Should I have him send me tubes and power them up with a light bulb limiter? Or would it just be better to have him trash the recto tube and 6v6 pair?