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

[Chuck H]

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?

If there's a tube with a plate to heater short in there it'll be bad. On the off chance that it's not, and the problem is intermittent, power the amp up with a light bulb limiter and knock on the tubes with the unit under test. Not a sure fire diagnosis to be certain, but better than nothing.

You mention the owner plugging the same tubes into it.?. Did you receive the amp with no tubes?

It's probably just a bad transformer. It happens.

[g1]

I think for a rectifier or power tube to have done that damage would have required over-fusing or tinfoil.

[Chuck H]

I think for a rectifier or power tube to have done that damage would have required over-fusing or tinfoil.

Well, given the choice I prefer tin foil.

[g1]

Sorry Chuck, I shouldn't have limited your choices like that .

[mort]

Sorry Chuck, I shouldn't have limited your choices like that .
[ATTACH=CONFIG]36165[/ATTACH]

In the hvac service field, I have run into a condenser fan motor that had been replaced at somoe point and whoever installed it just twisted the wires together and taped it with aluminum tape. It had been that way for years without an issue too. Just by luck the connections just happened to never touch each other or the cabinet chassis either, which was literally less than 1/2" away. I run into all kinds of stupid crap like that

[Chuck H]

What I've seen as often as not is cigarette pack foil (gold with the paper on the back). A real MacGyver move. Working with materials on hand

[Enzo]

We had a guy many years ago needing a 15A fuse for a pinball machine, and his box of 15s was empty, so he wires a 10A and a 5A in parallel.

[pdf64]

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.

I think that T type fuses seem to be the norm for line / PT primary fusing over in the UK / EU.

But T type fuses are often used in the B+ too; I think whether F or T may depend on several factors, eg B+ supply impedance, tube or silicon rectifier, before or after rectifier, before or after reservoir, standby switch implementation (if any).
I suspect it may be down to experimentation, eg what's the lowest value of F and T type that the circuit and its variables can accommodate, that can maintain reliable operation.

[mort]

Just heard back from Mercury and they are asking me to send them the transformer so they can inspect it to determine if it failed from their error or not. How would they be able to tell?

I totally understand not wanting to supply endless amounts of transformers to people that are using them incorrectly, but I have no idea how they could determine one way or other unless someone had s track record of claiming warranty, which I have never(granted, I have only placed 4 orders with them).

[mort]

HOLY CRAP -- Now I'm nervous. I was just checking back through my parts orders and realized I've made a mistake on ALL THREE of these amps that I've built. I'm not sure if it would cause this failure but it might! All 3 amps are identical, with the same transformers, adhering to the stock schematic except omiting the cap on the primary of the PT, adding a virtual CT for the heaters(2 100 ohm resistors), and one leg of the heater loop does not connect directly to ground. Otherwise they are bone stock, but I just realized that on all 3 of them I ordered a tube set with a 5U4GB instead of a 5y3GT. Only one of the 3 amps have failed, and the first guy to get one plays the CRAP out of it and hasn't had a failure. Should I call him right away and tell him to put a 5y3 in it? I don't mind doing that at all if it's gonna prevent a possible failure.

This is the schematic: http://www.thevintagesound.com/ffg/s...e9-a_schem.gif

[Justin Thomas]

HOLY CRAP -- Now I'm nervous. I was just checking back through my parts orders and realized I've made a mistake on ALL THREE of these amps that I've built. I'm not sure if it would cause this failure but it might! ... Otherwise they are bone stock, but I just realized that on all 3 of them I ordered a tube set with a 5U4GB instead of a 5y3GT.

I think that depends. What's the current rating for the 5V rectifier windings? Check your tranny specs. If it's only 2A, yeah, call your customers right away. If it's 3A, then you should be fine with a 5U4. And if your tranny mfr. makes them SO close to spec with zero overage, I'd think that's on them for not SLIGHTLY designing in fudge factor. All my opinion, of course...

Justin

[mort]

I think that depends. What's the current rating for the 5V rectifier windings? Check your tranny specs. If it's only 2A, yeah, call your customers right away. If it's 3A, then you should be fine with a 5U4. And if your tranny mfr. makes them SO close to spec with zero overage, I'd think that's on them for not SLIGHTLY designing in fudge factor. All my opinion, of course...

Justin

Current ratings aren't on the wiring docs they provide. I've just asked for them. But I'm probably ok on the heater current, as the problem is in the B+ windings. I'm still going to verify it. I'm wondering if the accidental tube substitution could/would cause over current on the B+?


Edit: the guy already emailed back [B+ 150mA, 6.3v 7a, & 5v 3a]

[Justin Thomas]

Then I think you needn't worry about the rectifier tubes. And even if a 5U4 can deliver more current, current isn't pushed - it's drawn. Now, if one of your output tubes shorted or drew too much current, or someone put in a 6L6 or other for fun, then it could cook the HV winding. But the rectifier won't SHOVE too much current through.

Justin

[mort]

That's how I was thinking about it as well.. seems the only differences would be output voltage, current capability**, and the heater draw.

[Enzo]

I have no idea how they could determine one way or other

They will look into the transformer, and they can tell if excess current was drawn over extended time, versus excess voltage puncturing the insulation between windings, and so on. They can see if a wire was nicked in the winding process or if the insulation has flaked off a winding wire. they can see if the points where the windings meet the wires to the outside could short to something. Etc.