Fuse blows for no reason?

[txstrat]

Hi folks,

I've noticed this in two of my amps recently.
I can play them and switch them on or off and everything is alright. But every 10th time or so the main fuse blows for apparently no reason. If I replace the fuse, the amps play fine until the next fuse blows, ten switchings later.
The amps are standard circuits with no relay or other techniques. Just the secondary, bias tap and heaters.
Today I changed the fuse in the two JJ 6V6s amp for a 1 amp slo blo (originally it had a 500mA slo blo).
The fuse didn't blow today but for the first time I heard a severe hum. Throwing the standy by switch on and of didn't help). Shut the amp off and on again and it plays fine.

Output tubes???

Any other ideas?

Thanks

[txstrat]

Found the culprit.
When the hum appears, one of the power tubes glows bright blue (internal arcing?).
Gonna replace it and see what happens.

[Gtr_tech]

A blue *glow* is nothing to be concerned with. Normal occurance with alot of output tubes. And....its a nice indicator of the presence of plate/screen supply voltages. Looks like this:
http://farm4.static.flickr.com/3058/...29067e35f5.jpg

If it looks like blue lightning inside the tube.....thats trouble. Means the tube is arcing and needs to go....
YouTube - tube arc
Extreme example of failed/arcing output tube....

[Chuck H]

I'm thinking tubes. Especially if both amps are using the same type/brand of about the same age. Do both amps also happen to have similar plate voltage?

It could also be an intermittant shorting of bias. When the bias fails the voltage goes up and the tube glows blue. It would seem odd that happens on two amps. But it's odd that this obscure problem is happening on both amps anyway. Are there any parallels in the bias wiring between the amps. It sounds by the description that this is happening on one side of a PP amp so that rules out problems with the bias supply itself. So maybe tube sockets? Bias feed resistors?

[txstrat]

I didn't make myself clear enough. Yes, arcing and way more bright blue than normal. I could literally "see the hum".
Could this be the culprit of the blowing fuse? I mean it blew while the std by was in off position.
Have replaced the tubes and will report later if that fixed it.

I don't know if this is the same thing with the other amp (two 6L6GC). But I don't mind the other amp yet. Have to see if it blows a fuse after replacing, but it's in the rehearsal room so it's gonna take me a couple days. Both run at roughly 420vdc.

Thanks for your thoughts.

[R.G.]

No fuse ever blows for no reason at all. Sometimes its because old age and flexing with getting hot has caused stress cracks, but there is a reason.

We had a big variac in the power supply lab once upon a time. It blew its fuse about every 15-20 times we turned it on. One day we scoped the input current with a current probe. Every turn-on it pulled a first-cycle peak of at least 50A. Every so often it would get to 200A, and some of them it hit 200A just as the fuse popped. We then scoped the power supply voltage and current at the same time. It depended on exactly what point in the AC wave the transformer was last turned *off* and at what point it was then turned back on. When the remanent magnetization and a turn on at a bad point in the AC power wave combined, we got the 200+A events. When they cancelled, we got the 50A events.

First-cycle inrush is a common problem with magnetic stuff, especially motors and big transformers. It is possible that this is where the issues started. The interruption of first cycle inrush with a fuse breaking causes some massive transients. It's conceivable that this could have caused some subsequent damage to bias circuit, power tubes, rectifier tube, filter caps, as these are all connected to the PT and the transients.

This will be worse with very old transformers (designed for 100Vac, not the 125Vac that's common today) and even worse if the recording studio or other venue has a high impedance neutral back to the power pole. That condition makes the two halves of a 240VAC phase (in the USA) imbalance so the heavier loaded side has less voltage, the lightly loaded side more voltage.

The big hum makes me think something got damaged.

Obviously I don't know exactly what's going on, but if it were me I'd look at and try to eliminate some of these.

[txstrat]

The big hum makes me think something got damaged.
Obviously I don't know exactly what's going on, but if it were me I'd look at and try to eliminate some of these.

You're right.
The hum was there when the tube was arcing inside. Must have been an arcing between (at least one) thing(s) which are not dependant on the std by circuit (I assume the heater), otherwise the fuse hadn't blown with the std by in off position.
The 500mA fuse blew instantly when I turned the amp on (std by off) so I coudn't make out any hum or arcing.
I replaced the fuse with a 1A and when I turned the std by into play position the tube started arcing inside accompanied by a strong hum. I changed the set of tubes and everything is alright now. Have played the amp frequently since then and it keeps playing without any issues.

Tonight I'll take care of the other amp and see what's happening. Think I'm gonna take it home for a thorough examination.