Topic: i keep blowing a fuse... (tube amp content)

[macdaddy]

same deal - one of my 4 el34 tubes gets hot - the same one - then it turns real bright orange, and then the fuse in the back blows...

problem is, i dont know anywhere here that i cxan take the am to have it looked at by somebody that knows what they are looking at - i wouldnt even know of such a place...

i switched out the 12at7 tubes, and one of the two 12ax7 (didnt have another one) - this lasted longer before the fuse blow, but i still had a fuse blow...

this is after hours and hours of flowless operation...

[Liquid Meat]

I am shooting in the dark here a bit, but I think that is an over biasing (bad cap/rectifier?). In any case, I wouldn't run it anymore until it gets fixed. What kinda amp is it?

[macdaddy]

jolida 302a

bought it used years ago - it was heavily modified...

+t

[cheshirecat]

Did you try swapping the el34s to the other side to see if the problem follows the tube or the socket?  That would be telling and help narrow things down.  Let us know what you find.

[macdaddy]

another round of Ts for the help...

i will try switching the tubes around...

i also have two new 12ax7 tubes on the way (the ones i amusing now are five years old, while the others tubes (4xel34 and 2x 12at7) are all relatively new...

i hear a whooshing/static/white noise/wave sound every once in a while, and it is the sound that precedes the fuse blow (i switch off when i hear the whoosh). and the whoosh does nto change with the volume - it is the same level regardless of volume...

could a bad 12ax7 cause the problem, but that problem visibly shows in the super bright el34, or is it a case of - the tube that glows differently is the tube that has the problem (in which case i should focus on the els and not the other tubes)..?

[Genghis Cougar Mellen Khan]

another round of Ts for the help...

i will try switching the tubes around...

i also have two new 12ax7 tubes on the way (the ones i amusing now are five years old, while the others tubes (4xel34 and 2x 12at7) are all relatively new...

i hear a whooshing/static/white noise/wave sound every once in a while, and it is the sound that precedes the fuse blow (i switch off when i hear the whoosh). and the whoosh does nto change with the volume - it is the same level regardless of volume...

could a bad 12ax7 cause the problem, but that problem visibly shows in the super bright el34, or is it a case of - the tube that glows differently is the tube that has the problem (in which case i should focus on the els and not the other tubes)..?

I'd probably lean towards the el34, one of the power tubes being problematic IF it's a tube issue, it's possible the tube could have a short or something, you never know.  Do you have any local tv repair shops/used electronics stores locally that might have a tube tester?  My amp wasn't acting right, I took the tubes to someone with a tester and found out one of my power tubes was bad (half the output), even one in my extra pair, so I'm ordering a new set this week.  The bias pot in my amp had a bad solder connection, to make a long story short, an internal problem probably put an abrupt end to that tube's life, the tube in question went red due to the bias pot issue.  Point being, even if one of your tubes ends up being bad, watch it closely after you replace it and make sure everything is working good, tube bias etc., I wouldn't rule out an internal problem if a tube is bad, it might be what caused the tube to fail.

Best of luck, I hope it's simple...

[BobW]

Bright orange power tube is a sign of high plate current.  Plate voltage high or bias voltage low, usually.
Bad tube can do it as well.
Get all 4 EL34's into a tube tester as soon as you can.
Blowing the fuse repeatedly will stress other amp components
can you get a different pair of EL34s to swap into that channel ?
swap the preamp tubes left and right

1) be sure that the speaker connections aren't shorted
2) swap left and right speaker wires to be sure it is not a shorted speaker driver
3) bias adjustment on the JoLida is strongly suspected, as it is user adjustable  - what does it measure ? How does it compare to the other channel ?  Remember, suspect low or missing bias voltage
4) yes, swap the preamp tubes ! 
5) JoLida is "push-pull", so usually if one tube is glowing, it is just that tube itself. swap left-right pair and see if it follows tube, but only do this once and if you have no access to a tube tester, which is the safest way to go.

The best thing about stereo is the ability to swap stuff left and right.
Power tubes should be swapped in pairs.

Be sure to unplug before swapping tubes for safety.
Keep one hand behind your back and away from the amp when doing the bias adjustment.
To this day, I still keep my other hand on my trouser belt behind me when making HV measurements as I was taught all of those years ago.

The one thing that "modern" vacuum tube amps are not is safe.
No US manufacturer would dare sell a bare-tube amp since the 30s.
I always fret my JoLi when the kids come to visit.

[cheshirecat]

Keep one hand behind your back and away from the amp when doing the bias adjustment.
To this day, I still keep my other hand on my trouser belt behind me when making HV measurements as I was taught all of those years ago.

Specifically I believe the 'rule' is to keep your left hand behind your back.  The thinking behind it is that in the event you touch something with HV the electricity does not have a path through your heart and you have a better chance of surviving.  Food for though.  Either way, let us know what you find.

[macdaddy]

i havent biased the tubes in a while - so you think that it could just be a bias issue..?

the only thing is, on one of the tubes, i cant get a reading - i know the electrodes are in the right place, but for some reason, the meter doesnt read...

[cheshirecat]

That could definately be an issue.  I had something similar come up with an ST-70, there was a burr that was shorting the bias connection to ground so the amp would come up, start to play a bit then go south and eventually arc the rectifier to the power tranny 

Could be a bad tube, could be that the amp has killed the tube.  Try a new set of EL34s, you can probably pickup a cheapo set at the local music shop to use for testing.

BTW, you said the amp is modded, do you know what the mods are?

[macdaddy]

oh yeah - i bought it on a'gon from a real nice guy who wrote to me in detail regarding the mods...

i will dig up all the details...

i probably should send the guy an email, and see what he says...

[BobW]

i havent biased the tubes in a while - so you think that it could just be a bias issue..?

the only thing is, on one of the tubes, i cant get a reading - i know the electrodes are in the right place, but for some reason, the meter doesnt read...

Would that be the same tube that's glowing ?.....
Digital meter ? Any sign of it reading polarity opposite of the other channel ?

[macdaddy]

i havent biased the tubes in a while - so you think that it could just be a bias issue..?

the only thing is, on one of the tubes, i cant get a reading - i know the electrodes are in the right place, but for some reason, the meter doesnt read...

Would that be the same tube that's glowing ?.....

nope.

Digital meter ? Any sign of it reading polarity opposite of the other channel ?

(i am clueless about alot of this stuff - thanks for the help )

[pfife]

mac, I have no advice, but I just wanted to express my condolences for your situation.  I know I'd be stressed-the-fuck-out about it.

[cheshirecat]

i havent biased the tubes in a while - so you think that it could just be a bias issue..?

the only thing is, on one of the tubes, i cant get a reading - i know the electrodes are in the right place, but for some reason, the meter doesnt read...

Would that be the same tube that's glowing ?.....

nope.

Can you provide more info on which tube is glowing (ie 1-4 left to right) and which gets no bias?... If one socket is grounding out the bias (ie you're getting no reading) you very well may get the the behavior you are seeing.  It might be worth popping the amp open and taking a look to make sure none of the pins on the sockets have anything loose or hitting the chassis.  Maybe take a quick picture of the inside and post if thats an option?  Might also be able to see if something has cooked itself inside the amp.

Oh yeah, be careful when you open that guy up, the power supply capacitors will most likely stay charged for a while (20-30 min?) so you might want to be cautious and let it sit (unplugged) for a while before taking the bottom cover off and poking around.