1969 Super Lead crackling noise and blown fuse

[koffo]

Hi,

Last year I built a Super Lead clone, in December it was finished and it was ok and played beautifully.
In the build I included also a PPIMV and a Zero Loss FX loop.
Everything was ok until one month ago when I started hearing a loud crackling noise from time to time, the it started regularly, and finally the OT fuse blown up.
After that I checked all the connections and soldering, found nothing strange, and after powering up again I found that V3 was blown too. I replaced the PI and the amp started working again, but after a couple of days, same loud crackling noise, and this time the mains fuse blew up.
I started checking all the soldering again, everything looks ok, checked the bias, I tried also to check if I had some faulty tube, but as soon as I put the tubes in another amp they are ok. My amp is still making crackling noises and keeps on blowing mains fuses.

I'm quite desperate...please help!

[danman]

Try replacing each tube, including the power tubes to see if one of them has gone bad. Also be sure that every tube fits snugly in the sockets. A loose or bad socket can cause this issue.

[koffo]

I already tried, but tubes seems to be ok, if I put them in another amp they work correctly. I also checked for loose sockets, also in this case everything seems to be ok. I suspect something related to capacitors or Bias.

[danman]

Did you actually try a different set of power tubes in this amp? Operating conditions and other variables may allow the current set of power tubes to sound fine in another amp but they may not play nice in the Superlead. You need to eliminate every possibility in a logical sequence to track down the issue. The fact that you are blowing fuses would make me want to take a thorough look at the power tubes first and then work my way through the power supply looking for problems.

[neikeel]

Danman has given you some useful pointers.

Unless you have a dodgy bias pot that is making intermittent contact I doubt that is your issue.

Filter capacitors cause ripple and hum noises or die completely with a short or open circuit.

Crackling is usually a poor/intermittent contact or failing valve. After that a poor valve socket contact.

First thing to do is check the tension of the tube sockets and that they are clean without signs of tracking (between pin 3 and pin2 commonly). It sometimes help to paint the pins of the tubes with contact cleaner/fader lube (I use Deoxit gold on a small brush) DON'T SPRAY it every where!!

Other things are a loose wire, a rusty failing fuse holder (seen that twice now - does exactly what you describe) or more annoyingly a failing component such as capacitor or resistor which is intermittent.

Can you replicate the crackle by thumping the head cab in use or (using a chopstick) by moving wires around?

Post some pictures if you are not getting very far.

[koffo]

Hi,

Didn't think about the fuse holder, I'll check it! For what is concerning the sockets, I already cleaned them, they are ok, no traces of rust or burn (the components are brand new!), and the tubes are pushed firmly into them. I also tried to hit the chassis with a plastic hammer in order to check if there are loose wires... But also here I had no luck. I'm starting thinking about the problem could be a coupling capacitor just after the phase inverter. Also in this case, the caps are brand new (and they are Synergy Royal...).
The next week end I'll do the following:
- check the bias circuit (with or without tubes?!?)
- change the coupling caps
- change the fuse holder
- test the tubes with a tube tester

As soon as I can I will post some pictures.

Thanks a lot for your help,
Andrea

[koffo]

Hi,

after a couple of weeks struggling with this issue I finally fixed the amp.
As usual, when the issue is difficult to discover the issues are more then one:
- The fuse holder was ok, anyway, I bought it on a US site (together with many other components), so I discovered that it is not 100% compatible with european fuses (I'm from Italy) because they are slightly shorter and do not provide a proper contact in the fuse holder. I changed the fuse holder to european type, much better but still blowing fuses
- A friend of mine helped me with a tube tester and I discovered that I had two faulty power tubes. I took a matched quartet of JJ 6CA7, one of them was making huge sparks inside just after powering up the standby switch. The other one had a more elusive issue, since it began sparking just after a couple of minutes of use

Now I'm running the amp with just two tubes since one week and it is ok, no more issues. Returning to the tube issues, even now the the remaining tubes are working if I tap gently with a pencil on one of them an hiss starts coming from the amp (like uncontrolled feedback) and then stops suddendly. The other tube does not have this issue.

In conclusion: the tubes were new and had about 100 hours of use... I'll never use JJ tubes again!