Why are my amps doing this?

[Christof]

Is it standard for an amp to burn resistors when a power tube goes out? I was not in the room when this happened so the actual cause of the fault I'm unsure of. Maybe a cap exploded and made the tube burn out or visa versa? Anyway one of my Electro Harmonics 6l6eh tubes is cooked and I found that one of the resistors burned up, a cap was bleeding oil all over and another large resistor had fallen completely off the PCB. My amps are Dared VP20 monos, which are obviously cheaply made. Any idea whats going on here? The amps are powering a pair of Cain & Cain Abbys.

Thanks for any help
CHris

 

[kevinkr]

This is not at all uncommon, it is most likely that one of your output tubes failed.. I am going to assume that the cathode bias resistor and capacitor are toast from what you say, and if there is a screen resistor for that tube then that is probably burned up as well. I have never seen one of these amplifiers, but it doesn't follow that it was cheaply made because a couple of components failed when one of your output tubes shorted out catastrophically. I would recommend the JJ 6L6GC over the EH product, but better still would be either NOS 6L6GC or 7581.. I am not sure either would fit the size constraints, another choice if there is room would be one of the new KT66 clones. Take a look at the other amplifier to see what component values were used for the bias and screen circuits. Also double check the grid resistors and coupling cap to make sure they are not implicated in this failure.

 

[Christof]

If a faulty power tube can wreck havoc inside the amp for future reference how do I prevent this (faulty tubes) from happening. I also had a pair of 350b monoblock that had a tube failure which resulted in burnt resistors. Both sets of amps were made by Dared. I am a cabinet maker and know very little about electronic/tubes other than the fact that these amps sound very nice with my Abby speakers. Would it be in my best interest to dump the tube gear and go strictly solid state or is this just a freak thing that happens?

Thanks for the help
Chris

 

[kevinkr]

God No! Walk slowly away from the solid state panacea! 😀 😀
This sort of failure should not be a normal occurence, but I recommend you stay away from current production power tubes and buy some NOS 6L6GC or equivalent as I suggested in my previous post.. Also try to find a good service tech in your area. I used to manufacture my own line of amplifiers and in my experience a tube amplifier is only as reliable as the tubes you stick in it. The best thing is that although they are a little more expensive they tend to last a lot longer. Another option would be to have a tech install fuses in the primary center tap of the output transformers - about 250mA will prevent nuisance blowing, but will open quickly when an output tube fails. I also recommend he bypass the fuse with a good film capacitor in the range of 0.1uF/600V - note that whatever cap he puts here has to survive the full plate voltage if the fuse opens up.

I have an SE amp here that has run on the same set of NOS tubes for over 7 yrs now without a single problem. (I actually used to manufacture tube amplifiers and it is not hard to design them not to self destruct when a tube fails.)

Try Kevin Deal, he may have some specific tube recommendations for your amplifiers.

 

[Apogee]

Christof

Jadis had a similar problem back when the Defy 7 was designed... From what I was told, the output tubes at that time suffered from very poor reliablity so they had to figure out a way to protect the rest of the unit.

The Defy 7 is a much larger amp but the principles would be the same...

I've attached a schematic so you can see how they went about protecting the output tubes with fuses...

Good luck,