6550 Tube Blew - A Few Questions

Dunno what amp you have, so take what I say with a grain of salt. It's possible that the 6550 arced and blew a bias resister. OTOH, maybe it would just be better to call the factory tomorrow and talk to the folks who know YOUR amp.

Ok, having said that, I'll tell you my tale. I own an ARC tube amp, which uses (8) 6550 winged C tubes. [As an aside, I later upgraded to the KT-120s.] Doesn't happen often, but maybe twice since I've owned the amp I've blown power tubes. Both times the tubes took out a bias resister which means that I had to call in my friendly ARC authorized service tech to replace the resisters. It's an easy fix.

Well, the last time I blew a tube, I called the factory and asked if I could run the amp without the tube or with a fresh tube even though I couldn't bias it. The factory said if I did, I would invalidate the warranty -- so I didn't.

In your case, call the factory. But I'm curious what would happen if you dropped in a fresh tube. If it biases, then maybe you didn't blow a bias resister and the amp might be good to go.

Gary (Hifigeek), an authorized ARC service tech, once metioned on the Forum that even if the bais resister didn't blow, the tube arc might have changed the resister's value. Another reason to ask the factory.

Good luck and let us know how you make out.

Ok, thanks for the info. I didn't know about the bias resistor blowing. I'll certainly call Jolida today and ask about it, and where I can obtain 0service if necessary.

Iirc, the same thing happened once before, and I didn't need to replace a resistor. But, I was thinking, if its just a resistor, shouldn't be too difficult to do myself if I can be told which resistor. Another reason to call Jolida and do some Googling.

Terry

When I owned VTL MB100 in this unit tubes didn't die 'piecefully' and once a goo chunk of PC board was fryed. It was designed so that if bias resistor blows the rest of the circuit stays untouched, but for some reason it didn't blow fast enough that caused bias supply caps go bad and literally explode and setting PC board on fire. I suspect that reverse B+ voltage 'visited' the bias supply due to the blown grid in the tube which is the usual situation with power tubes.

The tube has a-bit more pieceful ways to die, but you never know and need to check bias resistor.
Each resistor has color code but I would guess that in your case it may be 100Ohm. In any case you should refer to written notes or color code in order to get proper readings. Resistors are checked out of circuit with multimeter set to measure Ohms. It's enough to de-solder one lead and measure resistance. A blown resistor would show an open circuit(just like blown fuse). To mount resistor onto PCB or to the wireing of amp isn't a big deal even for the amateur. Check u-tubes like that http://www.youtube.com/watch?v=J5Sb21qbpEQ or similar. I'd emphisize the safety items you should have even for minor work with solder.

07-05-12: Marakanetz
When I owned VTL MB100 in this unit tubes didn't die 'piecefully' and once a goo chunk of PC board was fryed.

Thanks for the info. I can certainly solder in a resistor, but I need to know which one. I guess the first step would be to get a new quad of tubes, so I can see what is working and what isn't?

Then I can decide whether to ship it somewhere or open it myself. I like the sound of the amp, but if tube amps are _this_ much trouble, maybe a good SS amp is the way to go. :)

Terry

Invest in a DVM (digital volt meter) That way you can measure resistors when you need to. Be advised, sometimes there are no bleeder resistors in the power supply. In this case the power supply likes to hang on to its high voltage. If you are in L.A. I can take care of this for you. If not, ask the factory how long it takes for the power supply to discharge. You don't want to get zapped.