6550 Tube Blew - A Few Questions

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.

I have an analog meter. In fact one of the noticeable things about this tube that blew was the shaky needle of the meter when biasing it. It was the only tube that wouldn't stay constant but would go like a pendulum from a low to high voltage and back again. I guess it was getting ready to go.

I could certainly change a resistor, I'm not a stranger to a soldering iron and a PCB, but I've no idea what I'm looking at, without some assistance. I don't even know if the amp has been affected beyond the blown tube. I guess I'll order some new tubes and see what happens.