6550 Tube Blew - A Few Questions

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.

1. The simpliest way is to plug in the tube and if there's no bias voltage reading, the bias resistor is blown. What it simply means that cathode power supply has an open circuit and tube doesn't emit electrones to anode.
To ensure safety of remaining circuit and tube it's essential to visually inspect DC supply caps, elements and PCB itself around the blown tube with magnifying glass. DC caps may look like baloon or have traces of liquid waste around them. If that's the case you should replace them first.
2. Visual ID bias resistor. Locate bias resistor between cathode pins of the tube (usually pins 1 and 8 but check with any tube dealer who will share the pin geography of any tube you'll ask). Bias resistor most of the time 10Ohms and sometimes 100 Ohms and designed to get your voltage drop reading plus to protect the circuit by cutting off power from cathode. Tube may well work without bias resistor and you can bias it by measuring current instead FYI. To ensure that it's blown just test the sucker for open circuit.
3. If bias resistor is OK and your visual test is negative, consider your tube's gone 'piecefully' and you're safe to replace the new one.

Do all your tests with mentioned above dummy load resistor instead of speakers and connect your unpowered preamp or source to input.

Unfortunately not all of tube amps are plug-n-play when they blow tubes, but having been dealing with number of guitar amps seen everything. I hope that it's just a tube that you need to replace, but don't know precisely how Jolidas built. No surprise to me that they might be better built than VTLs and all you need to do is just to replace tube and worst case tube+bias resistor. If you'll be aquainted to the luxury to get schematics and circuit element description from manufacturer, you'll also benefit and make your life a-bit easier.

Continue to post what you find.