6550 Tube Blew - A Few Questions

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.

Hey, that was a great post. I think I found the resistors you said on Ebay (who has 50 watt resistors lying around? :) ), and I'll start on these tests as soon as the tubes arrive. Thanks a lot!

Terry

Forgot to mention that I prefer to bias amps on dummy load instead of speaker. When I owned VTL, I implemented switch to dummy load and used it even if I wanted to simply stand by the amp with no listening(only in case if I plan to listen to it multiple times per day).

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