6550 Tube Blew - A Few Questions


Hi. While listening to my system the other night, I noticed one of the power tubes for the right channel glowed extra bright, and static came through the speaker. I immediately shut down the amp, and haven't turned it back on.

Is it possible to check for damage to the other channel or other parts of the amp without installing a new tube to replace the blown one? I don't want to buy a matched pair and then discover that the amp has other damage.

Is it safe to test each channel with the other channel's tubes removed? Thanks!

Terry
rwinner
Connect dummy 50w/10Ohms resistor to each of the channel before testing.
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.
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
Well, I got the replacement tubes today, replaced the right channel with them, and am not getting power at all.

I checked the fuse, and it was still good. I checked inside, and could see no obvious damage or the smell of something burnt. What are some of the obvious things I can check before I bring it in for out-of-warranty repair?
Did you ID the bias resistor as mentioned above?
I believe that you have 2 tubes per channel and bias resistor is shared between two tubes. It's realy not hard to ID low-resistance resistor and test it for open circuit.
I thought of doing that, but I understood you to say that the bias resistor would not prevent the amp from turning on. I;ve already checked the filaments on the tubes, and they are intact. Should I also check the bias resistors?

Terry
Blown bias resistor would prevent tube or pair of tubes(for push-pull amp) to turn on.
Just checked the bias resistors. They were easy to spot - 5 watt 10 Ohm jobs. They tested for 10 ohms, all 4. I tested them out of the circuit. What else might I check?
It's the front switch. :0
Mazeltof
Yea, I was hoping it was something simple like that. It fooled me, happening right after the tube blew. I guess I learned a little about my amp, though. Thanks a lot!