Tube Glows / lights up

[guitartango]

Guys

How can you tell when a tube has gone? One of my EL84 glows brightly (like a xmas tree) and the amp sounds like sh***. Not got a spare tube so cannot test this out. My guess is that the tube need replacing.

Fender blues JNR with 2xEL84

[tunghaichuan***]

That doesn't sound right. EL84 tubes should have a faint orange glow from the filament and maybe some blue flouresence, but should not glow brightly. Is the large grey/black structure glowing bright red? If so you tube is about to go and could take out other parts of your amp. Get a new set of EL84s and try them out. I wouldn't play the amp as is.

If the new set of tubes doesn't cure the problem, I'd take it to a tech to get it sorted out.

tung

Guys

How can you tell when a tube has gone? One of my EL84 glows brightly (like a xmas tree) and the amp sounds like sh***. Not got a spare tube so cannot test this out. My guess is that the tube need replacing.

Fender blues JNR with 2xEL84

[jim p]

I am glad the previous post has warned you to stop using the amp. I kinda thought your tube is going bad and could lead to runaway with excessive plate current. So since first looking at your post found this site on the web
http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
Search through to the blue glow section to give you an idea of what is happening with your tube. Being the tube is in the push pull output section of the amplifier it is grid biased this is the worse case for a soft (loss of vacuum) tube. The ionization of gas in the tube will cause current to flow on the control grid turning the tube on harder leading to excessive plate current which can fry your output transformer. So as the previous post states shut the amp down, look into getting a new set of tubes and rebias the amp if needed.
If you had a cathode biased amp this problem is a little less because as the plate current rises so does the cathode voltage, but it still may not be enough to prevent major problems. I had a brand new EL84 tube that did this and it toasted my cathode resistor in a Valve Jr. It is one reason I am not so sure a high wattage rating cathode resistor is a good idea I think frying a resistor is better then frying an output transformer.
With the grid bias you have no cathode resistor so there is no added limiting built into the design of the amp except for the lower resistance value of grid resistor that is used with grid bias.
Hope you need nothing more then a new set of output tubes and this helps.

Note: I was just looking at the schematic for the Fender Blues Jr. and see there is no bias adjustment? They have a fixed negative voltage on the grids so maybe if you order a set of tubes from Tube Depot or Tube Store ect. you will need to tell them that the set is for a Fender Blues Jr. so they can match them for the set bias. Also you can surf the web to see if anyone has added bias adjustment to the amp no big thing just installing a pot in place of fixed value resistor. Would be nice if they put some resistors in the cathodes of the output for bias adjustment too but? Maybe Fender is doing a Mesa Boggie thing trying to make you buy there replacement tube by having fixed bias.

Note: I surfed the web on Fender Blues jr tube bias mod and by what I am running across Fender is overdriving the EL84 tubes on this amp. So if you are able or know someone that is I would think about adding adjustable grid bias to the amp, your new tubes will thank you.

[guitartango]

Thanks for the info, lets hope it's only a tube gone and not the amp.

[ibanezjunkie]

if one tube is really bright, and one is dull, couldnt that be a biasing problem?

[jim p]

In all the amplifier schematics I have looked at I have not seen any that use separate bias to the tubes in a push pull amp. That is why you need to get a matched set that should be matched for bias point and gain I would hope. On the tube bias being wrong in the Fender Blues Jr they are nice enough to post there schematics with voltages on there web site and they have a bias voltage of -10.7 with a plate voltage of 329 volts. Looking at a Tung-Sol data sheet for the 6BQ5 for two tubes in Class B they have a grid bias of -14.7 with 300 volts on the plate. Now the amp should be biased more positive then -14.7 for class A/B but I don’t think as high as -10.7 so I think you really could use adjustable bias on the amp. In posting on the fact that the Fender Champ 600 has too high plate dissipation by the numbers on there schematic 14.8 watts when the max for the tube is 14 watts someone posted back that Fender just runs tubes too hot. I can’t say that this is a fact but on these two amps it looks to be the case.
If you want more information on amp biasing at “Tales from The Tone Lounge” they have a bit on biasing a Marshall JCM800 that goes into some detail on it.

Note: Going by a table of cathode currents for various tubes in push pull amps the EL84 is suppose to have approx 22mA of cathode current when correctly biased. So class B would have the tube just at cutoff at -14.7 the transconductance of the tube is 11.3mA/volt using those numbers the bias voltage should be approx -12 volts so again running bias at -10.7 sounds like it maybe a little hot.

[guitartango]

Replaced the tube with a new pair and the amp sounds fine. Panic over

[jim p]

Glad to see you got your amp sorted out. I just got an old Carvin 100 watt tube amp going again and found with an old basic tube tester its problem was a blue tube. The 3 amp line fuse was blown when I started to look at it so figure the bad tube could have been drawing close to 1 amp. Can’t think that is good for the output transformer good thing the one on this amp is stout enough.
Makes me think with this in mind you may want to get a spare line fuse if the one in your amp has been weakened by high current from that bad tube you replaced.

[Radioboy950]

Replaced the tube with a new pair and the amp sounds fine. Panic over

Good to hear.
I once had a 6L6 turn blazing orange/red...not good.
I caught it before doing any amph damage.

[M29]

Good tip jim p,
Fuses are cheap it never hurts to put a new one in. I had one that partially blew and you could not see it under the end cap. It looked fine like a normal fuse. It drove me nuts for a long time. The amp had a very thin sound and was distorted until I changed the fuse just for the heck of it. Bingo!

[jim p]

Guessing what happened with this amp it probably would be best for the amp to have an internal fuse on the plate supply this amp does not. Also I posted on the thread “Clean sound from a Peavey Windsor” how with the circuit I added to gate the tubes on and off you could add a protection circuit if the bias supply fails. One could also be added using a low value cathode resistor on each output tube that would shut down the plate supply by adding a mosfet in series with the plate supply. Monitoring the cathode current this way would handle both cases. So with a comparator op amp on each output tube a flip flop and two mosfets you would be all set. Even hook an LED up to the output of the flip flop to indicate the fault mode. Plus having cathode resistors helps to bias the amp when you change tubes or to check the health of the tube set in the amp.