• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

suspect a bad tube-any simple checks?

fenderjazz

Member
2010-12-11 2:23 am
I have a Mesa/Boogie 400 plus amp that has 12 6L6GC power tubes. When preamp is powered up the heaters all light up and no unusual looking power tubes when they are powered up but when the tubes are powered down to standby I hear a loud popping sound accompanied by a blue flashing from an area where 4 of the tubes are. Sometimes one of the 8 amp fuses will blow. How much does it cost for a tube tester, and are there any simple things to check to try to narrow down which power tube might be faulty? Thank you, Roy
 

bob91343

Member
2010-03-11 10:43 pm
Tube testers are available for around $50 more or less.

I wouldn't assume the blue flash is coming from a tube unless you see it. Realize that when the unit is in standby, there may still be high voltage around, higher than when in operating mode, and that could encourage flashover somewhere.
 

fenderjazz

Member
2010-12-11 2:23 am
Bob, I have the cover off and I can see the flash, it (the flash and the pop) happens when the 6L6's are powered and the standby swithch is being pushed in. 50 dollars for a tube checker is something I would invest in. It would cost me nearly that much to drive to a guitar tech, wait a day or more and then drive back to pick up the tubes. I modified my Twin with the one Ohm resistors so I could bias my own amp with my multimeter. Is there a similar point on the 400+ where I could take a current measurement off of each tube or is the best thing to get a tube tester? What tube tester would be the best to invest in? Thank you, Roy
 

bob91343

Member
2010-03-11 10:43 pm
You can always put a sampling resistor in a cathode to measure current. Keep the resistance low enough not to affect performance.

Unlike so many, I believe a tube tester shouldn't be asked to do much. So in my opinion, the simplest of testers is fine. Measure emission, check for shorts.

However in your case you are seeing a flash which would probably only occur under high voltage conditions, which no tube tester supplies. So a tester won't help you.

Substitution is the best test anyway. Have spares and swap them in to see if problems go away.
 

quikie22

Member
2009-09-24 1:53 pm
probably when you are putting the tubes in standby, you are suddenly cutting off the loading on the power supply and causing the B+ voltage to go up, hence causing an arc/spark with consequent pop.
Sorry, but I am not familiar with the mod you have described. Did the problem arise after the mod?
 

M Gregg

Disabled Account
2010-06-28 11:04 pm
UK
I have a Mesa/Boogie 400 plus amp that has 12 6L6GC power tubes. When preamp is powered up the heaters all light up and no unusual looking power tubes when they are powered up but when the tubes are powered down to standby I hear a loud popping sound accompanied by a blue flashing from an area where 4 of the tubes are. Sometimes one of the 8 amp fuses will blow. How much does it cost for a tube tester, and are there any simple things to check to try to narrow down which power tube might be faulty? Thank you, Roy

What makes you think the tubes are faulty? If they work OK with HT on why should they short over with the amp in standby. The increase in HT in standby should not cause the tubes to arc / flash over.

Hopefully not inside the O/P Tx!

What is the value of HT in standby?
Where exactly can you see the arc?
As above does the fault move if you change the position of the tubes swap them over?
Are you sure its inside the tube and not on the socket or circuit board?

Remember a short from O/P Tx to Gnd through a tube can take out the O/P/ Tx. Don't be tempted to increase the rating of the fuse.

Regards
M. Gregg
 

fenderjazz

Member
2010-12-11 2:23 am
Mr Gregg, I wasn't too clear, sorry. The amp idles all day in standby. It blows fuses only when in "play" mode. Then the fuse will eventually blow a without even playing a note through it, just idling there in play position for maybe 10 minutes or so it is ok. It has a cooling fan for the tubes and that is working well. Its symptoms I have noted have been a loud popping noise when going from "play" mode down to standby, like when I change an input and that is when the blue flash is visible. I have not been able to find a schematic for the 400+ amp yet. The mesa site only has the 400 which is a different amp with 10 6l6's instead of the 12 mine has. Mine can take 2 ohm output, that seems to be the big difference between the 400 and the 400+. It is a pretty large monetary investment to buy 12 new 6l6 tubes so that is why I was hoping to test my tubes to see if that is the ONLY problem I have here. I do have a pair of known good 5881's I could try to place in there to see if I could find a change that might cure the problem, and no I won't be tempted to mess with that 8 amp fuse! I really need to find a service manual so I know how and where to take voltage and current measurements on this amp. I know how to do it on my Twin, but not this amp. Thank you, Roy
 
Quickie22-no I haven't modded this amp at all. I had just read about a mod where someone had a series of toggle switches rigged up to take out pairs of tubes at a time, I don't know a thing about it I just know it has been done by someone. The amp does sound bad lately, so I guess an entire set of new tubes is probably in order. It takes the 6l6GC's and I am thinking the less expensive 5881's would be ok for it. Thanks, Roy
 
Yipee! I got lucky, I changed out the outermost pair of tubes near to where I saw the flash and the amp is working fine again, sounds like it should and no fuse blowing. I am going to go ahead and change out the entire set of 12 and then see if I can find a way to match up the tubes I have left over into spares for my Twin. Thanks for the help, all. I still want to figure out how to check all the plate voltages and currents in this amp though. Roy
 
I still want to figure out how to check all the plate voltages and currents in this amp though. Roy

Schematic would help. To check plate voltage, simply look at the pin configuration of the tube and measure voltage at that pin. If the amp is fixed bias, you're done. That's your plate voltage. If the tube is cathode biased (resistor in series with cathode sets bias) then measure at the cathode for that tube, and subtract that from the voltage measured at the plate. If the output tubes are run in parallel, then you only need to measure the plate of one of the paralleled tubes.

To measure current, you need a resistor in series with each tube. If the amp is cathode biased, simply measure across the cathode resistor and use Ohms law to calculate current. If it's fixed bias you need to add a resistor. It can be in series with the plate or cathode - technically, matters not which, if it's a low value resistor. A 1R resistor should be fine.

Sheldon

edit: Actually the current sensing resistor is a safer in the cathode, because the voltages are low there. You can wire up a small pin socket for each tube, to make it easy to probe without opening the amp.
 
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Which fuse does it blow?

Usually when power tubes work they are good but,

If you are replacing tubes don't forget that :

6L6
6L6GB
5881
are rated for plate voltage up to 400 volts and are 25 watt tubes but :

6L6GC
6L6WXT+
6L6WGC
are rated for plate voltage up to 500 volts and are 30 watt tubes.

BTW those Fender Twin Reverbs are the loudest guitar amps I have ever heard.


 
There is one fuse on the front panel of the 400+ that is the one that had been blowing, PW. I blackfaced my Twin and changed to one 50 watt C12N and one 75 watt Eminence Governor so the sound is sweet, Right, the 5881 pair is just temporary, they will work in the 400+ according to folks that know a lot about those amps. Apparently they are biased very low for long tube life (ha ha). I am going to try to find 12 new tubes as soon as I can afford them. Thanks again, all. Roy
 
not trying to sound like a smart a#@

wow, flashing lights and blowing fuses usally means you have some open resistors in your bias circuts, usally caused by either a shorted cap, or shorted tube.

moving the tube group to another side of the amp would help determine if the tubes caused the problem, if that side of the amp starts to spark, and different fuses blow, you know that you have burned out some resistors on that side of the amp board.

simple tests with out a tube tester. pull all your tubes and mark where they cam from. find a way to get them tested. someone near you has a tester. what city/state. back to your amp board. with the tubes out and unit unpluged, check you resistors for open circuts, and your caps for shorts. open resistors are hard to identify by visual inspection but easy to find with a mult meter. while your in there with a meter, double check your diodes.

don't put the tubes back in til you get the tubes tested, and you find out what is going on , on the amp board.

main thing is if the fuses were never over sized, any fixes should be easy and cheap. I had an amp recently that was blowing fuses, root cuase never found, but an open resistor was, the fuse was oversized so I was worried about tranny damage.

when replacing resistors take note of the watt rating. if you pull out a 3 watt resistor, but only have half watt and/or 1 watt resistors, 4 series paralleled 1/2 watt resistors will give you 2 watts while keeping the same resistance. which would be fine for the short term. don't put a 5 watt cement resistor. if you line in the nyc area, I can lend you a tube tester. or you can bring you amp over, and we can go thru it real quick. good luck.