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Old 20th December 2010, 08:12 PM   #1
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Default suspect a bad tube-any simple checks?

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
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Old 20th December 2010, 09:19 PM   #2
zacster is offline zacster  United States
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Try swapping the tubes around one at a time to see if it follows the bad tube.
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Old 20th December 2010, 10:43 PM   #3
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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.
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Old 21st December 2010, 03:30 AM   #4
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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
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Old 21st December 2010, 06:28 AM   #5
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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.
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Old 21st December 2010, 07:49 AM   #6
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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?
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Old 21st December 2010, 12:48 PM   #7
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If the amp sounds good I would not worry about it. If the amp sounds bad you need to change the tubes whether they are actually bad or not.
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Old 21st December 2010, 01:36 PM   #8
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by fenderjazz View Post
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.

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Old 21st December 2010, 02:22 PM   #9
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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
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Old 21st December 2010, 02:27 PM   #10
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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
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