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Blew a kt88 in a mesa four 88......

#82
BogeyBass said:
what he said wasn't all that bad.
anyways that head has a dual bias setting switch. which could be rather convenient.

use one side of the bias switch for full power the other side for half.

most likely the tube was on its way out anyways and possibly little hot on the bias for half power.

I don't know the method they are using for half power. usually a voltage drop. sounds like they are keeping the voltage high and actually halfing the quad to a pair.

so usually in good practice the amp
would include a bias circuit that automatically changed for half power. don't know if it does or doesn't but the dual bias switch could be used to your advantage.

like I said if possible bias the quad to one setting and bias the pair on the other. use the correct cab impedance like you did. And with a new pair of tubes I wouldn't worry bout it to much. since you did bblow the tubes. check the screen resistors for tolerance and clean the tube sockets and check for carbon tracking. cause yes if this amp is actually getting 125 watts out of a pair. then yes arc over is pretty easy to get. the plate voltage has to be atleast 600 volts or higher to get that much power from a pair. And yes kt 88 are little unstable at high plate voltage. And you wouldn't want them biased to hot. be interesting to see the schematic for this amp. see if mesa actual used pretty common methods to tame instability with this tube type.
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Doesn't work like this at all, not even close. The bias switch and bias functions have nothing to do with 2/4 tube operation.

DO NOT MESS WITH HOW THE BIAS SWITCH OPERATES, period.

My comments were with respect to internal tube structure flash-over, not external to the tube.
 
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#83
well darn that sucks with bias thing.
still interesting option that it offers.

likewise though still good practice to check screen resistors and clean the sockets after blown tube. plate resistors to if it has them.

plate voltage has to pretty aggressive to get 125 watts out of a pair. hopefully a tube upgrade from factory tubes is all that was needed. And the op won't have to worry further about 1/2 power blowouts

be curious to know what plate voltages are being used
 
#84
If you knew anything about the KT-88, the maximum rated plate voltage is 800V, and 235mA plate current. It's in a different class than just about everything else.

The factory tubes are the best tubes available in the KT series, I recommend sticking with the factory tubes in these amps.
 
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#85
BogeyBass said:
what he said wasn't all that bad.
anyways that head has a dual bias setting switch. which could be rather convenient.

use one side of the bias switch for full power the other side for half.

most likely the tube was on its way out anyways and possibly little hot on the bias for half power.

I don't know the method they are using for half power. usually a voltage drop. sounds like they are keeping the voltage high and actually halfing the quad to a pair.

so usually in good practice the amp
would include a bias circuit that automatically changed for half power. don't know if it does or doesn't but the dual bias switch could be used to your advantage.

like I said if possible bias the quad to one setting and bias the pair on the other. use the correct cab impedance like you did. And with a new pair of tubes I wouldn't worry bout it to much. since you did bblow the tubes. check the screen resistors for tolerance and clean the tube sockets and check for carbon tracking. cause yes if this amp is actually getting 125 watts out of a pair. then yes arc over is pretty easy to get. the plate voltage has to be atleast 600 volts or higher to get that much power from a pair. And yes kt 88 are little unstable at high plate voltage. And you wouldn't want them biased to hot. be interesting to see the schematic for this amp. see if mesa actual used pretty common methods to tame instability with this tube type.
Click to expand...
Im not gonna get into terrirtory that im unfamiliar with as far as the kt88, but the bias switch: operate/bias then right beside it is a little plastic pot (not a pot.i hope u know what i mean) then beside it is the little led. Go check one out on google and hit images- but the manual sais only set the bias on full power mode. But i am quite curious myself about the voltage plates on what they read on half power on each tube as well as full power. Havent powered it back up yet but im gonna make sure that the bias wasnt set too hot. Cause typically on a guitar amp, most of the boogie guys tend to set their values a little hotter to get some extra tone out of their amps. Now they go through tubes like crazy doing that, but to each of their own right......im hoping this amp wasnt treated the same way and someone thinking glory tone and cranked this bass prodigys bias level out the roof.
 
#86
Do NOT attempt biasing in 2 tube mode, that will result in an inaccurate reading.

Nothing in this power amp is "conventional", the screen resistor uses a compensating network that's critical to the tube's stability. There is no conventional bias set cathode resistor because of the way the new bias set circuit works. To assume wrong and measure the bias across what looks like cathode resistors could have fatal results. The plate voltage is about 700 volts, nothing for a DIY'er or non-qualified technician to be messing with.
 
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#88
jastacey said:
Why doesn't the owner just box the amp up and send it back to the folks who built it, this is not a traditional designed tube bass amp, nothing like factory service, they have the knowlage and correct test equipment, shipping cost can't be that much.
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Waiting on tubes. (Will be here tomorrow) calling mesa when the tubes come in. And scuffed around reverb and bought another terror bass 1000 for backup.....lol. but yes that is my plan if mesa tells me to box it up. D
 
#89
agedhorse said:
Do NOT attempt biasing in 2 tube mode, that will result in an inaccurate reading.

Nothing in this power amp is "conventional", the screen resistor uses a compensating network that's critical to the tube's stability. There is no conventional bias set cathode resistor because of the way the new bias set circuit works. To assume wrong and measure the bias across what looks like cathode resistors could have fatal results. The plate voltage is about 700 volts, nothing for a DIY'er or non-qualified technician to be messing with.
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Just got off the phone with a guy named chris in customer service for mesa. First of all, very informative and understanding. I described in as much detail as to what happend and what power, ohm load i was running. He even put me on hold for a few minutes and im assuming, went to someone else for the same question. But what he told me was to go ahead and replace the tube (in pairs of course) and the slo blo fuse and it will not avoid my warranty. I also asked him if i had any more of the same problem, to give him a call back and they would probably ship the amp back to petaluma for further evaluation. Just a fyi of what i was told by your fellow customer service rep at mesa. More than likely, i wont be running high volumes again at half power in a 4 ohm load, from a 8 ohm cab. Im not trying to forcefully damage the internals of the amplifier to just simply get a new one. Just sketchy about what happend so make sure i dont repeat the same problem. Stay tuned. Gatta go get a slo blo fuse for it cause theyre calling for snow here tomorrow. And ill be dammed if im snowed in without a bass head to jam on! Cheerz!
 
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#93
D Rock 247 said:
U even know whats going on? Did u read the whole thread? Have u read the bass prodigy manual?
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Sigh...

Yes I know what's going on.
Yes I read the thread.
Yes I've read the Prodigy manual. I've owned one.

You were running the amp at full bore, popped a tube, then ignored lots of good advice for like 4 pages, then you finally came around when I told you that agedhorse was an engineer at Mesa. That about sum it up?
 
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#94
Bass prodigy back up and running with no problems. Havent had the urge to try half power mode again and probably wont. I should have also told you guys i have the footswitch as well: that has a solo boost engage on it and on the amp. And im starting to think that maybe, tapping the solo switch on at half power might also not be a good idea. Cant remember if i was on solo boost when i blew the tube at half power. Right now im kinda on pins and needles with this amp so not sure if its a keeper but she does sound pretty dang good when running ok. Ill repost in a few days and maybe try the 8ohm 212 cab but kinda worried about that one also. D
 
#95
Dave W said:
Sigh...

Yes I know what's going on.
Yes I read the thread.
Yes I've read the Prodigy manual. I've owned one.

You were running the amp at full bore, popped a tube, then ignored lots of good advice for like 4 pages, then you finally came around when I told you that agedhorse was an engineer at Mesa. That about sum it up?
Click to expand...
Dave W said:
Sigh...

Yes I know what's going on.
Yes I read the thread.
Yes I've read the Prodigy manual. I've owned one.

You were running the amp at full bore, popped a tube, then ignored lots of good advice for like 4 pages, then you finally came around when I told you that agedhorse was an engineer at Mesa. That about sum it up?
Click to expand...
You owned one hugh? Then tell me whats the next thing you do after unscrewing the top plate screws beforw lifing the top cover off......
 
#97
BassmanPaul said:
Normally one would scratch one's crotch! ;D
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Close! Lol. That was a good one. If i can trust its reliability, then its definitely a keeper. I just dont understand why someone would sell one if hasnt had any issues. Its one of the most versitile tube heads ive ever had the pleasure of owning. Was going to play with it and throw some 12at7s in it but it actually sounds fine as it is. I did rattle the walls a little bit just to see if the tubes would hold. When i rebiased it, seemed to me like it was 1 turn to far hot. But if i get a chance, you should try one out. There pretty amazing when they operate.....lol
 
#98
D Rock 247 said:
Close! Lol. That was a good one. If i can trust its reliability, then its definitely a keeper. I just dont understand why someone would sell one if hasnt had any issues. Its one of the most versitile tube heads ive ever had the pleasure of owning. Was going to play with it and throw some 12at7s in it but it actually sounds fine as it is. I did rattle the walls a little bit just to see if the tubes would hold. When i rebiased it, seemed to me like it was 1 turn to far hot. But if i get a chance, you should try one out. There pretty amazing when they operate.....lol
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When U get a chance. Whoops.
 
#99
D Rock 247 said:
Bass prodigy back up and running with no problems. Havent had the urge to try half power mode again and probably wont. I should have also told you guys i have the footswitch as well: that has a solo boost engage on it and on the amp. And im starting to think that maybe, tapping the solo switch on at half power might also not be a good idea. Cant remember if i was on solo boost when i blew the tube at half power. Right now im kinda on pins and needles with this amp so not sure if its a keeper but she does sound pretty dang good when running ok. Ill repost in a few days and maybe try the 8ohm 212 cab but kinda worried about that one also. D
Click to expand...
Glad it worked out for you. Welcome to the Forum. It's always nice to have a new member to share in all this fun.
 

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