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Blew a kt88 in a mesa four 88......
- Thread starter D Rock 247
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- bass prodigy mesa
Lol. Yeah man i popped a tube.....lol. wish it was from something earth shaking but was only on half power. Granted it was almost at full tilt of half power......needless to say before i blew my load-i mean tube, i hit a low e with the bass cranked, on bass boost voicing #1, and ive never in my life heard 2x12 cabinet sound like 4x15s in my life. It was epic.......lol.
- Feb 12, 2006
- 61,038
- 5
- 220,966
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Its 8 ohm. The manual calls for a 4 ohm input at half power. I actually took the backplate off the cab after we talked last night and checked things out, pcb and caps seems to be fine, all speakers were hooked up. Checked the ohm load of each speaker and tested the speakon which was reading 7.1-.2. But whats got me wondering now, cause when i removed the speakon from the cab, i believe i had it hooked up to the parallel out. Im thinking this might have been the issue all along. On my ampeg cabs, all the 1/4s or speakons are all input, so im just used to slapping a cord in the back without looking but on the mesa jackplate, its a different ballgame i believe. Just another round of mesa (bass) rookie mistakes but not sure. Thanks for following up.
It sounds to me like it was pushed too hard at a non optimal resistance.
It's hard to get annoyed at something when the directions say one thing and they are ignored or bent.
It's hard to get annoyed at something when the directions say one thing and they are ignored or bent.
- Feb 12, 2006
- 61,038
- 5
- 220,966
- Disclosures
- Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
Correct, the reason for using the 4 ohm tap at 8 ohms when using the 2 tube mode is to correctly match the plate load to the speaker load. With 1/2 the tubes, the plate load needs to be 1/2 (double the impedance) which is reflected from the load to the transformer primary by using the 4 ohm tap with the 8 ohm load. 1/2 the plate load with 1/2 the tubes = 1.
It's also possible that there was a flashover defect in one of the power tubes, more common with the KT type tubes because of the (much) higher operating voltages. The other side of this is that the KT tubes have a somewhat uniquely rock tone, very hard to duplicate with 6550's at this power level, and even more difficult with 6L6's without using a heck of a lot of them (and even then, the plate voltage is much lower so the transformer parameters will be quite different).
This is a big challenge in the quest for this kind of tone, and why there are not a lot of choices in this class of product.
It's also possible that there was a flashover defect in one of the power tubes, more common with the KT type tubes because of the (much) higher operating voltages. The other side of this is that the KT tubes have a somewhat uniquely rock tone, very hard to duplicate with 6550's at this power level, and even more difficult with 6L6's without using a heck of a lot of them (and even then, the plate voltage is much lower so the transformer parameters will be quite different).
This is a big challenge in the quest for this kind of tone, and why there are not a lot of choices in this class of product.
From your pic, you seem to gig alot also. Ever had
Yeah im not too worried about what happened. Its reading this manual: ok i had it matched to the right input at half power, checked the cab and it was ok. Lemme find a pdf of the manual on page 30 in the category of diagnosing power tube failure. Then i think agedhorse might understand why i wasnt too worried (at the begining of the thread) about replacing the tube and fuse. (gatta update my flickr acct to upload pics)
Mine was the last type of tube failure where it sais severe short. Then it tells me to replace "it" (meaning the tube i assume)and the proper slo blo fuse.
But if your like me when playing, i turn my amps up to the point where i can just tap string fretted lets say, on the octave fret. And if im close enough, im gonna get that sustain. Not a true bass player, so not sure if this is something i should even be talking about but i want that volume level (usually have it) so i can be dynamic with my strumming, plucking hand. Now i believe u hit the nail on the head about being too loud with a non optimal resistance. I can see that all day. But before i go gung ho on getting the amp back up and running, ill call mesa to make sure they give me the green light to do so. Even if the manual tells me otherwise, id rather be safe then sorry. Thanks for your input. Cheers
Another point if I may, using text-isms is also frowned upon. I find them disconcerting and off putting. Plain English is so much better for getting a meaning across. Especially for we throw backs who only use the device to make telephone calls. 
frowned apon? Hell thats 2nd nature to me. Ive been frowned apon since the day i was born......lol. but yes, no problem. I will strive for better punctuation and try to lay off the abbreviations. I guess whats next to be frowned apon is my spelling......cheersAnother point if I may, using text-isms is also frowned upon. I find them disconcerting and off putting. Plain English is so much better for getting a meaning across. Especially for we throw backs who only use the device to make telephone calls.
Bunch of helpful yet still grumpy old farts in here.
Helping U iz EZR when we dont have 2 tranlatize UR ?'s N2 English.
Lol, love it. But totally understood what you were saying btw........
sorry i did it again. Dont blame me for all the abbreviations, blame all these kids who started this mess and got me doing it now. And im not that young, i still remember rotary phones and ford escorts......
"Aoccdrnig to a rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteers be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe."
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.
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.
