Mesa Bass 400, power tube went out...HELP..!!

[suraj]

About 4 months ago,I recapped and retubed this amp. I also got an adjustable bias mod done to it. I've only been using this in my bedroom for a while, but when I took it to practice and crankedit a little, I blew a fuse. When I looked inside, I had blown the two 100ohm resistors that ground the heater supply. I replaced the resitors with higher wattage ones. All tubes were still lighting and getting hot and the amp worked fine after that. All this while I was using a Hartke 410 hydrive cab, 8ohms.

Now again when I took it to practice, my hydrive wasn't there, so I used this 2x12 loaded with car subs. Obviously not good sounding or efficient, so I had to crank it further just to get through my loud band practice. Before we started our set, the fuse blew..!! This time, a power tube went out(JJKT88). When I looked at it, half the getter became transparent and whitish.
My tech said this must have happened due to moisture on the tubes. He said due to the heat inside the tube and the moisture cooling it outside, the glass must have cracked..I didn't find any crack on the tube though, but yes there was moisture on the tubes. Its rainy season and its very hot and humid too.

Im confused what to do in the future so that my amp doesn't fail me. It failed on both practices so now im thinking of letting it go..but the tone is god like

Any suggestions, thoughts, something I did wrong.

PS: I used a hair dryer for a minute on the tubes so the moisture goes away before practice, and it still failed My tech also thought it could be due to physical damage of the tube, but the tubes have been in the amp with retainers on them. Its also in a home built shock rack...!!

[Bassmec]

If I where you I would replace the failed tube, if you cant find a crack then maybe what you had was, only a partial vacuum tube and leaked air in over time anyway.
Do you have your plate voltage and bias figures sorted?
You may be dragging more quiescent current than is good for your tube set and red plating them.
If you leave the amp in standby for a few mins it will dry its self with the tubes heaters, under most atmospheric conditions.
PS What was the total impedance of the car sub and how was your amp impedance set up. Was the amp getting very heavy mechanical shocks when hot.
Was the amp subjected to loudspeaker disconnections.

[suraj]

I will definately replace the failed tube, I just want to know what I did wrong. Yes my plate voltage and bias was redone. I got it done according to the suggestions in my last thread. Yes maybe the tube was defective. I can't return it now cuz im over tube depot's 90 day return policy. The car subs were 4ohms each, 300 watt rms, and wired in series to be 8ohms.

I always kept an eye out for red plating. I was never redplating them for sure. The amp was biased to give about 35mA to each tube with about 460 plate volts as per my tech. but after the tube went, I pulled it out and turned on the amp with 5 tubes, each tube showed about 60mA
I cooled it down again to about 30-35mA and the amps working fine. Now i'll have to order another tube from tube depot and tell them the bias point but then will it match..??

The amp never got heavy mechanical shocks when hot, and has never been subjected to loudspeaker disconnections.. Only twice, like a noob, i switched on the amp and realized I hadn't plugged in the speaker cable .. Once was more than 6months ago, and once was recent, I think 2 days before this happened. But then the amp still worked fine be4 the mishap. On the day of practice, I had to crank it up quite a bit for those inefficient car subs to come alive. The master was on 5, the input gain on the clean channel was full, and the lows were cranked from the rotary and graphic eq quite a bit. That kinda volume would have blown my hartke, but sounded fine through those subs. A min later, the tube went out.

Tube amps are supposed to be cranked, so that shouldn't be the cause.

[BassmanPaul]

The only way water can crack a tube is when the tube is hot. If the tube is damp due to condensation, running the amp will evaporate any dampness in short order. I suggest that a drop of liquid fell onto the tube from a glass or bottle. Once cracked the tube is toast. No one should ever put a drink on top of a hot tube amp!

Car subs are notoriously inefficient and are simply unusable for a stage or practice rig. Trying to drive a pair would be a task for a low powered head like yours. Tube amps can be driven certainly but only so far. You tried to tow a trailer with a motorscooter.

If you can get a single tube that will line up with the bias requirements of the rest then go that route. If not then a full set would be best. When you had the bias mod done did you just have one pot installed or more?

Paul

[suraj]

No I never keep anything on my amp. I can't actually, its in a 6space flight rack. The tube is gone I know. Maybe I cranked it a bit too much yes, but again my master was only on 5..I'd cranked the bass eq on the rotary and graphic, so that must have been the culprit. Anyways, i've read these amps blow fuses when cranked into inefficient cabs. But in my case a tube went out, and air has leaked in, so I doubt that has anything to do with me cranking the amp...

There is only one pot installed in the bias mod.
My tech did it. He replaced the bias resistor with a pot and added a resistor before the pot so that it can't be adjusted to zero volts. He also changed the pot value from 100k to 22k i think. I don't remember the value of the resistor before te pot.

Eitherways the bias scene seems fine. So should I just take this as a bad tube or something else is up with my amp..??

PS : I just installed a new set 3-4 mnths ago..!! I can't afford to change it again..!! I live in Mumbai, India and tubes are not available in my country. So I cant take lots of tubes and check which one will match with the others.

Before I purchased these from tubedepot, I asked that if one tube failed from a sextet, can I replace just that tube. Brian from Tube depot said that the bias point number written on my tubes is all he requires to give me a single tube that will match with the others. Is this possible..??

Edit : Thanks Paul for all your help in my amp fail threads..

[svp]

I talked to the top Mesa tech in my area about the bias mod and he first that you would need two because of the # of tubes in the amp and second, it is in his opinion a waste of money to do. the added life if gives to the tubes is not worth the mod.

[suraj]

I did the bias mod so that I can use KT88's in the amp. And I installed cathode resistors to get a reading from each tube so I at least know whats going on. From what i've read, a lot of people have done bias mods to the amp successfully. Maybe my tech did it in a different way. If you know exactly how to do the bias mod please explain it so that I can forward it to my tech and he can do it that way.

Are you saying the way my bias is done right now is incorrect and is causing the problems..??

[Bassmec]

If you get a replacement tube that matches the spec you will be fine.
I heard a tube crack in the back of a fender bassman ten when a musician lent the amp towards him to pick it up. a bead of sweat fell from his brow and I heard it go plink! as the drip fell directly onto one of the hot 6l6's.
There is no reason why you cant operate a tube amp for years with total reliability if you get a good set of tubes biased right and don't hook it up to any old passing cab of unknown performance.
5 on the master with a dime'd pre is flat out
don't run a bass tube amp at a volume level much higher than it will produce clean.

[Nightlyraider]

Suraj, with the amp's master on 5 you are cranking it almost as hard as you ever should. Once you get your amp sorted out I would focus on real bass cabs that will provide ample efficiency. The 400 should be getting silly loud by that point if everything else is set right. If you read the manual Mesa will even say that anything above 5 is pretty much useless electrical noise (and practice level should not be at amp maximum).

[BassmanPaul]

Originally Posted by suraj

Before I purchased these from tubedepot, I asked that if one tube failed from a sextet, can I replace just that tube. Brian from Tube depot said that the bias point number written on my tubes is all he requires to give me a single tube that will match with the others. Is this possible..??

Yes. A company who specializes in tubes can sort tubes into batches on a test jig. This will apply a certain negative voltage to the tube and read off the current the tube passes. These batches can be assigned an identifier. Once a tube has been in service it will drift as it ages but should still work with another of the same batch.

If Brian says he can supply a suitable single tube I'd take him at his word. I have no reason not to.

Good luck with the amp I hope you get it sorted.

Paul

[suraj]

I understand that I shouldn't be cranking it that much. Though when I say I have my pre dimed, im still getting clean signal as im using a passive bass. I usually use a Hartke Hydrive 410 which is relatively efficient. With that cab I can't take the master past 4 or the cab will start distorting and i'll damage it. I've even invested in a Genz Benz Uber 212 cab which is on its way, so I may not have to turn the amp past 4 ever..

But my main question is, did this happen because I cranked it too much, or was it just the tube that was gradually letting air in..?? My tube is definately not cracked, but has turned whitish from the inside. I doubt this will happen due to cranking the amp..correct me here. Thanks Paul and everyone else for all your help

[BassmanPaul]

If the getter, the silvery deposit inside the tube envelope, has turned white the tube has lost vacuum. It's leaking somewhere even if you cannot see the damage. While overdriving the amp can cause problems it's not usual for the tube to crack.

Paul

[suraj]

Yes the getter has completely turned white. But even after close inspection of the tube. there is NO crack in the glass..Obviously air leaked in from some other place. If this had to happen, it should have happened a month ago ;(..poor me would have gotten a free replacement..

Thanks to all
Peace

[T-Bird]

Hi.

The crack is most likely between the pins or around one.

I'm with Paul on the notion that cranking the amp doesn't crack the tube if everything else is in order.

Tubes are fragile little creatures and anything that's made out of glass is bound to crack, especially if there's a vacuum and pieces with different thermal expansion rates involved.

Regards
Sam

[Bassmec]

Originally Posted by T-Bird

Hi.

The crack is most likely between the pins or around one.

I'm with Paul on the notion that cranking the amp doesn't crack the tube if everything else is in order.

Tubes are fragile little creatures and anything that's made out of glass is bound to crack, especially if there's a vacuum and pieces with different thermal expansion rates involved.

Regards
Sam

That said, the set of gec kt 88's in my old simms watts only got replaced a couple of years ago and their date codes where october 1971.
And I still use the better pair of them in my marshall JCM800.
I am surprised about the original posters bias figures jumping up to 60 ma when originally set at 35 I would be keeping a close eye on the bias circuit and any component/tube drift.
I would get an octal plug bias probe to second guess the values of the added 1 ohm? bias shunts.
I think it sounds wise at this juncture to get separate big cap stabilized push and pull bias controls fitted.

[suraj]

My tech said he checked the bias before closing up the amp but I feel he checked the bias and then changed the resistor value before the pot which definately must have altered the bias. Thus the 60mA reading. My tech insists he'd checked the bias so I guess maybe the tube that went out must have caused the change in bias. Its stable now, but only running on 5 tubes. I think I'll have to pull one more out, so that there's equal tubes on each side. But im not getting crossover distortion with 5, nor am I using the amp outside my room till I get a replacement tube..

About the bias mod, I asked timv who's done lots of bias mods on these amps, and he says what my tech did is fine. He recommended 2 bias pots in a 400+ but not so necessary in a 400.

Im anyways gonna use matched sextets, and you don't get tubes in triplets, so i think one pot is good enough. Now that one tubes failed, i'll order a replacement with the same bias point value, so i guess i'll be sorted. Again two pots in the bias wouldn't make a difference. I'd need two pots if three tubes crap out and I buy 3 completely different tubes. Then it would make sense.