basey Posted May 25, 2005 Report Share Posted May 25, 2005 I am usually a lurker around here, but I've got a quick question for any of the tube guru's on here. I bought a Mesa Bass 400+ used from a gear trade in shop in town. It sounds good but the low end sounded a little flabby. So I replaced all of the tubes with new higher quality ones. (from lord valve). The tone sounds wonderful now. My problem is that now, when I first start the amp up and take it off standby ( after about 5 min) I get really loud pops for about the first 20 minutes. when it pops the volume reduces and then comes back after a minute or so. Any ideas? Quote Link to comment Share on other sites More sharing options...
unclej Posted May 25, 2005 Report Share Posted May 25, 2005 since the popping didn't start until you changed tubes that's the first place i would look. after your warm up remove the tubes one at a time and see if one of them stops the popping. replace that one with a known good one and see if that's the end of it. the only other thing that came to mind while i was reading your post is that it could be a bad cap discharging and then charging back up and then discharging again. that's all i've got basey but i imagine lk and the guys that actually know what they're talking about will be around soon. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted May 25, 2005 Report Share Posted May 25, 2005 My first guess is a bad coupling capacitor that doesn't like your new preamp tubes. Problem is that it shouldn't go away after a few minutes. Hmmm. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted May 25, 2005 Report Share Posted May 25, 2005 I have a '74 Fender Twin Reverb that made popping noises when it got warm. I pulled the chassis out and found a coupling cap that had broken because the stripboard had warped. Replaced it and the problem went away. Popping noises are almost always caused by arcing happening internally. You could pull the chassis and turn off the lights and you may be able to see what is happening very easily. Check your power tube sockets, too. If you have carbon trails between any of the pins, that socket needs to be replaced, and it will probably also fix the problem. Just don't try to touch anything in the amp with the lights out! And use chopsticks or something if you have to push wires around. DO NOT use a pencil! Good luck! Quote Link to comment Share on other sites More sharing options...
Pr3Va1L Posted May 25, 2005 Report Share Posted May 25, 2005 DO NOT use a pencil! ← lol Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted May 25, 2005 Report Share Posted May 25, 2005 What's so funny about that?! The graphite in a pencil WILL conduct electricity... Quote Link to comment Share on other sites More sharing options...
unclej Posted May 25, 2005 Report Share Posted May 25, 2005 paul..i could be wrong but that lol sounded like a voice of experience lol to me. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted May 25, 2005 Report Share Posted May 25, 2005 (edited) PaulM. VERY GOOD! Socket-arcing could very well be it. The burnt tracks go up in resistance and start to do their insulating job better. I haven't seen it ever that I can remember but it makes perfect sense. OK, I stand corrected. My bandmaster gave me a nasty little jolt/burn when I was poking around the low-voltage filament circuit and it decided I was the new negative. Forgot about that one. Jumped through about 1/8" of carbon-track to get me but it got me. Edited May 25, 2005 by thedoctor Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted May 25, 2005 Report Share Posted May 25, 2005 paul..i could be wrong but that lol sounded like a voice of experience lol to me. Ah, I see. Socket-arcing could very well be it. That or a faulty component is the only two reasons that I know of that will cause an amp to make popping noises. There may be one other scenario where it could happen, but I think that comes into play in this case. Quote Link to comment Share on other sites More sharing options...
basey Posted May 26, 2005 Author Report Share Posted May 26, 2005 thank you all for the good info and advice. I'll let you guys know what I find. but right now I'm goin to sleep. I just worked a 16 hour shift! Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted May 26, 2005 Report Share Posted May 26, 2005 Keep us posted! I'm curious to see what the deal is. Quote Link to comment Share on other sites More sharing options...
basey Posted June 1, 2005 Author Report Share Posted June 1, 2005 I've made a little progress. after checking the tubes and cleaning the pots and tube pins. I checked for burnt components and.... There's two burnt resisters ( i think they're resisters) Here's a picture of them I'm having a hard time figuring out what to replace them with they're too charred. Any ideas? I was able to find a schematic for the mesa bass 400 at www.tubefreak.com but that amp only has 6 6l6's and mine has 12. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 1, 2005 Report Share Posted June 1, 2005 That looks like a strong possibility. I am curious as to why they fried, though. Quote Link to comment Share on other sites More sharing options...
basey Posted June 1, 2005 Author Report Share Posted June 1, 2005 After a little more research, I think that they're the bais resistors. I'm not sure though. Unfortunately, now the amp pops once after it warms up and then the back six 6l6's start to turn red. would a burnt bias resistor have that result? Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 1, 2005 Report Share Posted June 1, 2005 Unfortunately, now the amp pops once after it warms up and then the back six 6l6's start to turn red. would a burnt bias resistor have that result? YES! It sounds like your bias supply has went south. The red glow is a dead giveaway that the tubes are moving way too much current. The funny thing is, usually, resistors don't burn up like that. It may be that the wattage rating of those resistors needs to be higher, and they skimped, trying to save a penny here and there during manufacturing. You could try replacing those resistors with some 1 watt flameproof ones and see what happens. All you need now is a schematic! Quote Link to comment Share on other sites More sharing options...
thedoctor Posted June 1, 2005 Report Share Posted June 1, 2005 Boy, Mesa is a bunch of a-holes about their schematics! Wow! You would think I was trying to date their daughter! Those shouldn't be your bias resistors because they are 2 and 5watt on the 400+. A little better picture MIGHT help but maybe not. There is a tie-point on the scematic of a non-+ that shows they could either be 2 watt resistors or HV diodes. They should not be diodes unless it is part of their standby circuit. Pretty screwed up without a schematic to go by. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 2, 2005 Report Share Posted June 2, 2005 Yeah, good point, bias resistors would usually be 3-5 watt. But, something is messing with the power tube bias. Something must be whacked for those mystery thingies to fry - maybe they're failed zeners? They are more sensitive to damage than the run of the mill rectifier diode is... BTW, are those new filter caps that I see? Quote Link to comment Share on other sites More sharing options...
lovekraft Posted June 2, 2005 Report Share Posted June 2, 2005 ...they could either be 2 watt resistors or HV diodes...I think you may have hit it, especially since the tubes are red-plating. I'm just guessing, but they might be the diodes for the fixed bias winding (yes, MESA uses a whole separate transformer winding with a full-wave rectifier for the fixed bias source). That's just a guess, but that's where I'd start looking. I absolutely hate working on MESAs - they're stuffing about 8 lbs of manure into a 5 lb bag, and it's almost impossible to trace anything out without a microscope and a search warrant. Good luck with it! Quote Link to comment Share on other sites More sharing options...
basey Posted June 2, 2005 Author Report Share Posted June 2, 2005 Thanks for the help guys. I'm getting there.. I've tracked down a schematic from mesa. here's the part for the power supply section Power supply schematic the resistors that are burnt are r311 and r312. I've also found about 5 wires that look like they've been chewed bare by some rodent or some thing. It looks as though they may have been shorting with each other. I think the resistors are 100ohm but other than that I'm not sure which resistor to replace them with. any help would be greatly apreciated. Paul , Nope on the caps, everything is original except the tubes. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 2, 2005 Report Share Posted June 2, 2005 R311 & R312 are part of the heater circuit. Can you measure your heater voltage and see what it is? If it's right at 6.3V +/-10% you're OK. Those 100 ohm resistors are to reference the tube heaters to ground and reduce the amount of hum contributed by the tube heaters. If the heater voltage checks out OK, then my bet is one some fried diodes having to do with the bias circuit. I really can't imagine some 1N4007s getting fried like that in this circuit, but I suppose it's not impossible - they must have somehow sustained a higher voltage than they can stand and croaked. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted June 2, 2005 Report Share Posted June 2, 2005 No, I believe the 4007s are the problem and the new tubes are drawing more current, for some reason. They are 2.5 amp @ 1000V and this is a misapplication for them. The two resistors in that circuit are 62k @ 2 watt and that is not what is in the picture. Course, I am going with supplied info, not sompthin I have seen. LK, you got it right! Mesas DO suck a bunch to work on. Quote Link to comment Share on other sites More sharing options...
unclej Posted June 2, 2005 Report Share Posted June 2, 2005 but they wire them up so pretty on the inside..and they're very liberal with the silicone. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted June 2, 2005 Report Share Posted June 2, 2005 Same applies to Pamella Anderson and she blows anything that she can find. Maybe it's the sillycone. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 2, 2005 Report Share Posted June 2, 2005 They are 2.5 amp @ 1000V and this is a misapplication for them. What should they have used? Quote Link to comment Share on other sites More sharing options...
basey Posted June 3, 2005 Author Report Share Posted June 3, 2005 I think I fixed it!! knock on wood. I desoldered the main board hook ups and removed it from th chassis. then I replaced the burned resisters. then I replaced the chewed wires. then while i had the pots taken off of the front panel I hit them all with red and gold deoxit, after that I put everything back together. I flipped the switch and then took it off standby ... POP...red glow... At that point I thought I was going to have to take it in to the tech $$$. But I thought I'd try the old tubes first before I gave up, so I switched them out. Amazingly, it worked great. so I began to replace the pairs of 6l6's one by one and found one of them had failed so I called up Lord Valve and he said he'd switch me for another good one. I have it running with one of the old tubes in it right now. It sounds better now than it ever has, and it doesn't hum anymore. My question is, do you guys think 1. that the tube failing caused all the popping and problems, or 2. something else caused the tube to fail? I'm hoping the answer is 1. I thought I'd ask before I pack it back into the rack case. BTW. Thanks again for everyones help. Quote Link to comment Share on other sites More sharing options...
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