Premier Rock Forum

hi.

my bandmate's amp (laney pro-tube aor , 50 watts) is having some volume/power issues that have confounded my rudimentary knowledge of amps. he plays it for about 10 minutes or so, and then the volume drops significantly, all of a sudden. he's got brand new power tubes and the pre-amp tubes seem to be working fine, so i'm pretty sure it's not a tube issue, and we've tried a number of things to eliminate the guitar, pedals, etc. as culprits.

right now, he's using the amp with a 16-ohm marshall something or other cab. i tried to run it out to a 4-ohm marshall 1960 something or other, so i turned the impedance dial to 4, and now it's not delivering any sound at all, but it seems to deliver when i switch it back to 16. something is amiss there, but i'm not at all sure what.

if any of that makes sense, does anyone have any idea what the problem might be?

are the volume and/or gain knobs scratchy when you turn them?

sounds like a problem with the output transformer, if it's got a dead tap and all.

how's the combination of output tube type, plate voltage, screen voltage, and bias voltage? everything in order there?

when it gets quiet, if you give the amp a little thud with your hand (firmly, but not super-hard) does anything happen?

have you used a multimeter to measure the resistance of your cabinets, to make sure everything's as it should be? how about to measure the primary and all secondary taps on the output transformer?

no, none of the pots are scratchy, and banging on it does nothing. as far as all the measurements and whatnot. i have not performed any of those tests, but he did just, like, in the last couple weeks, get the amp back from a technician who, i think, was tasked to diagnose the amp for any problems. i think the only thing that came back was that one of the power tube sockets was loose, but that has since been fixed.

sorry i have so little supplemental information.

scott wrote:when it gets quiet, if you give the amp a little thud with your hand (firmly, but not super-hard) does anything happen?

Had he said "it gives a little thud/boom sound back" you would say...?

skatingbasser wrote:

scott wrote:when it gets quiet, if you give the amp a little thud with your hand (firmly, but not super-hard) does anything happen?

Had he said "it gives a little thud/boom sound back" you would say...?

If physically jarring the amp causes it to change the way it operates, or how it's behaving or whatnot, then the problem is an intermittent connection, some solder joint or tube socket or such... Once the amp warms up a bit (i.e. 10 minutes of operating a tube amp) it causes some piece somewhere to expand and break that connection, or break it more than when it's cold, or something like that.

My guess, at this point, would be that something's awry with one or more of the tubes. Other than the performance of the tubes themselves, there isn't a whole lot that should change with temperature like that. Or to say it better, it is a known fact that the operation of tubes is directly affected by temperature, and that temperature changes as the amp is operated. Yeah, capacitors can be affected by heat, but they pretty much shouldn't. And I think that usually, "problem after 10 minutes" is gonna be heat related.

Have you tried operating the amp with a fan blowing nice and hard into the back of it? Does that change anything? Extend the time before it freaks out, or prevent it from freaking out?

f you play the amp, and have somebody sitting there watching the tubes, do one or more of them start to glow orange after a bit?

I'm still thinking something is mucked up with your output transformer.

Also, in all sincerity, if your tech is a normal amp tech, it wouldn't surprise me if he dropped in the new tubes and checked the bias and sent it out the door. If you don't play the amp at a loud, working volume, to put it through its paces, you're likely to miss a fucked up tube and think everything's okay.

I've bought brand new tubes, dropped them in an amp, biased it up, and plugged in to play, and everything was fine. But within maybe only 10 minutes, one of the output tubes exhibited a shorted element, started blowing fuses, etc. To an amp tech, on the bench, the amp would have left the shop performing just perfect. And as soon as we fired it up to play, it would have been like "WTF?!?!"

I would first check the resistance of the speaker cabs, and the amp's output transformer. Then I would check all the tubes with my tester(s). Then I would try firing up the amp and see what the voltages are like, on the output tubes' pins 3,4,5, and also the cathode current (cause that's how I normally bias amps). Then I would look at the DC and AC voltage values at each of the filter caps, to see if anything looked whack. If all of that shit checked out, *then* I would try operating the amp at a medium level, sitting behind it and watching the output tubes to see if any of them start to red-plate. And also, check the temperature of the output and power transformers with my hand every couple minutes or so. If that worked fine, then I'd crank it up to full, or near-full, and again watch the tubes and check the temperature of the OT and PT. And then if the amp was still getting all quiet, but not showing any signs of red-plating or overheating, then I would scratch my head a bit, and go through and check every little thing inside the amp, doing the "tap it with something non-conductive" test, to see if I could find a problem. And if that also turned up nothing, then I'd probably check the values of all the resistors, especially in the power section and the plate supplies for the preamp section, and then replace all the electrolytics unless they're all already new.

Then I would drink a berrs.

scott wrote:

skatingbasser wrote:

scott wrote:when it gets quiet, if you give the amp a little thud with your hand (firmly, but not super-hard) does anything happen?

Had he said "it gives a little thud/boom sound back" you would say...?

If physically jarring the amp causes it to change the way it operates, or how it's behaving or whatnot, then the problem is an intermittent connection, some solder joint or tube socket or such... Once the amp warms up a bit (i.e. 10 minutes of operating a tube amp) it causes some piece somewhere to expand and break that connection, or break it more than when it's cold, or something like that.

My guess, at this point, would be that something's awry with one or more of the tubes. Other than the performance of the tubes themselves, there isn't a whole lot that should change with temperature like that. Or to say it better, it is a known fact that the operation of tubes is directly affected by temperature, and that temperature changes as the amp is operated. Yeah, capacitors can be affected by heat, but they pretty much shouldn't. And I think that usually, "problem after 10 minutes" is gonna be heat related.

Have you tried operating the amp with a fan blowing nice and hard into the back of it? Does that change anything? Extend the time before it freaks out, or prevent it from freaking out?

f you play the amp, and have somebody sitting there watching the tubes, do one or more of them start to glow orange after a bit?

I'm still thinking something is mucked up with your output transformer.

Also, in all sincerity, if your tech is a normal amp tech, it wouldn't surprise me if he dropped in the new tubes and checked the bias and sent it out the door. If you don't play the amp at a loud, working volume, to put it through its paces, you're likely to miss a fucked up tube and think everything's okay.

I've bought brand new tubes, dropped them in an amp, biased it up, and plugged in to play, and everything was fine. But within maybe only 10 minutes, one of the output tubes exhibited a shorted element, started blowing fuses, etc. To an amp tech, on the bench, the amp would have left the shop performing just perfect. And as soon as we fired it up to play, it would have been like "WTF?!?!"

I would first check the resistance of the speaker cabs, and the amp's output transformer. Then I would check all the tubes with my tester(s). Then I would try firing up the amp and see what the voltages are like, on the output tubes' pins 3,4,5, and also the cathode current (cause that's how I normally bias amps). Then I would look at the DC and AC voltage values at each of the filter caps, to see if anything looked whack. If all of that shit checked out, *then* I would try operating the amp at a medium level, sitting behind it and watching the output tubes to see if any of them start to red-plate. And also, check the temperature of the output and power transformers with my hand every couple minutes or so. If that worked fine, then I'd crank it up to full, or near-full, and again watch the tubes and check the temperature of the OT and PT. And then if the amp was still getting all quiet, but not showing any signs of red-plating or overheating, then I would scratch my head a bit, and go through and check every little thing inside the amp, doing the "tap it with something non-conductive" test, to see if I could find a problem. And if that also turned up nothing, then I'd probably check the values of all the resistors, especially in the power section and the plate supplies for the preamp section, and then replace all the electrolytics unless they're all already new.

Then I would drink a berrs.

Scott, I love you!
Thug-hug?

Its the Output trans. I had a similar problem on this Marshall JCM-800 that we were using for bass. The guy was running the impedance wrong and it did the same thing that you describe. It was replaced with some Drake. I dunno what comes in the Laneys but I would check that first, actually I would just replace it. Also check the impedance selector switch if it has that feature. Like the marshalls do. That is all.

thanks for the advice. tubes, output transformer, impedance switch. got it.