my bassman is going to explode?

theshadowofseattle · Post by **theshadowofseattle** » Mon Jun 04, 2007 8:15 pm

Everytime I turn it on, wait for it to warm up, and then take it off stand-by, it makes a loud "BZRZSRZSAZRT" noise like a fart through Trent Reznor distortion and a LOUD hum becomes present. Then if play anything loud enough to trigger the next fuzzplosion, it sounds normal. Then after a bit, it fuzzplodes again and the bass is completely gone. Then it will happen again and be normal.

I get minor fuzzplosions if I apply pressure to the top of the amp. r it the t00bs?

euan · Post by **euan** » Mon Jun 04, 2007 8:23 pm

Microphonic preamp tubes. Though that noise was what was happening with my first Tiny Terror.

James · Post by **James** » Mon Jun 04, 2007 8:27 pm

Your head a-splode.

Though it's a 4x10 combo right? Still, the awesome punnage is worth the lack of accuracy.

filtercap · Post by **filtercap** » Mon Jun 04, 2007 8:46 pm

Is the exfuzztion similar to what you'd get if you plugged a cable into an input and then touched the tip on the other end of the cable with your thumb? It could be that a component is shorting to ground somewhere.

Your amp probably has a metal sheet lining the underside of the top of its cabinet. With the chassis screwed firmly in place, the flanges on the sides of the chassis get pressed up against this sheet, and then at least in theory you have a box of grounded metal shielding the amp circuitry all the way around. Pressing down on top of the amp may cause the now-grounded top sheet to flex down just enough to touch some component it isn't supposed to, like a terminal on one of the control knobs, and short it out. This happened with my amp once.

Or maybe a capacitor has gone bad and is internally shorting to ground from time to time, depending on whatever voltage/vibration it's being subjected to? That's all I got. Pour beer on it, maybe?

theshadowofseattle · Post by **theshadowofseattle** » Mon Jun 04, 2007 8:59 pm

filtercap wrote:Is the exfuzztion similar to what you'd get if you plugged a cable into an input and then touched the tip on the other end of the cable with your thumb? It could be that a component is shorting to ground somewhere.

No, it's WAY more powerful. It kind of sounds like a piece of sheet metal being ripped in half, like the sound effects used for Luke's lightsaber in ROTJ.

Post by **Mike** » Mon Jun 04, 2007 9:07 pm

1) Get one new 12AX7/ECC83 preamp tube and swap it into all the preamp slots one at a time replacing the tube the and leaving the others in place. If this finds a bad tube then voila it was microphonic

2) If this doesn't find the problem then you have to go to an amp tech.

theshadowofseattle · Post by **theshadowofseattle** » Mon Jun 04, 2007 9:09 pm

Do I have to get the same kind of tube as what's in it now? Like, brand or whatever. I hear all this crap about matched sets of tubes being important.

Post by **Mike** » Mon Jun 04, 2007 9:14 pm

no fuck that noise. Matched is a term for Power Tubes (the big 6L6s that are in there) and even then there's debate as to whether it matters - stop here if you're not interested: It's to do with the construction of the tube, in push pull amplifiers people say you want them to be matched so that they drive the top and bottom halves of the signal the same and can be biased to the same idle voltage - however, lots of people deliberately mess with the tubes to get them asymmetrical (although within safe operating region) and say the sound is better, more musical. It's a way to charge money really.

Preamp tubes (dual triodes like the 12AX7/12AT7 etc) don't need to be biased as they're in a preamp circuit that is self biasing or fixed bias. Your power tubes are too but that's a different story.

Short answer - no, just buy a single JJ or Svetlana/Sovtek/EHX/Tung-Sol 12AX7.

Post by **Doog** » Mon Jun 04, 2007 9:24 pm

bob wrote:Your head a-splode.

Are you feelin' lol strain
Meltin rock into rofflz

James · Post by **James** » Mon Jun 04, 2007 9:31 pm

theshadowofseattle · Post by **theshadowofseattle** » Mon Jun 04, 2007 9:38 pm

Homestarrunner is not funny.

Post by **Mike** » Mon Jun 04, 2007 9:41 pm

theshadowofseattle wrote:Homestarrunner is not funny.

Yeah you're welcome, Michael.

James · Post by **James** » Mon Jun 04, 2007 9:45 pm

theshadowofseattle wrote:Homestarrunner is not funny.

aen · Post by **aen** » Tue Jun 05, 2007 12:14 am

I like it.

theshadowofseattle · Post by **theshadowofseattle** » Tue Jun 05, 2007 5:16 am

Mike wrote:
theshadowofseattle wrote:Homestarrunner is not funny.
Yeah you're welcome, Michael.

I don't know if your advise was an elaborate trap to electrocute me yet. I'll thank you when I fix it without getting pwned.

Lanark · Post by **Lanark** » Tue Jun 05, 2007 2:42 pm

Mike wrote:no fuck that noise. Matched is a term for Power Tubes (the big 6L6s that are in there) and even then there's debate as to whether it matters - stop here if you're not interested: It's to do with the construction of the tube, in push pull amplifiers people say you want them to be matched so that they drive the top and bottom halves of the signal the same and can be biased to the same idle voltage - however, lots of people deliberately mess with the tubes to get them asymmetrical (although within safe operating region) and say the sound is better, more musical. It's a way to charge money really.

I seem to remember reading (when last I went through a retubing exercise) that part of the reason you want to match voltages is simply for wear. They'll simply last longer if they're all pulling the same amount of power and working at the same rate. If one runs a bit hotter it's going to wear faster than the rest and when it starts to go south the others will have to work harder and so on and eventually you've got a dead amp.

That may of course be completely dead wrong, but it was one of the few things I remember making sense to me when reading about tubes, so it stuck with me.

Preamp tubes you can change like your socks (assuming you have proper laundry habits). I recently spent an afternoon with the little box of 12AX7's I've amassed over time and did some swapping in various amps. They can make some really interesting tweaks to the overall timbre of your amp. More than I thought it would anyway.

filtercap · Post by **filtercap** » Tue Jun 05, 2007 5:15 pm

Lanark wrote:I seem to remember reading (when last I went through a retubing exercise) that part of the reason you want to match voltages is simply for wear. They'll simply last longer if they're all pulling the same amount of power and working at the same rate. If one runs a bit hotter it's going to wear faster than the rest and when it starts to go south the others will have to work harder and so on and eventually you've got a dead amp.

If you can find where you read that, I'm curious to see it. I'd have to alter that statement to say that power tubes will "last similar amounts of time if they're all pulling the same amount of power." Power tubes do wear out faster the hotter they're biased. But I don't see one tube's bias (or failure) having more than a very slight effect on how warm/cool the other tubes operate.

If one tube gradually starts running hotter, it will slightly reduce the voltage that's available to shove electrons through the other tubes, and those tubes will run a tiny bit cooler as a result. If one tube fails entirely, the voltage on the other tubes will rise a small amount, and they'll run a hair warmer. This rise/drop in voltage is going to be a fraction of one percent of the volts available, so it's really tiny.

Many amps (including Shad's Bassman, I think) have a control to balance the overall bias of the output tubes, or pairs of tubes if there are more than two. As a side benefit, balancing tubes/pairs cancels out some amp hum.