2:90 pops when 1/2 drive is engaged

[Jackie]

Hi,

I've been using my TriAxis preamp with my Marshall JVM410H's power section (which sounded AWESOME) for about a month now, and I decided to pull the trigger on a used Simul-Class 2:90, so I could really hear the "boogie".

I have a problem when using the 1/2 voicing. When I engage it, a VERY loud POP noise comes from the speakers, even with Level on about 1 and TriAxis on 5 (pops on any mode). When I engage the Deep/Modern modes, it pops a little as well, but those aren't something I'd worry about.

The unit is pretty old, serial no. less than 3xxx, so I'm guessing the problem is the power tubes, since they are Mesa's and labelled "STR-420", which are out of production - it was retubed like a year or two years ago and according to the previous owner, he played it very rarely to none at all in those years.
I've only run it in mono on channel "B" (turned down Level A and maxed Presence like the manual suggests), so maybe it doesn't pop as bad on the other channel.

Is there anything other than the powertubes that could cause this? Oh yeah - can the three preamp tubes found in the 2:90 be accessed without removing the chassis?
If its the tubes, I'll buy an octet of SED 6L6s and 3 Mesa 12AX7s, but that's 300€ worth of tubes, and I'm already tight on cash... :|

 

[tedsticle]

Yes......the 1/2 mode pops on mine as well. Only way around it is to always have it engaged, so if your doing a live show there is no pops. I only use it when I need to practice at bedroom volumes.....Live shows it is never on.

 

[Jackie]

Interesting. Does it pop like really obnoxiously loud? Mine sounds like someone unplugged my guitar while playing.

I tried both channels today at band practice volumes.

Channel A - there was a small "woooo" from the tubes after the signal died down (like when I did a sudden CHUNK and after that I got the "wooo"), so I think that channel might have at least a bad tube for sure. Some crackling in the speakers too.

Channel B sounded fine, slight crackle in the speakers when idle. AFAIK, that's also a sign of bad tubes.
Both channels had the humongous 1/2 drive pop.

I'll try swapping the quartet of the A channel and B channel, so I'll be able to rule the actual channel circuitry from the equation. Though the more I think about it, the more it seems unnecessary.

 

[Jackie]

I'll just play w/o the HALF DRIVE feature until I have the money for new tubes. Then, I'll just replace all the tubes (including the 12AX7s) and the fuses to be triple sure, then we'll see.

I'm still interested if there is any other thing besides old tubes that could cause the popping.

Video of my pop (actually sounds like someone dropped something heavy in the background xD)

tedsticle - have you ever changed the tubes? So it pops despite the tube change?

 

[tedsticle]

Yeah....I've retubed 2 times since I've had the amp. It's going on 10 years old now. The tubes in it now are a little over 3 years old, so it's close to retube time again. I have always noticed the pop. I'll mess around with it today, because I know with midi switching it helps tame the pop instead of switching by pressing the loop switch on the Triaxis. I just can't remember how bad. One of the combinations works well though. Like 1/2, and Modern on together, then switching both off at sametime?........I'll post back my findings.

Your pop does sound a little different then mine, but not by much. It could be the camera though. Either way, any pop sucks.

 

[Jackie]

It sounds more muffled in person, and it is VERY loud, painful to hear, like I'm about see a tube fly out the back of my rack any minute.

I can tell you it's no better/worse if you use MIDI. I use my BOSS GT-6 as a MIDI controller and it's just as loud, which makes sense, since you do the same thing (send "tip to ground" signal from the TriAxis' switch jack to the 2:90 voicing jack).

That's a bit of a dissapointment... I was sure it was just old tubes... then again, there's still hope that yours isn't as bad. I hope there isn't like a batch of 2:90s with this pop and mine's one of 'em (I suspect mine's about 10 years aswell).

 

[Jackie]

Well, I pulled all the tubes and reseated them, cleaned out the whole unit with an air compressor (yes, I was gentle).

The pop got no better or worse. I even swapped out all the 12AX7s with known good ones and again, it was no worse or better.

Any news, tedsticle? I'm too stubborn to give this up and just say "meh I just won't use the 1/2 drive mode". I bought this thing so I can use 120% of it.

Maybe my bandmate will give me the 6L6s from his ENGL that he'll be retubing as routine maintenance (his can't be worse than mine!!!), that way I'll be able to see if there is a problem with the power tubes or the unit itself. If the tubes plead guilty, thank god. If they don't... :evil:

EDIT:
A little addendum (click to make it bigger)

The inside of a 2:90 - I've yet to find one on the 'net, and since folks may, in time, want to change their 12AX7s/ECC83s, it might be nice that such a pic exists. Plus, the TriAxis had his nude shot done by Mesa and the 2:90 is probably jealous :lol:

 

[tedsticle]

I find that, well, your screwed....lol. You either need to keep 1/2 drive on at all times or off at all times. If your amp is sounding fine and there is nothing weird going on with the tone, I'd say your tubes are fine.

 

[Jackie]

That's too bad. I called an amp tech and he also said that if it sounds good the tubes are fine and its probably a relay problem. I'll get it fixed as soon as I can, because it is a feature I'd like in my arsenal very much. In the meantime, I'll just remove the TriAxis Switch 2->Half Drive cable and play without it.

 

[zeeman]

WOW, that is loud.

Mesa suggests using unshielded cables for the switching.

Are you using unshielded cables?

zeeman

 

[Jackie]

Yeah, it's pretty unpleasant. I'm afraid it might blast a speaker some day :?

To answer you question - I'm using short patch cables that are meant for connecting stomp-pedals (so I'm pretty sure they're shielded). But I think it doesn't matter which cable I use - I have a "tip to ground" footswitch that I plugged in the 1/2 Drive jack to rule out cable/TriAxis switch jack problems. It was the same with the footswitch.

 

[Snoopy551]

HI Guys

That is so weird.
THe pop sound is like a charging capacitor.

That is bizarre the amp tech told you, that is a relay problem.
Following the schematic, that is a ldr which switch the half drive.
(But that is the same effect of relay switch).

Keep in touch,
i would lik to understand the probleme.

 

[zeeman]

Yeah, it's pretty unpleasant. I'm afraid it might blast a speaker some day :?

That is a possibility. :?

I have a "tip to ground" footswitch that I plugged in the 1/2 Drive jack to rule out cable/TriAxis switch jack problems.

I am assuming that this was with a shielded cable - for the sake of sanity, I would still try it with an unshielded cable.

Before I barrage you with a bunch of other questions, can you please list what else you have tried?

Thanks.

zeeman

 

[tedsticle]

Here's something I've always wondered about. Since it said Unsheilded cables in the manual. I first connected them with Speaker Cables that said unshielded on them(or is all Speaker Cable unshielded)? I got the pop then, and I get the pop now with guitar shielded cables...

 

[Jackie]

zeeman - The footswitch I used was definitely with an unshielded cable. I don't think that's so important anyway.

I already said what I tried, but I'll write it again:
- tried the channels separately, to see if pop occurs on both. Pop occurs on both channels
- used all sorts of cables; shielded, unshielded, all known good cables (my father uses them for his pedalboard and they work fine); no better, no worse
- ruled out the TriAxis by using a Yamaha FC5 "tip-to-ground" footswitch. This footswitch works fine despite it being for a keyboard, I used it to switch channels on my old Roland Cube 60 (no problems there). Also tried using a different number switch jack. No dice.
- pulled and reinstalled all the tubes, checked them for microphony (no problems), exchanged the 3 12AX7 positions, even swapped all three of them with known good 12AX7s. Again, no better no worse. Also cleaned the entire unit.

I sorta ruled out bad power tubes aswell, the tech said that if they were bad, the sound would be shitty, there would be a lot of noise, they'd have an irregular glow, and if the cause was indeed bad tubes, all of the three voicings would pop as loud. The Modern pops a little, so does Deep, but that's barely audible, like hitting yourself on your leg with your fist (weird analogy, but it's about as loud). Quite frankly, I'm 95% sure its not tubes, and I hope its not, since I don't have 300€ lying around right now, budget is still depleted from all the Mesa gear :mrgreen:

Snoopy - the tech didn't check out the video, I just described the problem to him, we talked on the phone.

I'll probably take it to the tech I, maybe after our gig in January, or I'll take it first thing after holidays and use my JVM+MXR 10-band EQ for the gig. He's a good tech and I trust he'll be able to fix it.

 

[Snoopy551]

HI Jackie

I found that on internet that is very interesting.

Popping is almost always an arcing problem. The high voltage in the amp has found some path that cannot stand the high voltages and discharges suddenly through that path. The arc current is high, but cannot be sustained by the power supply, so the voltage drops a little, the arc extinguishes, and it takes some time for the power supply voltage to build back up to where the arc will start again.

Popping is often associated with the time when you flip the standby switch. In standby, the current drain from the power supply is less, so the voltage rises, causing more voltage stress. When the standby switch is thrown, the higher-than-normal voltage can break over things that stand the normal stress of operating voltage.

Tubes
The tubes themselves will sometimes develop internal, intermittent arcs. Do some tube swapping. Start at the front (preamp) end of the amp and pull a tube, listen, pull a tube, listen. When you find one where pulling it makes the popping stop and a new tube makes it quit completely, you're done.
Arcing power tube socket.

The B+ may be arcing across the surfaces of the output tube sockets themselves. This is often the case when an amplifier has a lot of dust and dirt inside it. In some cases, the arc can be started by a few seconds of playing without a load on the amp, which causes large spikes on the plates of the output tubes. Contamination of the tube socket surface can let an arc get started, and the arc itself burns the surface of a plastic tube socket body. This leaves a carbon residue in the path of the arc, burned remains of the trail of the arc; the carbon residue is itself somewhat conductive, so in the future, there is a ready made path for the next arc. You have to replace the socket if this is the case.

Intermittent switch
A switch, often the standby switch, can develop internal arcs
Intermittent breakdown of coupling cap or a popping resistor
Sometimes a signal coupling capacitor just can't take it anymore, and it starts breaking down intermittently. Resistors, particularly those delightful, brown sound carbon composition ones, sometimes develop internal pops. Proceed as for tubes. In this case, a new replacement tube will NOT make the noise quit permanently, and you have to figure out which component is causing it.

Intermittent breakdown of output transformer or choke
The filter choke, if your amp has one, and output transformer are connected to the highest voltages in the amp. If they are old (can you say "vintage"? I thought you could) and if they get hot, the insulation on the wires inside can start being intermittent. Internal shorts that clear will cause popping. Shorts that don't clear will pop a fuse, usually. Sometimes it'll just cause smoke.
broken resistor or capacitor or R/C lead/ wire
This is one where the problem is not caused from the B+ breaking down insulation. Mechanical damage can break a part and leave enough pressure on the pieces so it mostly makes contact and kind of functions. Vibration will cause it to open momentarily, causing a pop.
Heat from resistor or output tube melting solder
This is a fun one. Some of the power supply dropping resistors, output tube cathode biasing resistors or the connections on the output tube sockets themselves get so hot that they melt the solder that attaches the leads. Even more interesting is when they just soften it so it gets grainy and any vibration (speakers, anyone?) makes a cold, grainy solder joint. This bad joint can pop and arc, sputter, hiss, rectify AM radio, do lots of nead stuff. Once you find the bad joint, you'll also have to find out why it was so hot.
You can sometimes leave the amplifier turned on and turn out or dim the lights in your workroom, and see arcing happining. On pops that happen when the amplifier is touched or jarred, you can (gently, now) tap the chassis with a rubber hammer or wooden stick, being careful not to break anything or touch the amplifier with your hands as you do this. This often makes a mechanically-motivated arc happen, and you can see where it is, and deal with it when the lights are on. BE VERY CAREFUL NOT TO TOUCH THE INSIDES OF THE AMP IN THE DARK - THE HAZARDOUS VOLTAGES ARE JUST AS DANGEROUS IN THE DARK.

Keep in touch

 

[Jackie]

Wow, that's way too much info.
I'm not quite sure if that applies to my problem though, since you can't really "pull a tube, listen" unless you have a pop when you engage standby.

 

[SonicProvocateur]

Sounds like a bad LDR.

 

[Jackie]

How hard is it to fix, if it is?

 

[SonicProvocateur]

Easy tech repair if it's just a LDR. Parts from Mesa. I'd call Pentaluma and double check though. They are very friendly. You are overseas? If you call em now and make sure thats what it is you could have the parts ready after your gig.

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