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Squishy sound? What am I hearing?

mikeswals

mikeswals

Supporting Member
Nov 18, 2002
18,672
20,193
Seattle / Tacoma
#1
Kinda has me baffled. For equipment background, I have two Mesa PH210's and two PH212 cabs. Been using a Mesa Prodigy since mid summer.

I mainly gig with my two 210 cabs, all's been fine, casino work, amp is on for 4-5 hours usually. The 212 cabs mainly stay at home and rarely only come out only a couple times a year. A week ago decided to bring just one 212 and the mentioned Prodigy. First couple of hours the rig sounded fine. After hour 3 the attack started sounding squishy and by hour 4 it was hitting distortion pretty easy. I didn't hear any of these noises two weeks prior with the Prodigy and both 210 cabs.
So I figured the power tubes being original and over six years old are probably done, but I still didn't touch them yet.
Yesterday hit a thrown together jam with some new players. I brought the Prodigy and only one of my PH210 cabs. We were playing loudy and for 3 hours...and I didn't get any squish or distortion, so that tells me the amp and its tubes are fine.

Ok, so seems the drivers in that PH212 are doing whatever I was hearing. But never heard that before and why did it take 3 hours before doing it? Casino work is not loud at all-they keep low stage volumes, the amp volume is only at 9'o-clock and the bass knob setting is actually cut from center.
I suppose I need to take the grill off and check for any rubbing.

​
 
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#4
yourpost.JPG


What I see.

My first thought when hearing strange noises from an amp using an active bass is "my battery is dying." If that's not the issue I don't really have much more to offer and hopefully someone else can chime in and help you out.
 
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#6
There are a couple of possibilities I can think of, the main one is that there is driver damage on the P212. This usually happens when the cabinets are overpowered and the driver suspension becomes damaged, resulting in the drivers no longer being tuned to the cabinet because of a large shift in the TS parameters. I don't think you could do this with a Prodigy, but if the cabinet was purchased used, or you may have used it with a very big amp, it's possible (especially if it's 4 ohms).

Do both of your 212's do this or only one?
 
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#7
For those who want to be able to read it:


Kinda has me baffled. For equipment background, I have two Mesa PH210's and two PH212 cabs. Been using a Mesa Prodigy since mid summer.

I mainly gig with my two 210 cabs, all's been fine, casino work, amp is on for 4-5 hours usually. The 212 cabs mainly stay at home and rarely only come out only a couple times a year. A week ago decided to bring just one 212 and the mentioned Prodigy. First couple of hours the rig sounded fine. After hour 3 the attack started sounding squishy and by hour 4 it was hitting distortion pretty easy. I didn't hear any of these noises two weeks prior with the Prodigy and both 210 cabs.
So I figured the power tubes being original and over six years old are probably done, but I still didn't touch them yet.
Yesterday hit a thrown together jam with some new players. I brought the Prodigy and only one of my PH210 cabs. We were playing loudy and for 3 hours...and I didn't get any squish or distortion, so that tells me the amp and its tubes are fine.

Ok, so seems the drivers in that PH212 are doing whatever I was hearing. But never heard that before and why did it take 3 hours before doing it? Casino work is not loud at all-they keep low stage volumes, the amp volume is only at 9'o-clock and the bass knob setting is actually cut from center.
I suppose I need to take the grill off and check for any rubbing.
 
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#9
the other problem could be thermal compression. of the drivers. Are the drivers in that cab neo? Neo drivers are more prone to thermal problems. The larger magnet on ferrite drivers can absorb and dissipate heat much better. The signs of thermal overload are the drivers sounding fine at the beginning of the set and gradually losing low end and volume. You turn up and it just makes things worse. I wondered for years why my bass sounded crummy at the end of a set. This problem is WAY more prevalent than you'd think. As Duke says "thermal compression is the dark secret we bass cab makers don't want to talk about". I've moved to all ferrite drivers except for a neo 12 that is very efficient and designed for lower power tube amps. Everything else is ferrite. I don't think that ferrite drivers sound better--they are just less likely to experience thermal overload. Also---a multiple driver cab will dissipate more heat. My 4x5 cab will dissipate more heat than my single 10 cab--both are ferrite and move about the same amount of air but there is more magnet weight on the 4x5 and the heat is more dissipated.
BTW--the chief designer at Eminence is the guy that educated me as far as neo vs ferrite
 
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#10
agedhorse said:
There are a couple of possibilities I can think of, the main one is that there is driver damage on the P212. This usually happens when the cabinets are overpowered and the driver suspension becomes damaged, resulting in the drivers no longer being tuned to the cabinet because of a large shift in the TS parameters. I don't think you could do this with a Prodigy, but if the cabinet was purchased used, or you may have used it with a very big amp, it's possible (especially if it's 4 ohms).

Do both of your 212's do this or only one?
Click to expand...

I only brought one 212 cabinet that night. I can try to test them both at home, but setting the volume to club level for three hours might not go well with neighbors.
I do have a Strategy, but I have not played that head with these cabs in well over a year. been using primarily my PH210 cabs for nearly everything.

Mike Arnopol said:
the other problem could be thermal overload of the drivers. Are the drivers in that cab neo? Neo drivers are more prone to thermal problems. The larger magnet on ferrite drivers can absorb and dissipate heat much better. The signs of thermal overload are the drivers sounding fine at the beginning of the set and gradually losing low end and volume.
Click to expand...

The drivers are ferrite, little under two and a half years old. These were the latest 300w ea 12" version that Mesa was using, much different from the neo they used early on.
 
#11
I would suspect either low voltage at the gig sight or the output tubes on your amp are going into thermal runaway. The tubes may just be totally worn out as well.

When I was stationed in Turkey I wound up playing in a cover band. I had left my SVT back in the states because I didn't expect to be in a band. I brought my Fender Twin, which was loaded with JBL E120s, for a practice amp. Well I wound up getting in a band so I had to use what was available.

I had retubed the Twin with Groove Tubes and I started experiencing the same problem you are having. The amp was making good power when I first turned it on, but as the night went on, it would start to compress and distort at a much lower volume. I started turning the amp off during breaks and that helped some, but by the end of the next set the problem had returned.

I noticed the tubes had developed a purple glow, which I believe can be an indicator that the vacuum is weak, and the tubes are gassy. Gassy tubes tend to go into thermal runaway because the control grid overheats and starts emitting electrons. Basically the control grid is acting like a cathode. This happens progressively as the tube overheats, and as the grid current increases it pulls the bias down so tube current goes up. Basically a positive feedback loop is formed that slowly spirals out of control.

Once a threshold is reached the tubes quickly go into thermal runway and begin pulling as much current as they can. During this state, the tubes are not capable of making very much power. If the amp is left on indefinitely the tubes may get so hot that the glass envelope melts. If this is the problem, hopefully your amp will blow a fuse before the transformers fail. In my case, I found that aiming a fan at the tubes helped quite a bit.

I own a boutique amp that is not stable without a fan blowing air on the tubes. This amp has a design flaw in the bias network. I suspect in the your case, it's more likely that the one of the tubes is gassy. However if one tube is gassy, the reverse grid current pulls the bias voltage down, so all tubes on a given bias network will go into thermal runway. If the tubes have broken over into thermal runaway for any significant time, they are most likely ruined.
 
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#12
mikeswals said:
Kinda has me baffled. For equipment background, I have two Mesa PH210's and two PH212 cabs. Been using a Mesa Prodigy since mid summer.

I mainly gig with my two 210 cabs, all's been fine, casino work, amp is on for 4-5 hours usually. The 212 cabs mainly stay at home and rarely only come out only a couple times a year. A week ago decided to bring just one 212 and the mentioned Prodigy. First couple of hours the rig sounded fine. After hour 3 the attack started sounding squishy and by hour 4 it was hitting distortion pretty easy. I didn't hear any of these noises two weeks prior with the Prodigy and both 210 cabs.
So I figured the power tubes being original and over six years old are probably done, but I still didn't touch them yet.
Yesterday hit a thrown together jam with some new players. I brought the Prodigy and only one of my PH210 cabs. We were playing loudy and for 3 hours...and I didn't get any squish or distortion, so that tells me the amp and its tubes are fine.

Ok, so seems the drivers in that PH212 are doing whatever I was hearing. But never heard that before and why did it take 3 hours before doing it? Casino work is not loud at all-they keep low stage volumes, the amp volume is only at 9'o-clock and the bass knob setting is actually cut from center.
I suppose I need to take the grill off and check for any rubbing.

​
Click to expand...
tenor.gif
 
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#14
Probably not the tubes if the amp only does it with 1 cabinet. In fact, I would leave the tubes alone, you have a known good set of robust tubes, you probably have another 2000-5000 hours left on them.
 
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#18
mikeswals said:
I used a computer at work, and when I copied and pasted my original post to continue working, thats when the font somehow changed.
I've seen my font in responses in this thread are normal.

I was going to try deleting my thread, but the user tools won't let you do that anymore.
Click to expand...
Just change the font color on here to black at your regular computer, and you're solid. Sorry about your cab.
 

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