50/50 power imbalance

[chirp]

Has anyone experienced a power imbalance where one side does not keep up with the other?
I've tried switching out the tubes (with others) and those seem fine.
I think the amp may have been this way for quite a while.
Any thoughts out there as to what the issue might be?

 

[Henz]

If rolling power tubes doesn't help, you may have to open it up . There are high voltage levels inside. If you don't feel comfortable, take it to someone. I would start with the 12xa7's inside for the PI of each channel. Not so much the input tube. But it cant be ruled out though.Then go from there.

 

[chirp]

@Henz;
Thanks for the reply and suggestion. I could check the 12AX7s at the inverter stages, assuming that they are the last ones for each channel.
Besides checking for the glowing strength against the other 12AX7s, would the idea here be the substitute method?

Another clue to the problem that I didn't mention is the crackle and hiss that have occurred from standby to operation. I think that that issue would point to the input tube(s). Should that not yield any significant changes, I'll have to take the long march to the repair shop.

 

[Henz]

You're welcome. No. The signal goes through the input tube then to the PI tubes , then to the power tubes. And yes, roll the 12ax7's to see if it helps. Some preamp tubes may glow brighter than others. Just as long as they glow. When you said power imbalance, are you suggesting that one channel is always louder? And is it always the same channel after rolling? When powering it up, does the crackling sound happen just briefly? Does it give off a strange odor ? I'm assuming the hissing or white noise is always present. My 2:90 had that crackling issue on initial power up briefly and it would give off an odor. It ended up being a failing carbon comp resistor in the B+ voltage. You will definitely smell it if this is your case. I wish that I would have gotten a 50/50 instead of the 2:90. Much less maintenance and based on the schematic , pretty simple and straight forward. It could be a tube as well. Start with the input tube (V1). Then on to the pi tubes. Remember, even though the amp is turned off and unplugged, high voltages could still be present. When removing tubes inside, just be careful. Keep your other hand in your pocket. If all fails, then the amp would need checked to see if all voltages are ok plus measuring of resistors and caps which means the amp needs to be live. That requires prodding around with probes. Take it to a tech if that's out of your knowledge. Be safe!! Let me know.

 

[chirp]

...definitely try all of that.
I didn't detect any odor though, and although it always has a slight power imbalance when I power it up at home,(always the "right" channel when facing it),
I took it to rehearsal and had to crank that channel up to 8 just to match the 3 on the left. I knew something was up. The crackle was at startup at rehearsal; never had that before. I did however experience it on regular amplifiers and it is usually a symptom of a faltering preamp tube.
I can't know for sure until I bring it home and go through the process you described above. I'll get back to the Board most likely on Wednesday with an update.
Thanks again for offering up your experience.

 

[chirp]

I think I got something... the PI tube on one side seemed to be the problem one. I moved the driver tube over to replace that PI tube and put a fresh tube in the input 12AX7, and it appears to have fixed it—no screechy and more balanced output. A couple of rehearsals should test that solution out.
Man, that's a heavy amp. I'm considering a case with wheels to cart it around.

 

[Henz]

That's great news! I hope it solved your issue brother. Blessings. Jim

 

[chirp]

Thanks for your help... Bro! I would've not thought about the preamp tubes without you.
We'll see if the fix holds up.

 

[Henz]

You're welcome. I'm happy I helped.

 

[frankpaush]

... I just enter here because it seems to be a close relative to my 50/50's problem:
channel A is a about half (numbers on the volume knob) as loud as channel B.
Changed the four big tubes => nothing changed
Changed the 3 ECC83 types=> nothing changed
Checked for cold solder points => everything perfect as far as I can see
Checked for burned parts or leaking caps => at least nothing can be seen

I tried to find a proper schematic and failed (somehow), all I could find is the "standard" files with hardly readable component descriptions ...
And: all part/position numbers seem to be hidden under the part on the board ... or is there a chance they are noted on the back side?? If not: is there a board layout image somewhere?

As you can see: I would appreciate some assistance ...

 

[Henz]

Try this. http://guiguijones.free.fr/Electronique & Bouine/electronique/Schema Amplis Effets etc/Amplis/Mesa Boogie/mesa.htm . Mesa is notorious for hiding the component numbers on board. I had to do lots of tracing in my 2:90.

 

[frankpaush]

Try this.

thanx, but the same gifs we find everywhere

 

[frankpaush]

... not much life in this platform ..or do I ask the wrong questions?

 

[Henz]

I understand your frustration. That's the only schematic that I could find. I don't understand why Mesa is so notorious for their un readable schematics. P!us lots of errors in them. Techs at times need to be able to understand what they look at. I completely traced my 2:90 out. To my amazement I only found one error. But as you mentioned, some of the components hide the number number it. Try to measure all the components you can based on that blurred gif. See if any resistors may have failed. The color bands on them will help.