Loss of power from 2:90

[Minoin]

Hey Guys,

I just moved my 2:90 poweramp from a gig last night and turned it on at home. But... The A-channel lost about 95% of it's power, while channel B just acted smoothly.
Initially, I thought about a bad powertube, but after pulling and checking them one by one (offcourse 1 pulled at a tie) they didn't seem to cause the problem.
So, I swapped cables and freshly cleaning them with contact-spray, nothing seemed to help. Could it be one of the three 12AX7?
Could someone have any clue?
Thanks in advance for replying!
Cheers

 

[faser]

I would rule out the 12AX7s given that their only duty is to control the presence of the power amp.. I think that the first thing that you have to do is to swap your tubes with a fresh set.. Sometimes it is quite difficult to spot faulty tubes just from their appearance.. So, keep playing on the second channel until you buy the new set (remember to turn the volume of Channel A all the way down) and the latter may alleviate your problem.. After all, this is the only thing that a non-tech can do as a means of troubleshooting..

 

[crane]

The 12AX7s are input tube (V1) and power tubes drivers (V2 and V3). Your problem may come from one of these 12AX7.

 

[TOOLGUY]

Ok,I posted on the other thread as there is yet another person sort of having the same problem as us.
My problem is more like yours (channel a dropping about 98% for me).As I stated I tried swapping the 12ax7 tubes and it did not fix my problem.I also tried another pre-amp and going direct.
Try cycling the standby switch and/or power off and on when the A channel drops out and see if it kicks the sound back on (it does for me at times).Guess one of us should try a set of new 6l6 tubes to see if it fixes things...Keep me/us posted.I will do the same.

 

[prsrose]

Same problem in mine at one point (posted my own thread) - i replaced the power amp tubes and correctly wired my effects loop and it seemed to solve the volume/tone drain. But still having problems with consistency in tone etc.

 

[TOOLGUY]

So would you say it was the 6l6's?
My b channel is rock solid through all of this.

 

[TOOLGUY]

Fresh octet of 6l6s in yesterday... Did NOT fix the issue.
A channel will work for a few and drop out. If I cycle the standby or power switch it will come back on for a bit.
Looks like I need to send it in for repair.
Can I send it to Mesa for non warranty work?
Or am I better off sending it out to a local repair service?
Anyone know of a good place in the states (if not mesa) that does
good/cheap work (I know good and cheap should not be combined lol).

Btw.. Been getting my tubes from Bob over at eurotubes. The 6l6s I got for the 2:90 sound AWESOME,much better then the Mesa tubes that were in there. I can't put into words on how much better it sounds. Just more "tubey" I guess.

 

[TOOLGUY]

Still trying to fix my 2:90.
All voltages check out ( yes I read everything there is to read on amp safety 400+volts = death :shock: ) .
Having two problems.
I can't get my A channel to drop out "on demand" to check voltages when it's acting up.
When it does act up as soon as I probe around the v3 (a channel inverter tube) it will pop the channel back on. I checked every cap/resistor ect for cold joints and sans from re-soldering what I can from above (not pulling the pcb in this thing) I don't know what else to try. I got to speak to a tech at Mesa last night (can't believe I actually got through to a tech-Mesa Rocks!) and he gave me a few ideas to check on but said there were no "common problems" with the 2:90 so it may be hard to home diagnose.
One point of interest is/was the v3 LDR to switch the amp into half mode. He said they can go faulty and sort of jam up in a state of neither off or on causing all sorts of problems. Later tonight I may try and jump it or clip it (I am fairly sure I can do that and I will just loose half mode).
He also said to check around c25 c24 caps for leakage. Said he had one a while back the had some fluid leak under the cap and was causing all sorts of power problems.Looks like this one may be a hard fix unless I can find a local tech that has had his hands one one of these with the same problem.

A question I have is does anyone else’s 2:90 "pop" when they switch the amp into half mode? I thought the reason for using ldr switches over a relay was to avoid this.

Thanks in advance.

 

[prsrose]

Still trying to fix my 2:90.
All voltages check out ( yes I read everything there is to read on amp safety 400+volts = death :shock: ) .
Having two problems.
I can't get my A channel to drop out "on demand" to check voltages when it's acting up.
When it does act up as soon as I probe around the v3 (a channel inverter tube) it will pop the channel back on. I checked every cap/resistor ect for cold joints and sans from re-soldering what I can from above (not pulling the pcb in this thing) I don't know what else to try. I got to speak to a tech at Mesa last night (can't believe I actually got through to a tech-Mesa Rocks!) and he gave me a few ideas to check on but said there were no "common problems" with the 2:90 so it may be hard to home diagnose.
One point of interest is/was the v3 LDR to switch the amp into half mode. He said they can go faulty and sort of jam up in a state of neither off or on causing all sorts of problems. Later tonight I may try and jump it or clip it (I am fairly sure I can do that and I will just loose half mode).
He also said to check around c25 c24 caps for leakage. Said he had one a while back the had some fluid leak under the cap and was causing all sorts of power problems.Looks like this one may be a hard fix unless I can find a local tech that has had his hands one one of these with the same problem.

A question I have is does anyone else’s 2:90 "pop" when they switch the amp into half mode? I thought the reason for using ldr switches over a relay was to avoid this.

Thanks in advance.

Ive had similiar problems.
My half mode pops as well.

When I was having channel a/b problems like you are having, I too retubed. It didnt fix the problem. What I did find out was that I had the effects loop wired wrong. Try disconnecting your effects loop/effects all together, and just run the triaxis stereo and/or mono into the channels to see if it works normally that way.

I also found that when using my effects unit, the whole rig goes a little more quirky. Oddly.

 

[mrcomputer]

A question I have is does anyone else’s 2:90 "pop" when they switch the amp into half mode? I thought the reason for using ldr switches over a relay was to avoid this

Yes. 2:90's pop when shifting between full and half power. This is normal.

jbw