Dual Rectifier Rev G drops volume after a while

[Doctor Blue]

Hey, guys, my first post here.

I have a problem with a Mesa Boogie Dual Rectifier Rev G amplifier. After playing on it for a while (more than an hour), the volume of the Orange channel drops suddenly, after a few seconds it recovers. Then it does this intermittently. On the RED channel I don't have this problem.

I recently changed all the tubes in the preamp. Although I thought the problem was fixed, unfortunately it came back.

I did not change the power tubes, nor the rectifier ones. I'm thinking of doing that too, although I don't think they'd drop the volume on just one channel if it was from them...

A friend suggested I should tighten the socket pins, because they turn loose after a while and especially after it heats up and dilation occurs. I did that, the problem remains.

Any suggestions? Thank you.

(I Attached an audio sample, be careful if you listen through the headphones, it starts with a low volume)

 

[meesa_jarjar]

I would start by checking the vactrols and channel volume pot.

 

[ricardo loma vista]

I had a similar problem on a MB Express. I did all the usual tube and cable troubleshooting with no success. Finally took it in for repair. It was a bad CH2 gain pot that would periodically short to ground when warm.

 

[CoG]

OP, dunno if you are still out there having this problem, but I have seen it on both a rev g dual and a tverb where there's a big resistor near the PI that's starting to heat up and desolder itself and/or cause its trace to lift. It's not a hard fix but it's a pain to troubleshoot, took multiple trips to the tech in my case because it wouldn't happen unless you played the amp for a good bit. Fix was new component on longer legs to keep it off the board...

 

[Doctor Blue]

Thank you for the input. I swapped the tubes with EL34s from my Marshall and the issues are gone. I will buy a set of matched 6L6 quad,

 

[FedWar]

OP, dunno if you are still out there having this problem, but I have seen it on both a rev g dual and a tverb where there's a big resistor near the PI that's starting to heat up and desolder itself and/or cause its trace to lift. It's not a hard fix but it's a pain to troubleshoot, took multiple trips to the tech in my case because it wouldn't happen unless you played the amp for a good bit. Fix was new component on longer legs to keep it off the board...

Thanks for this man,
I have a similar issue on my tverb where I need to raise the master after 20 to 40 mins and then it stabilizes.
I have to raise it by a good bit (a single digit) in order to match the previous volume.
I've never understood the reason but I've always suspected a overheating issue and this might be it!

 

[FedWar]

@CoG Do you have a pic of the resistor you changed or can you identify it on the schematic that is freely available on the internet?
Thanks!

 

[CoG]

@FedWar no, I don't, amp tech did the work.

 

[Doctor Blue]

The issue is back. I have changed all the tubes. It keeps dropping volume, drives me crazy!

 

[domct203]

What tubes are you using in the preamp, specifically V3? Are they Mesa branded? JJ’s? Tung-Sol reissue? Mullard reissue?

Russian tubes, especially New Sensor Corp brands (Sovtek, Electro-Harmonix, Mullard, Tung-Sol) can’t handle the voltages in the cathode follower (V3) and can cause volume drop outs. JJ’s, Chinese & the current Mesa branded (JJ) 12AX7 are what you want in this position.

https://boogieforum.com/threads/did-you-get-the-memo.25102/#post-186780