Volume drops on Dual Rec, please help;

[Lurch]

I'm running a preamp into the Return jack of the Effects Loop of my Dual Rec. (using the Mesa as a clean power amp) and after it warms up I'm getting some intermittent random drops in volume.

I changed the power tubes, tried different guitars, checked the preamp with a different power amp, etc. no luck. Next I'm going to change the preamp tubes.

Anyone had a similiar problem or any ideas what it could be?

Than you for any help!

 

[mbmesa]

I had a similar if not exactly the same problem with my Tremoverb. It happened at a band practice. I checked all the tubes and they were all properly seated. I pulled out V1 and put it back in but then did not retry it until the next day when I got home.

When I turned it on at home, all was fine and its been fine ever since at home. Either taking out V1 and reseating back in did the trick of something got knocked back into place during the transport back home.

That's a bummer because I don't know yet what it was but I assume it had something to do the with the pre-amp tubes. Get yourself a known good 12AX7 and start swapping them one by one until you hopefully find the bad one.

 

[Boogiebabies]

I have had this problem on every Dual Rectifier I have ever owned.
The V4A is the effects send. It sees 418 volts to the grid and 210 volts to the cathode. It's a tube eater. At best the tube will work for a little while with a volume drop and then completely cut off. Replace it with the highest quality tube you can afford and keep a spare at all times.

 

[Lurch]

Thank you guys for the help!

I did change the preamp tubes and I thought I had it beat. It worked perfect for about 45 minutes during practice and then all of the sudden it was there again, but not as bad.

Strange thing I noticed about the tubes- I always check the color of the shiny top because I've had a couple cracked ones before that turn white/cloudy at the top. I used two sets of brand new tubes all with shiny tops and when I took them out 1 tube from each new set now has a blacker/smoky colored top. I wonder if it is burning that tube and if it has an internal problem?

Also, I got a reply on another board that say's this:

If your running it into the return, thats bypassing the pre-amp tubes.. so its not that..

It could be the rectifier tubes... unless, like **** near everybody else (including me), your running on silicone diode mode

The problem may be in the grounding being out of phase with the effects.. and, sadly, if its that, theres no fix other than moding the mesa..

The parrallel effects loop and the grounding system in the rectifiers have always been an issue with the effect loop

the new dual rec's with reverb have a SERIES effects loop... ( I WONDER WHY THEY CHANGED IT!!!)

The OTHER item could be the power transformer.

Could that be the problem? I am having some ground loop issues that I've remedied by using an isolation transformer in my A/B/Y pedal. It's in a slightly complicated rack stereo setup, so multiple grounds could be a problem.