Express 5:50 Plus - Intermittent Volume Drop/Muffled Tone

I have a 5:50 PLUS head that has developed a problem - right when I turn it on, or sometimes when I remove it from standby, the volume is lower and the tone is very muffled - almost like rolling off the tone on a guitar. Usually it goes away shortly. Today it happened several times coming out of standby.

I have put in new 6L6's today thinking they might be old (they are 1 year plus with regular gig use) but it still exists so I am leaning towards a pre amp tube. Any

Any ideas which one might be the culprit?

Thank ypu

Sounds like a preamp tube problem to me.

By all means, swap tubes. Replace one at a time til you know the culprit. Be wary of the driver tube as well.
Hopefully that is all.
I did have a problem with my Stiletto, which had a defective power supply cap that caused similar symptoms, especially when coming out of standby. No power, wimpy tone. Often turning to standby and then back on would correct it, temporarily.
But tubes are still most likely.

Old BF Shred said:

By all means, swap tubes. Replace one at a time til you know the culprit. Be wary of the driver tube as well.
Hopefully that is all.
I did have a problem with my Stiletto, which had a defective power supply cap that caused similar symptoms, especially when coming out of standby. No power, wimpy tone. Often turning to standby and then back on would correct it, temporarily.
But tubes are still most likely.

Click to expand...

Yesterday I had it on an hour or so and it was perfect. This morning before work I replaced the V4 (the driver) tube and played it for an hour tonight, periodically taking it in and out of standby with no problems....

I wish I had my thinking cap on last time it was acting up so I could have switched channels to see if it prevalent on both channels....

What does the driver tube do?

You know, there was a post not too long ago, sorry can't recall exact thread, about a weak driver tube in the Express amps.

IIRC the 5w setting will display problems, most noticeably, if the driver is going. And not so much with higher power settings. Did you ever notice the problem change with a different power setting?
The driver is the last stage that "drives" the power tubes. They usually last a long time though not as long as the other preamp tubes (they run at a bit higher current).
But hey, if the problem is gone and don't come back, you solved it!

The bad news is, if it is a cap, it will happen again as it was very intermittent in my Stiletto.
Time will tell, but having a defective cap is pretty darn rare, especially since Mesa uses top notch parts!

Old BF Shred said:

You know, there was a post not too long ago, sorry can't recall exact thread, about a weak driver tube in the Express amps.

IIRC the 5w setting will display problems, most noticeably, if the driver is going. And not so much with higher power settings. Did you ever notice the problem change with a different power setting?
The driver is the last stage that "drives" the power tubes. They usually last a long time though not as long as the other preamp tubes (they run at a bit higher current).

Click to expand...

I had that problem with my 5:50. It was fine in the 50 watt mode but wouldn't work in the 5 watt mode. The tube functions differently in the 5 watt mode than it does in the 50 watt mode.

Don said:

Old BF Shred said:

You know, there was a post not too long ago, sorry can't recall exact thread, about a weak driver tube in the Express amps.

IIRC the 5w setting will display problems, most noticeably, if the driver is going. And not so much with higher power settings. Did you ever notice the problem change with a different power setting?
The driver is the last stage that "drives" the power tubes. They usually last a long time though not as long as the other preamp tubes (they run at a bit higher current).

Click to expand...

I had that problem with my 5:50. It was fine in the 50 watt mode but wouldn't work in the 5 watt mode. The tube functions differently in the 5 watt mode than it does in the 50 watt mode.

Click to expand...

I went ahead and put in a new Mesa driver tube. Played the amp for an hour or so in the all wattage modes with no problem. Have a gig tonight, will see what happens.