Mesa Maverick Hum

[Asura]

My 212 Maverick is a late serial number and is in great shape. I only acquired it within the last year, but it seems like it got very little use before that. It gets played loud with a band at least once a week and it sounds good, but there's a low buzz that plagues the clean channel and it's slowly driving me nuts. I would love any help people can provide, I am not very experienced with tube amps and I am near the end of my understanding. Here are the facts of the case:

1. The hum seems like 60 cycle hum, and it only occurs on the 'Rhythm' channel. The hum is low in tone and the pitch seems like an A#.
2. The hum is completely unaffected by the amp's 'Volume' control (this is the equivalent to most amp's 'Gain' knob. Turning the gain up adds a hiss on top of the hum, but I would expect hiss from high gain and it seems normal. On the Lead channel, the hiss increases alongside gain, and while the hum is there it's much much fainter.
3. The tone knobs do not affect the hum at all.
4. The Reverb knob adds a different high frequency buzz on top of the low hum, but it's not that loud or disruptive. It seems unrelated, and while I'd like to fix it, right now it doesn't matter. The buzz added by the reverb is the same on both channels
5. The master volume knob increases the volume of the hum, as does the final 'output' knob.
6. The hum gets worse during the first 10 minutes or so the amp is running and then levels out once everything is up to temperature.
7. (Edit) SS or tube rectifier does not impact the hum.

I have gone through each preamp tube and replaced it with a brand new tube to test them. There's no change to the hum from doing this, though when you swap a new tub into the v4 slot (which is related to the Rhythm channel as I understand it) the hum goes away for a few seconds as the new tube builds up heat.

The power tubes seem fine (though they are old). As per the manual, I pulled them one at a time and flipped off standby to see if it impacted the hum, it did not.

I have used the search on this and other forums, other people seem to have had similar problems, but the specifics of this one seem slightly different and it strikes me that someone with more tube amp knowledge could put these clues together in to a next step or two. Thanks so much for any help you can provide!

 

[domct203]

Does it hum without anything plugged into the input or fx loop?

Does it hum no matter where you plug in for power (home, band practice, different circuit, etc)?

Dom

 

[Asura]

Yep, nothing needs to be plugged in, and the gain can be at 0, it’s specifically the master and output that increase its volume. It seems like it should be a ground thing but it happens regardless of where it’s plugged in. And it seems like if it was an issue with the wiring it was plugged into, it would impact both channels equally?

 

[austin sonar guru]

Hello. I was just examining Maverick schematics and noticed that the Rhythm channel uses 190v dc power supply, while the Lead channel utilizes 170v. This is achieved by adding additional resistors in series to the circuit when you switch to the Rhythm channel.

I appreciated the details you provided.

Maverick's can experience problems with the numerous vactrols (optical couplers) on the circuit board. They can fail and still partially work. RY3a and RY3b are the ones I'd check......

Additional things to check if the Vactrols are ok:

There are 3 extra resistors in the Rhythm power supply section (170v). The schematic shows 4.7K, 6.8K and 6.6K resistors in that section.
They should be located on the left portion of the circuit board (to the right of the 4 large 30uf 500V Filter capacitors).
Hope this helps.

 

[Asura]

Very specific thank you. It’s a bit beyond my technical ability to address that myself, I’ll have to speak with a tech.

 

[austin sonar guru]

That approach makes a lot of sense. If you have the time - it would be nice if you could post a follow up of what your tech found.

There are so few posts on the less common Dual-Rectifiers, Maverick and Blue Angel. Such great amps. I've owned a Blue Angel since 96.

Trivia note: It's not well known that "Dual Rectifier" in the case of the Blue Angel is due to Preamp DC supply being solid-state rectified and DC power amp supply being tube rectified. On the Maverick it's more straight forward because of the switch.

 

[Asura]

I’ll definitely check back in. I love the amp, though I am a little fearful of this turning into a large repair bill or endless quest to fix something impossible to find.

 

[tbonesullivan]

I'm pretty sure that V4 (as well as V1) are only used by the Rhythm channel, so with that a good tech should be able to isolate the hum. The amp is definitely old enough to have some of the electrolytic caps, resistors, etc develop problems.