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.

 

[Asura]

Hey everyone, just got this back from the tech; the problem is (kind of) solved.

The low hum seems to have been a tube issue, apparently one of the tubes had been inserted into the socket incorrectly? Frankly this confuses me as the hum existed for a while, and then I popped all the tubes in and out in an effort to test them without any change in the hum, but so be it, three new preamp tubes and the hum is basically gone.

The amp remains generally noisy though, and while the really loud hum is gone, a less prominent buzzing is on the clean channel. Strangely, the buzzing goes away when the volume (gain) knob is all the way up, and is otherwise consistent through the rest of its travel.

The pot is a bit scratchy when moving it as well. The tech told me this is not due to dirt, and that Mavericks are noisy amps (this forum does seem to bear that out). He said he could lower the noise floor but it would take a lot of time to track down the issues.

I don’t really mind the buzz where it is, and particularly at the shop using clean power it seemed pretty subtle, but I am curious about the fact that it disappears entirely when the knob is turned up all the way.

I’ll probably reach out to him about it tomorrow but wanted to post an update here and see if others had similar experiences with knobs acting this way.

 

[Asura]

The more I mess with it the clearer it is that the hum is diminished but still there and that’s a bummer. It really doesn’t present on the lead channel, still the same old issue. It will work for playing with a band, but given that the tech (who is well regarded in the area) seemed to think it would be difficult and expensive to track, I am feeling wary to dump more money into this thing, particularly given that so many people seem to have issues with noisy maverick clean channels that resemble my own issues.

 

[austin sonar guru]

Your culprit is likely the 12AX7 in v4. V4 is the 3rd and 4th stage of the Maverick preamp.

I would try changing it to 5751 or 12au7 and listen to the idle hiss go away. I prefer the 5751 but they are harder to find and more expensive than a 12au7. Put the 12au7 in V4 and test.

I believe your noise should go away or be greatly diminished.

 

[Asura]

Thanks for the suggestion, I’ll give it a shot when I have the chance. This guy did replace v4 with a new ax7 so it’s at least fresh and presumably working right.

 

[Asura]

Bumping this thread with a new development.

I opened the amp and probed with a chopstick while it was running. I have found what is either the source of the hum or the part of the circuit that is responsive to it.
It’s the wire from the volume pot for the clean channel, highlighted in this picture. Tapping this wire is microphonic, which is not true of anything else in there. The wire is pretty tightly placed, but moving it slightly changes the tone of the hum a bit. Tapping the board next to where this wire connects is also microphonic, but doesn’t change the hum.

The lead channel pot is attached to the same bundle of wires. Its wire does the same thing but much less intensely, which matches what I hear when the amp is running.

Anyone have insight or thoughts on this?

 

[Asura]

Interestingly, my amp has the same symptoms as user bleachedfan. For all I know it’s the same amp, as I got it used, but I think it’s more likely this is some common problem in the circuit.

A relevant thread
https://boogieforum.com/threads/volume-knob-crackling-not-what-you-think.65681

I have decided that it must be possible to fix these amps and I am going to figure this out.

 

[ICR]

I build and repair amps. I have had my Mesa Maverick since the 1990s and bought it new.

I think the hum is a design flaw. I have looked at the circuit and tried a number of things (tubes, capacitors, grounding) to get rid of the hum with no success. I still like my amp, and try to use it at volumes where the hum drops below the noise floor. For example, using with an attenuator.