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**Jazz P Bass** · 08-31-2012, 01:28 AM

First off, does the amp hum with nothing plugged into the input jack?
It shouldn't.
With nothing plugged in, do any of the controls make the amp hum?
Second, it would be nice to know the hum voltage level.
If you have a meter, set it to read volts/ac, hook the meter leads to the speaker connectors & measure the hum.
Zero is ideal.(not gonna happen on a tube amp)
4 mv's is great.
6 mv's is ehhh.
10 mv's is annoying (something is not correct)

**soundguruman** · 08-31-2012, 02:14 PM

Originally posted by Chevy View Post

I posted a thread long ago about a hum problem, and never got it resolved, thought I'd give it another shot here.

The .50 Cal+ seems to generate way too much 60hz hum (or it could be 120, not sure) under normal operating conditions. I've replaced all the power supply filter caps, and checked the power supply under full power, and there's very little ripple, so power supply seems very good. I've replaced all the preamp tubes with new ones. Using a scope and a "noise grounding" test probe with a large audio coupling cap with one end grounded, I've isolated the source of the hum, it seems, to V2B or V3B (V2B is right after the tone stack, and V3B is the stage right after that... the reverb "driver"). I checked carefully under bright light for any possible black burnt streaks or areas in and around these tubes on the pcb, and see nothing bad there (thinking perhaps conductive pcb). None of the R's or caps seems to be burned or otherwise conducting to a trace underneath them.

Can this be a heater hum problem? There is no way on earth I would've designed this board the way the heater paths are... it seems the design/layout just begs for problems with electromagnetic noise. Two questions:

1) Has anyone perhaps tried to raise the DC potential of the 6.3V heater supply on one of these amps? If yes, what values of parts did you use, and how high would you raise the DC potential? (it would be a positive voltage, right?)
2) The 6L6's also run off the same heater supply; applying the new DC potential to the center of the two 100R reference resistors would raise the heater supply on the 6L6's as well... is there a problem with this?

Thanks kindly for any experience / advice on this!

Best,
Chevy

Well sure, all high gain amps also amplify hum and hiss, depends on if the noise is coming from the amp itself or your guitar and effects pedals.
Your guitar and effects pedals produce lots of hum and hiss, and that is amplified thousands of times.
But DC heaters are always a consideration when you are going for a huge amount of gain.
And sure, the AC from the filaments IS being amplified, along with the guitar signal. Hence, the hum, which you already experience.

There are all kinds of DC heater adaptations the best of which can be found in McIntosh stereo preamp schematics.
But the DC needs to be like a battery, with no ripple, to get the least 60 cycle noise, and that presents some quirky design problems.

You have a switch on the input jack. This closes a contact to ground, when the guitar is unplugged... The contact is connected to the output of the tone stack.
Yes, the tone stack.
This grounds the output of the tone stack, when the guitar is unplugged, to kill hum and hissing noises.

You have another switch, on the input jack. This closes a contact to ground, to ground the input of the amp when the guitar is unplugged.

If the switch is not closing, the amp will hum when the guitar is unplugged.

It's usually one of three things:
A. A resistor laying on top of the circuit tracks is arching down to the board.
B. There is arching on the board between a preamp grid track and a high voltage track.
C. There is arching between a high voltage track and a heater track.
The arching produces a carbon spot. The carbon spot IS an extra resistor in the circuit. The carbon must be completely removed by drilling, cutting, etc...
The arching usually occurs in the preamp section, between two circuit tracks. It may be UNDER the resistors.
LIFT the resistors and check carefully under each one.
The arching, can produce a carbon spot through the fiberglass, inside the board. NOT visible on the surface.
This can be found with a light and a magnifier, OR by removing the parts from the circuit and testing for leakage (with an ohm meter) between circuit tracks.
The typical leakage is 2.2 megaohms...between heater track and plate track, OR between grid track and plate track.

There's something else:
You see the solid buss bars (uncovered wires) that are soldered to the controls and jacks? The ground buss?
Look where the wire is threaded through the control terminals. The soldering at that point may be cracked or defective.
Check very carefully that the solder connections on those buss wires have not cracked, where they are attached to the controls...
Some of these wires were dirty when they were soldered. The solder never took to the terminals.
The wire may "appear" to be soldered, but the connection is not solid. The ground connections may be lose or defective.

LOOK under the board...
Check to see that the wires from the PT have not broken off the board. Check very carefully, as the wires on this amp between the board and the chassis break very easily. The center tap wire of the filaments may have broken off the board.

CHECK the effects send return jacks. The internal switches must be functional, or there will be HUM.

This IS one of the hardest amps to fix, by far. The design is, well, horrible.
It takes an expert with nerves of steel, test equipment, and infinite patience to make one of these amps work right.
If you succeed, you are definitely on my "respected" list. most techs run away from these amps, and it's easy to see why.
(there's a lot more problems than you currently realize-you are in for a fun, fun time)

**Chuck H** · 08-31-2012, 03:48 PM

Isolate the problem. Pull V1. Still humming? Pull V2, etc. Once you've isolated the problem to a specific circuit it may be easier to find a solution.

Have you tried other preamp tubes??? I remember a build I did about a year ago where I turned it on for the first time and there was an unacceptible amount of hum. This has NEVER been the case with this particular model. So I opened it up and started inspecting grounds. Nothing amiss there. Once you've ruled out the impossible, whatever is left, however improbable, must be the truth. So I replaced V1 and the hum went away. It was a brand new tube. Fresh out of the box!

It might be helpful to know if it's 120Hz or 60Hz. An easy way to tell is simply that 120 sounds a tad like buzz/hum whereas 60 is unmistakeable low frequency hum. A signal gen is the surest way.

There's nothing to lose elevating the 6.3V supply. Just build a divider off the hot side of the standby switch. Use a 220k in series with a 22k to ground. Use a 100uf/100V cap across the 22k (- to ground) and connect your 6.3V CT where the two resistors join. This is good for about 45V of elevation. Which should be plenty in most cases. Since this amp is a PCB I have NO idea how difficult this might be. Some guys suggest building the divider off the last preamp node. This makes NO sense to me. Why bring any extra heater components and circuits that close to the most sensitive stage in the amp!?! Bah! Use the other side of the HV rail for sure.

It might also help to know what a normal hum level is for this amp. Early Mesas do hum. They were vintage style hot rod amps built on PCB's. Without the benefit of more evolution in high gain circuitry, a bit of hum was unavoidable. As is noted, the layout and PCB tracing are clearly problematic. And lastly...

Some Mesas have hummy SS components. I had a Mesa that used a SS effects loop that way notorious for becoming hummy. There was an online fix but when I called Mesa about it they said "Just send it to us and we'll fix it for free." Who could say no to that!?! My amp came back with the SS effects loop module replaced (with an upgraded circuit) and new filter caps. The amp was over ten years old. So I thought the new filters were an especially nice touch. The .50 cal does have a SS components for the EQ, bias supply and switching. If any of this is not decoupled well enough or is failing in some way that could be a problem. Mesa has a rep for being stodgy. And in some ways they are. They also have a rep for confounded, dry sounding circuits. And that's also true in some cases. But they were pivotal in the development of modern high gainers and have always been especially accomodating every time I've had to work with them. That said...

Call Mesa! They may be very aware of particular hum issues that these amps develope in age and may already have a "fix" for it. Nothing to lose by checking and you don't get anything if you don't ask.

**km6xz** · 08-31-2012, 04:44 PM

I agree with Chuck on this one, call Mesa, they are good with taking time to help.

I also agree with the recommendation to isolate the source since there is not enough information provided as where the problem might be. Don't replace or alter anything, repair and modding are two different animals. Changing anything with hopes of the problem disappearing is less a possibility than just making things a little, or a lot, worse by introducing more uncontrolled variables.

First off, find out what type of noise and whether it is normal for that model in your working environment. Does the hum occur the same with other playing locations? Does the reverb sound OK, you mentioned the reverb tube. An open reverb pan coil or wire would certainly introduce hum.
You have spent a lot of time replacing things which did not need to be replaced so step back from the soldering iron and diagnosis it rather than changing anything. Go back to the very beginning and isolate the factors that contribute to the noise, such as whether the input jack being shorted has any impact, pulling V1, any control settings that change the level or tone of the noise, a measured noise level at the outputs with all controls normalized( set with gain down, tone controls mid point) and another measurement taken with gain in normal setting(write down the gain control position so you can reproduce the same conditions again).
Report back with your findings if the results do not lead you directly to the circuit stage that is impacted.

**Chevy** · 09-01-2012, 07:23 PM

Thanks for all your feedback... much appreciated. This may indeed be something to call Mesa about...

OK, got some more details for you, just to answer questions and clarify things. I'm only at this point talking clean channel... not the lead channel.
With nothing plugged into the amp, the Master volume on 10, Gain (first volume control) on zero, I get about .4V AC rms hum out of the speaker jack, 4 ohm speaker load. It's pretty annoying. Putting the scope on things, I can see a significant amount of 60 cycle on the plate of V2B, perhaps .01 to .02 V p-p, this is the first stage after the tone stack; this is with the Gain at zero. The gain pot is right there, just before V2B in the amp. So with nothing feeding V2B, it has an obvious 60 cycle on it's plate. There is no detectable 60 cycle on the 100K (power supply) feeding V2B. I can't see any ripple on the cathode of V2B.

So.... this 60 hz comes out of nowhere and appears on V2B's plate. Can't see any problems in the area of this tube in terms of burned traces or anything.

Heater hum problem?

**soundguruman** · 09-01-2012, 10:43 PM

Originally posted by Chevy View Post

Thanks for all your feedback... much appreciated. This may indeed be something to call Mesa about...

OK, got some more details for you, just to answer questions and clarify things. I'm only at this point talking clean channel... not the lead channel.
With nothing plugged into the amp, the Master volume on 10, Gain (first volume control) on zero, I get about .4V AC rms hum out of the speaker jack, 4 ohm speaker load. It's pretty annoying. Putting the scope on things, I can see a significant amount of 60 cycle on the plate of V2B, perhaps .01 to .02 V p-p, this is the first stage after the tone stack; this is with the Gain at zero. The gain pot is right there, just before V2B in the amp. So with nothing feeding V2B, it has an obvious 60 cycle on it's plate. There is no detectable 60 cycle on the 100K (power supply) feeding V2B. I can't see any ripple on the cathode of V2B.

So.... this 60 hz comes out of nowhere and appears on V2B's plate. Can't see any problems in the area of this tube in terms of burned traces or anything.

Heater hum problem?

There is a way for 60cps AC to reach the plate, there may be a burned spot, a carbon spot between plate and heater track.
You will not see this until you take the board out and look at the component side...
once the carbon forms, it's a resistor. Imagine a resistor between filament track and plate track...what would happen?
Well for one thing, AC would appear on the plate. Even with the standby turned off. Just heaters on.
maybe it is coupled by a 2.2 meg resistor, or somewhere about, caused by the carbon...

I realize that this is a real pain, but that might be the only way to fix it.

**Jazz P Bass** · 09-02-2012, 12:18 AM

[QUOTE=Chevy;273631]Th
With nothing plugged into the amp, the Master volume on 10, Gain (first volume control) on zero, I get about .4V AC rms hum out of the speaker jack, 4 ohm speaker load. It's pretty annoying. QUOTE]

0.4 Vac hum? (That is 400 millivolts. If you measured correctly, that is bad)
A good amp should read 0.004 Vac.(that is 4 millivolts)
How about with the master at zero?

**J M Fahey** · 09-02-2012, 12:20 AM

Sorry but I see Chevy doing his homework, he has already detected that

Using a scope and a "noise grounding" test probe with a large audio coupling cap with one end grounded, I've isolated the source of the hum, it seems, to V2B or V3B (V2B is right after the tone stack, and V3B is the stage right after that... the reverb "driver").

and

Putting the scope on things, I can see a significant amount of 60 cycle on the plate of V2B, perhaps .01 to .02 V p-p, this is the first stage after the tone stack; this is with the Gain at zero. The gain pot is right there, just before V2B in the amp. So with nothing feeding V2B, it has an obvious 60 cycle on it's plate. There is no detectable 60 cycle on the 100K (power supply) feeding V2B. I can't see any ripple on the cathode of V2B.

So.... this 60 hz comes out of nowhere and appears on V2B's plate.

This is *very precise*troubleshooting.
Dear chevy, please post here the amp schematic, specially the preamp area where V2B lives, to suggest some specific tests.
Thanks.

**Chuck H** · 09-02-2012, 01:02 AM

On the schem I'm looking at,V3B is the effects loop send/reverb/dry summing stage. Not the reverb driver. Though the reverb is placed between V2B and V3B. I don't know if I'm looking at the the right schem or not. V2B is indeed the tonestack recovery stage. Thing is, there is nothing in the way of a hum source that could be isolated to V2B plate. Several stages share the same power supply node, there are no independent controls on this circuit and the only place for an outside noise source to get in is the reverb circuit. But then you should have WORSE hum on the grid of V3B.

Since it's 60Hz, the most likely source is the filament circuit. And I did ask, but I'll ask again, did you try a known quiet preamp tube in the V2 spot? New doesn't count unless you've used it in another amp and it's "known quiet".

Can you confirm the amp circuit. If I have the right schematic link I'll post it here for all to see. And...

What are your voltages on V2B?

**Jazz P Bass** · 09-02-2012, 03:19 AM

50 Cal+ schematic

Attached Files

**soundguruman** · 09-02-2012, 05:47 AM

You can call mesa, but they won't help you.
They will tell you to go to a service center. That's all.
They have known about the problems for many many years. And still won't admit the cause.

After complaining about resistors laying on top of circuit tracks, and burning, for like about 30 years...
They started to leave space between the resistors and the board. That took, like 30 years. No, I'm not kidding.
In the newest mesa amps, they actually started to leave a space. Thanks, Mr. Smith.
But I also see in the new amps, 54 mechanical relays. No I'm not kidding, there's 54 relays inside the amp.
(you don't believe it, look for yourself) LMFAO.

NOW look at the space between V1 and V2. You see R104 and R105?
These are the resistors that are laying on top of the plate track.
That's where it burns. Right there, under those two resistors.
Seen it happen plenty of times.
FYI: mounting those resistors to the opposite side of the board is a good idea, when you replace them.

Do you live in a humid climate? Where it rains a lot? Or near the ocean?
How did I know that?
When the humidity is high, that's when the board arcs. In the desert, this won't happen.

**Chuck H** · 09-02-2012, 08:50 AM

SGM has been hammering this for the entire thread. And there's no "clear" answer to the cause yet. I'd look into it.

... But I'd also be interested in whether or not a known good tube has been tried and what the voltages are.

**J M Fahey** · 09-02-2012, 11:30 AM

Dear Chevy, *hope* you are referring to the same circuit as was kindly posted by JPBass. Please confirm.
If hum appears at V2B's plate, but it does not come from the supply line, there's only 2 paths for it:
1) filament to cathode 60Hz coupling, which so far looks like the higher possibility.
To test it you can
a) try different tubes there. Quoting from my poor memory, cheap chinese tubes or some types of Sovteks had *much* less hum than JJ's or other Sovteks, so try it.
b) don't know how filament voltage reaches this tube, if through PCB tracks it's not easy to modify, but I'd try to disconnect the original wiring and add a couple twisted wires straight to the PT or something similar. .... or ..
c) connect those new wires to a 6V DC source and see what happens.
Although I think the best option is to try many different tubes there, after all Mesa has been producing this amp for ages, as-is.
2) the other possible path is through poor grounding at the Vol pot.
I would disconnect the wire coming from it to V2B's grid and ground said grid straight to the ground pad of its cathode resistor. Any change?
By the way, MB schematics are famous for having errors , either through carelessness or on purpose.
I *think* V2B's cathode resistor is actually 1K5 , although it's written as 15K (ridiculous). Please confirm.
3) if you have , say, 20mV Hum at V2B's plate, you *must* have 50X as much on V3B's plate. Do you?.
Good luck.

**Chuck H** · 09-02-2012, 04:15 PM