MarshallJCM800 2210 hum

[Tom Phillips]

Great! Progress! 3.2VAC on the first filter node means that the filament supply and the B+ rail are connected together somehow...

Consider also that this may just be the normal ripple on the supply line. If the B+ rail is 450V DC then the ripple calculation would be
(3.2/450)*100 = 0.7% which seems within the normal range for the first filter node on that amp. (This assumes that you are referring to the node that feeds the OT center tap)
Substitute your actual supply rail value for the "450" in the equation to calculate the exact value for your amp. The % ripple should decrease dramatically as you make measurements on the subsequent nodes that feed the rest of the amp. If it does not then you have a power supply filtering problem. By the time you get to the pre-amp I'd expect the power rail ripple to be down to around 0.001%.

Please post a link to the schematic you are referencing.

Cheers,
Tom

[Chuck H]

That's a good observation. That 3.2V looked suspect for the filament supply. It just happens that the typical ripple and the 1/2 filament wind voltage are nearly the same. Back to square one I guess

[Joe L]

If the tubes are plugged in and providing a load, the reading is normal. If not, it's indicating a problem.

[J M Fahey]

1) Can somebody post the amp schematic so we all refer to the same one?
Would hate to suggest something based on a different revision or something.
We have been chasing ghosts for some time now, so having the room brightly lit will sure help.

2) what multimeter are you using?
By any chance, is it one of those with only 2 AC scales?
Usually 200V and 750V or thereabouts.

You should use one of those which have AC scales down to 200mV or something.

[big_teee]

I agree with chasing ghosts.
Hum is a very hard thing to fix on the Telephone, or by forum.
It still sounds like a ground loop, or lack of ground to me.
If it were mine, I would start moving and rewiring grounds to see if you change anything.

[Chuck H]

If the tubes are plugged in and providing a load, the reading is normal. If not, it's indicating a problem.

Capitol! Pull the tubes and THEN check for AC on the HV rail.

[Chuck H]

And still... An issue that's being disregarded is the low filament voltage. AC mains check out. The unloaded voltage on the filament wind is 6.3. That seems low, doesn't it. Only five and a half volts on the filament supply when loaded. Why? I think everyone is assuming this is normal for this amp!?! Is it?

[Tom Phillips]

Regarding the low heater voltage. The Heater windings of both the original and the replacement power transformers measured ~6.3V unloaded. That's low to start with and will sag down further when powering the tubes in the amp. Therefore, I say that the lower reading with the tubes installed is "normal" for this PT and probably due to a mistake in the design or manufacturing of the PT. I believe that a problem with the meter was already ruled out as reported in a previous post.
Therefore, at this time, I think the low heater voltage is a diversion and not the root cause of the hum.

I think that the owner reported that the hum started after a previous tech changed the choke and the hum remains if the old choke is swapped back in. That's a significant clue.

[jvm]

Here's a link to the schematic guys. JCM800 2210 Marshall Amps Info & Schematics
I am taking my AC readings with a Energy Concepts 50200A Bench meter. It has a 200mv scale. I am refering to the first node as the cap at the bridge rectifier, HT fuse and the OT center tap. With tubes installed, I have 2.8 VAC at first cap, then .6Vac and .5Vac at the second and third. I pulled all the tubes and now measure .5VAC at all filters.

[Tom Phillips]

I pulled all the tubes and now measure .5VAC at all filters.

0.5VAC at all filters is too high. You should be down to the tens of milli Volts by the last node. Probably a loss less with all the tubes pulled. Something is not right with the filtering. Maybe an open filter cap. A cap can that's not grounded, something miss wired etc.
Tom

[jvm]

I don't think it would be a bad cap. The hum was present when I got the amp and the first thing I did was all new electrolytics. I have been moving grounds with no change. I've also studied online gut shots and the schematic. I don't see any apparent wiring issues.

[Tom Phillips]

Understood. I know it feels like walking in circles when a thread gets this long.

It appears, based on your ripple measurements, that the multistage power supply filtering in your amp isn't functioning correctly.

I suggest the following next step.
With the caps all discharged you could verify the grounds from each filter can negative lug to the chassis.
Measure the resistance from the lug (right where it enters the can) and chassis ground (not the solder lug at the can screw but at an independent ground point and use the same one for all measurements) We are looking for a missing or high resistance connection.

That's by best thought as of now.
Tom

[jvm]

Thanks, Tom. Yes, it definitely feels like we are running circles. Lol. I greatly appreciate all the help from everyone. With caps discharged, I measure .4 ohms from each cap ground lug to the AC line chassis ground.

[Tom Phillips]

So much for that idea.

Note: Since they all measured the same at .4 Ohms I suspect that you'd get the same reading if you touched your two probes directly to each other. In other words .4 Ohms is the combination of the test lead resistance and the zero offset error of the meter. Some meters have a function that allows you to null out the offset. Otherwise just touch the leads together and do a manual correction when measuring very low resistance values.

[Chuck H]

How about the resistance between filter nodes? There should be resistance between each one. But mistakes can happen when things like the choke or filters are replaced.