Marshall 40th Anniversary Super 100 JTM 45/100 hums.

[Randall]

Got a re-issue MArshall in for a humming problem. It is also unstable at high gain setting on both channels, causing a high squeal.

The amp is confusing me because on back is is listed as model JTM 45/100, and Limited Edition Super 100 Amplifier, and on front it says MK II. I have not been able to find a proper schematic a of yet, one I grabbed at Metro Amps is not entirely correct.

Anyway, when the inputs are up a bit I have 120 HZ 6.2 VAC hum on the outputs. At about 75% full the amp goes into oscillation. The guy says he was playing bass through it at not very loud volumes when it suddenly made a horrible noise. He shut it down, and ever since it makes the humming noise. I have probed around, and determined the preamp tubes are not the cause, as I have swapped them around with a new one. If I remove the second pre tube, I cannot make it hum nor squeal. I know there is a rule of thumb regarding where to look for 60 Hz vs 120 Hz hum, but I don't remember what it is. The pre plates are clean, and nothing shows any visable signs of stress.

I could use some help on this one guys. Thanks

[Chuck H]

6.2V of hum is pretty enormous! The fact that it's 6.2V is suspicious but I'll avoid the obvious correlation to filament supply voltage since the frequency is 120Hz. 120Hz hum and oscillation usually mean either bad filter caps or a bad filter cap ground. Check filter cap grounds. If they're good try paralleling a good cap across the filters and see if that doesn't change anything.

If it's actually 60Hz hum it could be a bad power tube/s bleeding filament AC onto the signal path via the power rail (that is failing to decouple it because it's bad or has a bad ground which may also be causing the oscillation).

My guess is a bad ground. I think that amp has the HV rail nodes set up to filter only out of phase stages. That would negate a bad filter causing the oscillations as it does with some amps. A bad ground may cause failing decoupling at more than one node which COULD result in oscillation.

[Randall]

After checking all grounds, hitting all solder points on the top side, more swap testing, my poor eyes finally noticed one of the KT66s is not lighting up. I was swapping out different combinations of pairs and found quite a large difference in AC at the plates with different pairs. The three working tubes are pulling within 3 mA of each other at idle, yet the level of ripple is different with each. They are Russian big bottle Gold Lion, of which I have no experience.

Plus, I gotta wonder with all the iron in this beast (Ginormous PT, and two OTs) if some hum is just gonna happen from all that inductance, but I may be wrong about that. Anyway, a new quad and we go from there.

I want to get this right because this customer is the guitar player in a big name hard rock band, and he has some very cool amps that will be needing service.

[Diablo]

Why don't you try it with 2 output tubes and see if the hum goes away?
If it does, that confirms you have a bad tube causing the problem.

[Chuck H]

I was thinking the same, but refrained from posting because of the "two OTs" comment. Not sure how the amp is wired, but it's probably possible. Randall knows better than we do.

I'm having a hard tine envisioning how a bad power tube could cause the oscillation though. Perhaps an anomaly with the NFB loop? The important thing is to get the amp repaired and working "properly" in a timely fashion. That way Mr. Big Cootie will continue to use Randall's service! If we can trouble shoot and solve for the instability problem while he waits for a new quad of tubes that can only help expedite things

[Randall]

I am still fussing with this amp. I replaced the burnt Gold Lion KT66 with the same rating from the same vendor. Biased it down quite a bit, checked everything I can think of and it still hums. The diode rectifier feeds the standby switch. When it's open, the supply is clean, when it's closed I have about 20v p-p saw on my scope. I don't have any KT66s on hand. Do I dare popping a quad of used 6L6s in there just to see if it changes anything? Here is the schematic I am working from, but it's not entirely accurate.

http://forum.metroamp.com/download/f...5bb0151715f9cc

[Heliotricity]

Do I dare popping a quad of used 6L6s in there just to see if it changes anything?

Is this one of the reissues with the really high plate and screen voltages? If so, make sure whatever you put in there can handle the voltages, otherwise you might end up with additional problems! I've had some issues with Gold Lion KT66 tubes, JJs too. They work for some folks but I keep away from them.

I was researching a ways back trying to understand the differences between some of the Marshall reissues, the super 100JH, the 40th Anniversary JTM 45/100 and the 1959 HW. I dug this up about the 40th anniversary, maybe it will help.

The 40th anniversary has a special Drake potted mains transformer, which is a replica of the Radiospares De Luxe power trans (and has not been used in any other Marshall reissue), a pair of 50 watt output transformers (probably same as fitted to the JTM45 reissue head but not certain), and JTM45 spec pre amp mixer caps and tone stack...

[Randall]

Is this one of the reissues with the really high plate and screen voltages? If so, make sure whatever you put in there can handle the voltages, otherwise you might end up with additional problems!

Yes, it is. Plates are at 535 v presently. Pins 1 and 8 are tied together at the socket on this amp. Since there is no pin 1 on a 6L6, I just wonder if it's OK to swap them for a moment at idle to see if the hum changes.

And, it is the 40th Anniversary version re-issue.

[Chuck H]

I just re read this thread and caught this:

Anyway, when the inputs are up a bit I have 120 HZ 6.2 VAC hum on the outputs. At about 75% full the amp goes into oscillation.

Does "inputs up a bit" mean that you need to turn up the volume/s before the hum is present? You say you've eliminated the preamp tubes. Ok, good. That doesn't mean the hum isn't in preamp circuitry though. You also say that "The pre plates are clean". Does this mean you don't see any hum on the preamp? And you also say "If I remove the second pre tube, I cannot make it hum nor squeal". These last two statements seem inconsistent. Maybe a test parameter was missed? I don't think the hum is in the power amp at this point. Straight up, if you can stop the hum by removing a preamp tube then the hum is either that tube, in that tubes circuit or prior. I'm thinking a faulty ground in the area between the first and second preamp tube circuits.

The dead power tube was a coincidence and a red herring. That sort of thing happens. Don't let it dissuade you from practical trouble shooting.

[Randall]

I have probably been somewhat inconsistent in some of my reporting, and for that I apologize. I did conclude some hum was eliminated by tweaking the pre amp leads a bit, but not the larger part of it. I now think this amp has three problems. Dead tube, fixed. An oscillation when turned up to 8 or more, probably from lead dress and probably there from day one (customer says he has never had it past 5, so for now I am not worrying any further about it). And hum from the KT66s. I went ahead and swapped a quad of used 6L6s for a short bit, and no hum. So I think these Gold Lions have been the problem right from the get go, and I have chased my tail over it for quite a spell.