DSL PT physical hum

[drewl]

Marshall JCM2000 DSL reissue that has a pretty nasty physical hum when powered on, audible without standby turned on.
It wouldn't bother me, but this is a picky owner.

So far loosening the mounting screws and moving it slightly can diminish the hum, I still need to disconnect the secondary connections to see if it's a loading problem, but I doubt it and think maybe shimming the end nell and chassis connection would be the way to go.

What do you think?
Cheap trans, just the way it is?

[J M Fahey]

If it's induced hum in the OT, rotate one of them 90 degrees and check again.

If mechanical, it deserves a long refreshing bath in marine varnish or similar (or any oil based "synthetic" paint leftover, the kind that air dries in a couple days and stinks the room) .

[drewl]

Can't rotate the trannies due to mounting, no choke on these and the trannies are at opposite ends of the chassis.

[xtian]

In my experience, some transformers are "born that way": unbalanced from the factory. PT arrives, unbox and put on bench, apply power, and it hums like a fish tank pump. Send it back.

[Leo_Gnardo]

Run across some similar JCM2000 and JCM900 PT's that vibrated and that cause the chassis to emit a hum/buzz, very annoying. In most cases tightening the mounting bolt/nut reduced it to an acceptable level

In a MusicMan 130HD 2x10 earlier this year, had a similar problem, and it wouldn't go away no matter how much I torqued down the mounting or damped the bell covers. Owner was aggravated about it as I was, and he had an identical amp that did not hum the same way. Solved it this way: opened the mounting holes a little then used silastic grommets between the nut/bolts and chassis, grommets glommed from reverb tanks, they're kind of translucent white and seem to be made of silicone. Got a hundred of 'em at least, knew they'd be useful for something someday. The price was right, the problem solved to everyone's satisfaction. Hum/buzz was reduced to practically nothing. Might be worth a try on your Marshall JCM2000.

[Gregg]

I had the chance to replace couple of those. They just have a design flaw. The bobbin inside is split in two which is not recommended for a 200W PT. I replaced those with same specs PTs ordered from the local transformer factory. Because they were properly wound they are 1/ dead quiet and 2/ do not heat up.

[drewl]

This is one of the newer reissues, they have cage nuts riveted to the chassis for mounting.

They use Loctite on the screws, possibly to reduce noise.

The end bell covering is causing alot of the noise.

[nickb]

I had the chance to replace couple of those. They just have a design flaw. The bobbin inside is split in two which is not recommended for a 200W PT. I replaced those with same specs PTs ordered from the local transformer factory. Because they were properly wound they are 1/ dead quiet and 2/ do not heat up.

I do believe this is the crux of the issue.

I had a long email chat some time back with a guy who has designed transformers for Marshall. The problem is that the EU regulations specify creepage and clearance distances that result in a split bobbin requirement. There are constraints on the physical size of the transformer so the space for the windings is reduced. Rather than use thinner wire they opt to run the core at a higher flux density which in turn leads to buzzing.

[J M Fahey]

I guess that's the point.
We are going to see more and more split coils for higher primary to secondary insulation (think kilovolts) .

Edit: that said, I run iron at insanely high Gauss values , and have no buzz problems ... but I dunk finished transformers in a can of somewhat thinned transformer varnish (so it gets into every small space better) and let it there for an hour, although no bubbles are visible after 5 or 10 minutes ... guess OEM factories can't wait that much when they have to deliver a large order ....... or they vacuum impregnate only the bobbin but not the fully finished transformer.

I have solved many a small but annoying buzz by dripping some transparent car lacquer paint (the old Nitro or Acrylic type, not the fancy new ones) , think similar to old Duco or Dope paints, between core and bobbin until it starts coming out from the other side.

Yes, sometimes I make a mess of it, because I try to avoid unmounting the transformer but do it on site.

Silverface era Fender transformers, besides big, heavy and overdesigned, were often assembled and then *filled* with polyester rosin, through the bell holes, which turned them into a solid brick.

Hum ???? Buzz ???? What's that?

[Gregg]

The problem is that the EU regulations specify creepage and clearance distances that result in a split bobbin requirement. There are constraints on the physical size of the transformer so the space for the windings is reduced.

That's strange because I'm in EU and I ordered from an EN61558 certified factory and they did it on a single bobbin.
I don't understand why those regulation would apply to the PT and not to the OT which is wound on a single bobbin?

I have solved many a small but annoying buzz by dripping some transparent car lacquer paint (the old Nitro or Acrylic type, not the fancy new ones) , think similar to old Duco or Dope paints, between core and bobbin until it starts coming out from the other side.

The ones I replaced were very well impregnated both core and bobbin but it didn't help.
At one point I decided to keep the iron but disassembling the core was not possible without taking extra measures.

[J M Fahey]

That's strange because I'm in EU and I ordered from an EN61558 certified factory and they did it on a single bobbin.

Well, you *can* pass tests and meet regulations by properly isolating primary and secondary: try *3* layers of 0.1mm Mylar (2 will do, the 3rd is an extra step) , the strips cut slightly wider than necessary so they press against bobbin walls (no gaps are allowed), vacuum impregnate so you reach the farthest points ....... that said split bobbin meets and exceeds Specs even without trying, even dry ; it ends up being the Factory decision , of course always passing tests.

FWIW here in Argentina our IRAM rules are in fact cut and pasted from European ones (in the old days the DIN ones) so I meet them all the time.

I don't understand why those regulation would apply to the PT and not to the OT which is wound on a single bobbin?

Mandatory Electrical safety rules apply to anything that is connected or related to mains, and have unexpectedly high standards because they do not intend to protect user from 120/240V electrical shock, but from consequences arising from an electrical lightning hitting a power line during a storm.

There *are* discharge protectors, neon and spark gap dampers, etc. , but they reason that an up to 10X the mains voltage peak can reach you, even if for a millisecond or less, punch insulation and cause a mess.

So a typical test voltage is 1500V DC for USA lines and 3000 VDC for European ones.

Since such rules have zero tolerance, and they kick you out of the game for good, some play it extra extra safe and test transformers, primary to secondary, with scary 3000V AC which of course means close to 5000V peaks.

FWIW I'm thinking that I see a discarded Neon sign transformer in a pawnshop or something I'll buy it, no kidding.

Getting 5000V DC is easy, of course.

The ones I replaced were very well impregnated both core and bobbin but it didn't help.
At one point I decided to keep the iron but disassembling the core was not possible without taking extra measures.

"Beauty can be skin deep"

FWIW a friend of mine has a Transformer factory, makes tons of small 2 way radio chargers, Bank security camera supplies, etc.

Once I was visiting him (I buy wire and laminations from him at OEM prices) I saw him dipping them for a second, putting them in a wire rack, and 15 minutes later dipping them again (with air drying varnish).
His explanation: "earlier I did it the right way, took a long time to impregnate, even longer to dry up , and this @#%^*+:"& customer always complained, now I double dip it for seconds, get a thick *visible* layer , nothing inside ... and he's happy